Twist and Washout in the Trike Pilots Handbook and Why Billow was replaced

Published by: Paul Hamilton on 2nd Jan 2017 | View all blogs by Paul Hamilton

 

Background:

 

When the FAA Weight Shift Control Aircraft Flying Handbook was being written, one of the objectives was to standardize the terminology so that the WSC trike could most easily be understood by existing and new pilots.

 

 

 

The term billow was initially used by hang glider manufacturers with the original Rogollo wings to add material so the wing was not flat. The nose angle was 90 degrees and the sail was designed to be 95 degrees. At this time this was considered billow so the term "billow" has hung on over the years.

 

 

 

In fact with my hang glider design background,  I personally used this term in the manual along with wing "twist" and "decreasing angle of attack" towards the tips. My FAA review team asked "what is this billow term?. We do not see it in any credible aerodynamic description". The dictionary term was interesting and did not look like any thing related to sail design:

 

 

 

billow

 

[bil-oh] /ˈbɪl oʊ/

 

noun

 

1. a great wave or surge of the sea.

 

2. any surging mass:

 

billows of smoke.

 

verb (used without object)

 

3. to rise or roll in or like billows; surge.

 

4. to swell out, puff up, etc., as by the action of wind:

 

flags billowing in the breeze.

 

verb (used with object)

 

5. to make rise, surge, swell, or the like:

 

A sudden wind billowed the tent alarmingly.

 

  

 

It was explained that all these ancient "Tribal" terms from old times/technology needed to be updated/modernized to commonly known aerodynamic principles. I was initially perturbed/irritated with this but I moved on.

 

 

 

rIt was also brought to my attention that the current FAA reference 2005 for trikes (Lucian/Hal Trikes- Flex Wing Flyers) Page 3-29 Flex Wing Flyersdoes not have the term "billow" anywhere. It uses the common aerodynamic terms "Twist" and "Washout" as the concept was introduced. Again on page 3-39 Flex Wing Flyers the word twist and washout were used to describe turning. No reference or term "Billow" anywhere in the book or  anywhere in any credible aerodynamic resource i could find with an exhaustive search.

 

 

 

I was convinced/forced to comply that both "Twist" and "Washout" were credible aerodynamic terms and we did not need to invent billow to confuse the issue.

 

 

 

So I added in the Aerodynamics section page 2-3 the common aerodynamic terms twist and washout and addressed the term billow to transition everyone over to the established aerodynamic terms on page 2-3:

 

 

 

Wing twist is the decrease in chord angle from the root

 

to the tip chord, common to all WSC wings and ranging

 

from 5° to 15°. This wing twist is also called washout as

 

the wing decreases its angle of attack from root to tip. The

 

term billow was originally used for the early Rogallo wings

 

as the additional material in degrees that was added to the

 

airframe to create the airfoil. It is still used today to define the

 

amount of twist or washout in the wing. The WSC may not

 

have twist/washout when sitting on the ground, and must be

 

flying and developing lift to display the proper aerodynamic

 

twist characteristic of WSC wings. [Figure 2-6]

 

 

 

 

Again on Page 2-13 twist and washout are described in turning on page 2-13:

 

 

 

Longitudinal Axis— Roll

 

Turning is initiated by rolling about the longitudinal axis, into

 

a bank similar to an airplane using aileron and rudder control.

 

To turn, shift the weight to the side in the direction of the turn,

 

increasing the weight on that side. This increases the twist on

 

that side while decreasing the twist on the other side, similar

 

to actuating the ailerons on an airplane. The increased twist

 

on the side with the increased weight reduces the AOA on the

 

tip, reducing the lift on that side and dropping the wing into a

 

bank. The other wing, away from which the weight has been

 

shifted, decreases twist. The AOA increases, increasing the

 

lift on that wing and thereby raising it.

 

Thus, shifting the weight to one side warps the wing (changes

 

the twist) to drop one wing and raise the other, rolling the

 

WSC aircraft about the longitudinal axis. [Figure 2-24] More

 

details on the controls that assist wing warping are covered

 

in chapter 3, which should be considered with use of the

 

controls in the takeoff, landing, and flight maneuvers sections

 

of this handbook.

 

 

 

 

Again on Page 3-9

 

 

 

Roll Control System

 

Control bar movement from side to side controls the roll about

 

the longitudinal axis. The wing attachment hang point allows

 

the carriage to roll around the wing keel. Thus, it can also be

 

looked at from the carriage point of view, when the control

 

bar is moved side to side, the wing rotates around the wing

 

keel relative to the carriage. [Figures 2-31 and 3-19]

 

It would fi rst appear that moving the control bar to one side,

 

thus shifting weight to the opposite side, could alone bank

 

the aircraft. It is true that shifting weight to the right would

 

naturally bank the aircraft to the right and put it into a right

 

hand turn. However, the weight alone is not enough to provide

 

adequate roll control for practical flight.

 

As weight is moved to one side, the keel is pulled closer to

 

that side’s leading edge. The actual keel movement is limited

 

to only 1 to 2 inches each side of center. However, this limited

 

keel movement is sufficient to warp the wing, changing the

 

twist side to side (as discussed earlier in the aerodynamics

 

section) to roll the aircraft [Figure 2-24] by changing the

 

lift side to side. Simply, the shifting of weight from side to

 

side pulls the keel toward the leading edge on that side and

 

warps the wing to roll the aircraft.

 

Besides the keel shifting relative to the leading edges and

 

crossbar, overall roll control is adjusted by the designers to

 

fit the mission of the wing through sail material/stiffness,

 

leading edge stiffness/flexibility, amount of twist, amount

 

of travel the keel is allowed, airfoil shape, and the planform

 

of the wing. [Figures 3-20 and 3-21]

 

 

 

 

 

So there we have it, why we used Twist and washout instead of billow and how the wing turns from the shift of washout and/or change in twist...

 

 

 

 

 

Comments

228 Comments

  • monty stone
    by monty stone 11 months ago
    I knew it ! Sorcery was afoot!
  • Doug Boyle
    by Doug Boyle 11 months ago
    A question on the WSC Written Test is: what is the purpose of the keel pocket? Does anyone remember the correct answer per FAA?
  • Gary Crayne
    by Gary Crayne 11 months ago
    The Keel Pocket is a vertical patch of sail fabric, usually located aft of the CG with the purpose of serving as a yaw stabilizer, keeping the wing from wandering left and right along the normal (yaw) axis.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Doug,
    Good question since it is not as relevant. Note that after 2008 when this book FAA Weight-Shift Control Aircraft Flying Handbook was published the FAA has failed to update the question bank. Pretty pitiful. And this question could be eliminated since it is not as relevant, however it does apply to some trikes as referenced by diagram Figure 2-33 where the P@M has a keel pocket AND vertical tail. Now it could be modified to add of wing tip rudders and/or wheel rudders.

    Answer with longitudinal stabilizer is wrong since that is roll, roll stabilizer is wrong since it is roll, the best/correct answer is "Act as a yaw stabilizer, keeping the wing from wandering left and right"
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Yes Gary correct we posted about the same time....
  • Doug Boyle
    by Doug Boyle 11 months ago
    That is the correct answer per FAA, but as Paul pointed out it is not relevant in modern design. There was a time when keel pockets stood tall and shifted from side-to-side with the sail; even before the crossbars started floating. But even today the pocket for the keel is generous, and allows for the movement of the sail to be off-center. The other additions to roll control are now numerous and the combinations allow the contributors to be fine-tuned to miniscule amounts.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Directional stability or yaw stability as I recall. Doug.
  • Gary Crayne
    by Gary Crayne 11 months ago
    Doug, you asked a question. I gave you the answer. If you only want a response from a specific member of the site, please address it so.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Gary and all,

    Many times different answers from different people provide different perspectives which is good for the best learning experience which I hope is an objective here.

    Best to all perspectives and opinions.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Gary's answer is a perfect answer. I think Dougs point is the FAAs answer is far from helpful to a new sport pilot grasping the keel pocket.

    To add further to the answer, the keel pocket also controls reflex by the cut angle of the keel pocket. Or how about its purpose is to hold the back of the fabric wing down to the frame! :-)
  • wexford air
    by wexford air 11 months ago
    Could it be a pocket for the keel?
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    I should be known that the FAA does not provide any correct choice for the answers. They provide three choices only. It is up to the student or test prep provider such as ASA, to provide the correct answers and the explanations. I provided the answers and explanations for the ASA test prep and Prepware Knowledge/written test answers. I have seen a provider provide incorrect answers on some of the trike multiple choice questions. Pretty funny actually. Luckily the knowledge test is super easy and hard to fail. You can give most of the questions and the answers. None of my students have ever failed it....
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Paul, I think it is the written test that Doug was referring to that does have an answer because it's multiple-choice
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Yes I know Doug was asking about the knowledge/written test. I should have said above that the FAA does not provide the CORERECT CHOICE for the knowledge test questions which I just clarified above.
  • Doug Boyle
    by Doug Boyle 11 months ago
    The purpose of the question was to stimulate the discussion for the edification of all readers and posters. Both the question/answer on the FAA Written test is cause for some to doubt their understanding of the subject matter. Rather than remaining in the dark due to whatever reason, the answers provided by all should bring most up to speed. Thanks everyone!
  • Joe Hockman
    by Joe Hockman 11 months ago
    Just to clarify, when I inquired about the description of billow in the FAA manual, I did not appreciate that there was a consensus process involving multiple reviewers. It certainly makes sense to me that folks from the fixed wing world would not be familiar with the term billow as an aerodynamic word. I guess I don't view it as an aerodynamic word either but more as a "characteristic" of the wing that does impact aerodynamics. I also recognize that the dictionary definition of billow is not that helpful either in the wing context. I accept that washout or twist or wing warping are the preferred terms. I think it should be clear that billow may only be relevant with fabric wings. Of course it applies to trike or hang glider flex wings, but it also applies to PPG or PPC wings, and there is even a tiny bit of billow in the "wings" the wing suit nuts use to do a "controlled" plummet. But that billow does affect aerodynamics and it has a big impact on the stability of an aerodynamic state.
  • Doug Boyle
    by Doug Boyle 11 months ago
    Originally, I understood "billow" to refer to the conical shapes in the Rogallo design - which was not much more than a parachute with penetrating capabilities. This preceded "shifting" since the cone shapes remained attached at the keel. Nose angles increased as billow decreased, and performance gains were realized as the aspect ratio increased. Not until the sail became somewhat detached from the keel and rose above, did things start getting "shifty". Hence, Billow Shift. Throw in some more nose angle and further reduce the billow and the crossbars started to float. Along came some camber in the battens (originally used to reduce trailing edge flutter and cupping) and the resulting aerodynamics started to transform the heretofore modified parachute into an aerodynamic marvel!
  • Larry  Mednick
    by Larry Mednick 11 months ago
    In my opinion billow shift is a type of wing warping. Billow shift has to do with an aerodynamically formed trailing edge changing shape that alters virtually 100% of the shape of the entire wing tip to tip changing the angle of just about every batten. In fact it is mid span battens that do plenty of the work to make the wing roll since that area of wing is quite large with a long cord. The mid span battens in fact move the most if we measure trailing edge height change, but it is the shortest battens that change the most angle. So to me billow shift is not all about just the wing tips changing washout.

    Washout or twist is something that serves the purpose of making the wing stall first at the root. And in the case of our flex-wings it is one of two mechanisms for creating pitch stability (the other is airfoil shape)

    If the FAA wants to get rid of a term, I vote we get rid of the term "twist" it is completely redundant. Washout is not the same as billow although billow creates washout.

    It is only due to the delta wing shape of our wings that the billow in the sail creates washout. If our leading edges were not swept, the lowest AOA would occur mid span.

    This is a unique aspect of our wings.

    Wing warping is not always billow and Billow is not washout although Billow creates washout and billow shift is wing warping. Wing warping changes the washout and changing the washout causes the wing to roll.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    While the keel pocket certainly plays a role in yaw stability, it wasn't conceived for that purpose at all. It was intended to increase roll rate and reduce roll pressure, which It assuredly did, for the reasons Larry mentions above - assymetric lift from billow shift. It took some time after before the extent of its role (ha! a pun!) in yaw was appreciated. Something no one has mentioned is that to some degree it crossfeeds yaw into roll.

    It was originally intended as part of a foot operated roll system. Those whacky guys from Down Under would try anything...
  • Joe Hockman
    by Joe Hockman 11 months ago
    Well this is bit strange but I completely agree with you Larry. Billow shift can be viewed as a type of wing warping but it is not the same as washout. Billow shift does enable greater washout to occur or viewed another way the magnitude of washout is controlled by the extent of the billow shift. I also agree that twist is no different from washout and is actually redundant. I see little value in using the term twist except for the visual image it creates to understand changes in chord angle from root to tip. I also agree that washout and airfoil characteristics like amount of reflex of inboard battens are the primary mechanisms to ensure pitch stability. The description of billow as the aerodynamically formed trailing edge arc from root to tip that is dynamically changed through weight shift induced billow shift is spot on. It may well be that we do not have enough FAA reviewers that are truly knowledgeable about the unique aerodynamics aspects of FLEX wings. These characteristics discussed here do not apply to 3-axis fixed wing aircraft and by the way they do not apply to rigid wing hang gliders like the ATOS.

    Doug you are right there has been a significant evolution from the early battenless Rogallo wings to our modern HG and trike flex wings. Many of these changes dramatically improved performance characteristics like L/D. But frankly I think there are many additional innovations and refinements that we will see in the years ahead. In a sense you can think of the STARS system from P&M and the roll trim system Larry developed as innovations that attempt to control or modify the nature of wing warping for aerodynamic advantage. But I can easily imagine future refinements that will further improve selected performance characteristics. After all one of the beauties of a FLEX wing is that the design and engineering can be flexible.

    Bryan, I was pretty sure that the keel pocket was not originally conceived for the primary objective of imparting yaw stability. I found your story about Bill Moyes tinkering with this idea very intriguing. Actually Bill has made some rather remarkable contributions to flex wing design and he came up with arguably the holy grail for HG towing with the Dragonfly.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    I agree with Larry, Joe, Bryan and Doug. The flex wing has gone through many evolutions and it is common knowledge the "basics" of how the flex wing turns. Along with other factors, it warps, changes billow, twists, changes washout, mutates, transforms, distorts, deviates and the terminology list goes on. I am not necessarily in favor nor disapprove of the terminology billow or billow shift. I am simply explaining why it is what it is in the FAA manual. Hopefully everybody understands that the terminology does not change the aerodynamic principles which I think everybody agrees on.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    So billow controls washout. More billow more washout.

    BONUS QUESTION: What other design aspect controls washout??

    Hint: not tip twist, not haulback tension, not leading edge tension.
  • Doug Boyle
    by Doug Boyle 11 months ago
    A: Trailing Edge tension (in the case of the Bionix wing, for example)
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Good answer Doug, but that is still billow. Variable billow.
  • Leo Iezzi
    by Leo Iezzi 11 months ago
    Larry, ( I'm out of my league here) but are you referring to the tension system on a P&M? ( Stars)
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Leo, good answer!, But what I am thinking of, all Wing's have.

    Hint: it is not a mechanical mechanism.
  • Leo Iezzi
    by Leo Iezzi 11 months ago
    Well, I'll go with my first though......I'll take Battens for 1000 Larry. >< haha!
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Leo, reducing the camber or adding a rebend/reflex especially in the battens near the tip will reduce the angle of attack at the tips somewhat. But there is a much larger factor that I am thinking of that controls wash out other than the sail cut/billow.
  • Tony  Castillo
    by Tony Castillo 11 months ago
    The curvature (camber) of the battens control washout as well.
  • Tony  Castillo
    by Tony Castillo 11 months ago
    and the twist of the end cap in the wingtips also control washout.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Great answer Tony, but I excluded tip cap rotation in the original question.
  • Tony  Castillo
    by Tony Castillo 11 months ago
    floating cross bar?
  • wexford air
    by wexford air 11 months ago
    Has keel pocket size been mentioned? This will allow the sail root to rise relative to the wing tips
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Wexford air, there is one other major contributor to washout not mentioned yet but, keel pocket height was exactly what I was thinking of. More specifically, is there a way to raise the root trailing edge without the use of a taller keel pocket? And if so, what do we call that?
  • Doug Boyle
    by Doug Boyle 11 months ago
    A floating keel perhaps...

    On the Bionix wing design which uses a tall keel pocket on its "variable billow" system (Corset), the trailing edge "floats" and changes height in a miniscule amount. When it's relaxed the height is lowered compared to when it's tight. Three straps beneath the keel and attached to the nose battens, spread and lower as the "washout" is increased in the tips. When the wing is static you can see the effect in the tip area.

    One day when I have the wing off the trike I may try to stand the wing on its kingpost and video the overall effect to the sail.
  • wexford air
    by wexford air 11 months ago
    Variable geometry hangblock? That's mechanical though......
  • wexford air
    by wexford air 11 months ago
    And not all wings have that....mmmmmmm... Reflex
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Doug, Wexford - you guys are both talking about but not saying it

    Hint: the control bar length controls this ________
  • Joe Hockman
    by Joe Hockman 11 months ago
    A few other variables affect washout.

    1) keel flexibility. Not uncommon to have double and triple sleeving in vicininty of hang block with less sleeving toward nose and toward aft end. Usually at least one sleeve in rear of keel where haul back cable connects but there may or many not be a sleeve all the way to extra reinforced hang block area. This enables keel to have some slight flexibility aft of the hang block area in N-S and E-W directions. Flexing may actually be very minor but any flexing may contribute to billow shift and washout.

    2) Sail spanwise tension, not necessarily only at trailing edge (which is important) but in mid chord section. This could enable or resist more washout. Related would be batten flexibility. Usually very small to negligible flexing in camber area that forms leading edge of airfoil but some flexing midspan toward aft tips can enable more washout. Typically high batten string tension fights against this flexing.

    3). Variable anhedral. Larry, I may be wrong but in your roll trim system is not a greater amount of anhedral added when wing trimmed for fast speed and reduced anhedral when trimmed slow. If so more washout would be possible with less anhedral and less washout with more anhedral.

    4}. Variable leading edge flexing. Don't believe any US or most trike wing manufacturers use this yet. Leading edge tubing (and especially outer leading edge sections) with an elliptical cross section (slightly squashed circle) can be used with longitudinal tubing axis in plane of keel with minor axis of tubing in Z axis or perpendicular to keel. Upshot, greater resistance to flexing toward aft end of keel that makes it harder to pull back haul back cable and keeps tight spanwise sail tension. However in Z axis, outer sections can flex more which clearly will assist with greater washout and roll responsiveness. This can be done with special elliptical extrusions of AL alloy or with CF tubing. A good graphical representation of this is figure 2-30 in the FAA-H-8083-5.pdf manual. Please note that the amount of anhedral (inboard of wing strut junction) vs the amount of dihedral in outboard section of wing is a bit exaggerated in this figure but in any case it gives more of a multihedral effect which imparts some desirable aerodynamic aspects and yes more washout. I believe Aeros is playing with oval or elliptical tubing in their high end HG and it could be that Wills Wing is also experimenting with this.

    I think those are the primary additional variables that can affect washout.
  • wexford air
    by wexford air 11 months ago
    Aha!
  • Doug Boyle
    by Doug Boyle 11 months ago
    The length of the control affects anhedral...and the CG location vertically.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Bingo Doug!! Although Wexford and Joe hit it as well. Anhedral!

    If sail cut creates billow and billow controls the height of the back of each batten, then anhedral controls the height of the front of each batten. Then we can also say, increasing anhedral will reduce washout mid span and increase washout at the tips more.

    Leading edge stiffness, mentioned by Joe, is one of the major factors as well and I am surprised that no one has mentioned one last ingredient for controlling washout.
  • Joe Hockman
    by Joe Hockman 11 months ago
    Well, washout sprog setting or configuration can also have an affect. Out board or in board sprogs can be set to "enforce" a specific amount of washout. In some cases they can be set to be active in normal S&L flight. So setting could limit ultimate washout for a particular wing loading.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Not what I'm thinking of Joe.

    Hint: This controls the "curve" and found on both strut braced and cable braced wings.
  • Craig Dingwall
    by Craig Dingwall 11 months ago
    This is way above my pay grade but really interesting, I'm going to say is something to do with Winglets (although not sure how that fits with the last clue?) but there is a relationship between the effects of these and washout (I think :-) )
  • Joe Hockman
    by Joe Hockman 11 months ago
    If you are talking about the curve associated with the airfoil then batten string tension or flip tip tension is critical since this tightens the sail chord wise and helps to resist flattening of the airfoil spanwise. But I already mentioned this above so you must be talking about another "curve". Maybe the curve associated with the billow? Or maybe you are referring to the curve on leading edge of airfoil due to the leading edge tubing bending when haul back cable is tensioned?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Craig, winglets do change the aerodynamic washout and in some cases restrict physical washout by locking up the tip area so it can't flex up like it would if the winglet wasn't on it.

    Hint: does the undersurface fabric play any role in washout control?
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    How about another hint... Why does a single surface "generally" handle better than a double surface?
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Great discussion to help all those ENGINEER TYPE understand the many factors in trike wing design. TO ALL NEWBIES OR PILOTS FOR THAT MATTER,
    Do not worry, you do not have to understand all these advanced details of trike wing design, just the basics in the trike manual. But be glad/thankful you have the manufacturers working on all these factors and pilots providing feedback for everyone.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    That's a good hint Paul. Yes you are absolutely thinking of the answer I was after. Except the last words in your hint need to be corrected to Double surface.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Roger corrected
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    It is interesting where the trike manufacturers sit with VG (variable geometry) variable wing washout/twist (and of course the ancient tribal term billow). Air Creation uses Le Corset to vary the washout/twist. P&M aviation uses the STARS system to vary the twist/washout. The Hang glider manufacturers are using the cross bar tension method and more recently a similar cross bar extension method to vary the twist/washout (more tension/less twist). This increased sail tension was used over a decade ago on Hang Gliders and well refined today. I had one of the first variable twist Wills Wings with the lever that could cut off a finger if not operated properly. Now they use a chord/pulley system for all different types of settings.

    All these are great innovations to optimize low and high speed flight characteristics. High speed low twist, low speed more twist: both for many good reasons. Larry/Abid/Aeros and others have decided to keep it simple with a movable hang point for trim adjustment. Larry's variable anhedrial/dihedrial system built into the hang point fore/aft adjustment does provide some VG wing adjustment to optimize for high and low speeds.

    Yet this Hang Glider cross bar tensioning is not widespread for trike wings. I have a 16 meter Northwing here on a Maverick single place trike that has North Wings VG cross bar tension system on it. I suspect we will see more of the trike industry utilize the cross bar tensioning method in the future. However along with levers/pulleys/arms/actuators comes weight, complexity, failure modes and the time, money, testing and certification to get it out there. There are many challenges to get the wing/trike undercarriage working correctly at modern high and low speeds, now add VG and it gets more complex.

    It will be interesting as to how trike wings evolve in the future. My fourth hang glider wing design (that never got built) had the pilot in the wing and twist grips that changed the twist of each side of the wing individually by moving the sproges. I started triking and my desire to design and build wings turned to the pure pleasure of flying and passion for teaching others.

    Thanks to ALL 2016 and upcoming 2017 who put the time, effort, resources, devotion and passion into designing, building and supporting hang gliders and trikes.... And thanks to all here brainstorming (brainics and basic questions/comments) and helping all to understand the details that make our wings fly....
  • Tony Ford
    by Tony Ford 11 months ago
    I've been called worse Monty! I'm glad to have these "Masters of Trikes", to teach us "Apprentice" flyers.
  • monty stone
    by monty stone 11 months ago
    Me too, Tony!
  • monty stone
    by monty stone 11 months ago
    paul/larry,et al, great informative discussion! my comment got 'deleteioned' but i mentioned that my 12m double surfaced handles smoother and carved turns easier than my 14.7m s/surface, which 'notices' mini- bumps more than the 12m, which ignores them. what do you mean by 'better'? also, how come i can still get 'P' factor without 'tail-feathers? also they both climb at max 1k per min but the 12m bar about neutral, the 14.7 i have to pull into my gut to prevent a 'wheelie'? (i should address to DEAR ABBIE of trike-dom !)
  • wexford air
    by wexford air 11 months ago
    Of course the beauty/problem with flexwings is that everything is inter-related, if you change one setting, you will affect something else as well so that has to be catered for. Perhaps the VG cross bar tension has detrimental effects to different trike/engine combinations or return so little benefit that is not worth it. That said,I really can't wait to see what the next 10 years development brings and hope we see a growth in this type of flying. I love it
  • wexford air
    by wexford air 11 months ago
    And I want to see the purchase price of these machines come down to my level as well if that's ok. Or perhaps my income to rise to their level, ya that sounds better ☺
  • monty stone
    by monty stone 11 months ago
    hey wex, you don't ' have' to order chrome hub-caps and white-wall tires! make sacrifices !
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Monty and Tony,
    Sorry to call you guys brainics, please accept my apology.

    Monty, note I said "generally" so the single/double surface statement does not apply to every wing.
    Wexford, you can get a flyable trike for under $10,000. Full range for all budgets. I know many who can not afford a $9,999. If you have a trike you are lucky.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Monty you are correct that P factor is caused by the rotating airflow over the tail of a fixed wing, but the prop creates asymmetric thrust when it is not perpendicular to the relative wind. For the second reason we experience P factor in trikes.

    Paul's hint is spot on. There is a fundamental reason double surface wings do not billow shift quite as well as a single surface. But someone is going to have to cough up the answer before we discuss or Paul and I are going to keep feeding you clues.
  • wexford air
    by wexford air 11 months ago
    Answer? Sorry I missed the qn Larry. The reason they dont billow as well? The bottom surface lifting the x-tube?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Question was what major "thing" controls washout. So far we have:

    A bunch of small things
    Sail cut
    Anhedral
    And what is the 3rd major "thing" that controls washout?

    Yes the cross tube does interfere with twist, but the cross tubes end mid span and do not interfere with the outboard section of the sail.
  • Neil Scoble
    by Neil Scoble 11 months ago
    The under surface stitched to the top surface restricts the movement of the top surface, that is why you have a fully floating unconnected top and bottom surface on the Rival S ?
  • Joe Hockman
    by Joe Hockman 11 months ago
    Oh wow, isn't so great to have a "Trike God" here that can straighten every one out? NOT. This sort of disrespectful and condescending behavior has been often repeated on this forum. From the days of Toby SkyDog and before all the way to today. One of the reasons many have discontinued their membership here and moved on to more professional, friendly and respectful forums. This sort of behavior is also why many that periodically do come hear choose not to provide their input/thoughts/comments.

    "Great discussion to help all those ENGINEER TYPE understand the many factors in trike wing design. TO ALL NEWBIES OR PILOTS FOR THAT MATTER,
    Do not worry, you do not have to understand all these advanced details of trike wing design, just the basics in the trike manual."
  • Craig Dingwall
    by Craig Dingwall 11 months ago
    Yes Neil that's all I can think of relating to a relationship with the bottoms and top fabric, that and that there are also the stiffening battens in the bottom.
  • wexford air
    by wexford air 11 months ago
    It must be obvious but sorry, I'm just not able to click right now! Apart from what Craig and Neil say all I can think of is the set or backwards bend of the leading edge tips
  • John Glynn
    by John Glynn 11 months ago
    Wing loading
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Yes Neil!!! You are talking about it now. That is what I was thinking of. Who knows what that is called???

    Joe, with all due respect your posts are so technical I doubt most followed it (I think I did and it was very good) . I think it's a fair statement Paul made to say basically to those that are completely lost right about now, this is a bit above required knowledge. And Paul knew the answer that everyone missed up until his post, so I can only assume he was not lost in the conversation and stating that those who are, don't have to worry about knowing all the stuff that you and I and others are writing about. But I have really enjoyed this little blog and it has gotten my creative juices flowing. I am inspired by this discussion. Some really amazing thoughts are above. Good stuff!

    Now who can name what Neil described?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    John I think Wing loading is up on the board. If not that is pretty large factor as well for sure.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    White Eagle, yes, excess length between the front and back cables can aid in additional keel shift and billow shift. Reduced length on the wing struts increases Anhedral and effects washout which is up on the board.

    Can you name what Neil is talking about?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Btw regarding VG. Yes it's as good as being able to change Hang block position in the air. Adjusting speed trim and roll trim on the fly is awesome stuff. Tensioning the Haul back during flight is on many hang gliders and now on the Northwing soaring Trike. I personally view it as optimizing performance while sacrificing handling. My view is a have a powerful engine, I like my wing "soft" all the time. I'll pay for better handling with a few extra drops of fuel. Anyway that's kind of why I haven't focused on it myself. But I think the more things you can tune in flight instead of landing, adjusting and test flying and then Repeating until happy the better. So I think it's something that could be in the REVOs future, and if so it will be an upgrade for the existing fleet. But don't hold your breath on that one. Working on other "priorities" as we speak.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    If I offended anyone please accept my apology. Not my intention. My intention is to let the newbies not be overwhelmed with the high level trike design this has blog has evolved into.

    All the high level wing design guys here and those with practical experience Joe, Larry, John, Wexford, Doug, Tony, Leo, Monty, Gary, (hopefully I did not forget anybody) please understand all this trike design is not easy to understand for the new student/pilot just getting into the sport. Simply, the new student/pilot is trying to understand the basics (such as the WSC handbook) foundation before they try to comprehend the engineering of wing design. That is ALL. If anybody interporated this as me not saying Joe was smart at engineering discussed hre , big mistake. After that they more to a greater understanding through learning and flying, eventually they can gain the knowledge to participate in high level design discussions such as this. The basic building block approach to learning.

    So please understand that the newbies need to build the foundation of knowledge as in any sport or discipline and the new student pilots do not need to be wing designers.

    You start at the 101 and you work up to the 404.

    If any one took this personally and thought that I was trying to say THEY were not smart. NOT TRUE. Not said and not meant.

    My comments are to assist the newbies/student pilots. Do not read anything more into it than that.
    Hopefully this clears things up and we can more on to positive posts.

    Usually when I write things, I go back and look at them to clean them up but sometimes I do not. I will try to more sensitive to everyone feelings and read things a second time.

    Thanks to all.

    Again if I offended anyone with my intent to communicate with the newbies/student pilots, I am truly sorry.
  • Craig Dingwall
    by Craig Dingwall 11 months ago
    Larry, your right this has been a great discussion. I have enjoyed everyone's comments and descriptions and read and re-read some with interest. Better than that, it has also led me to some other informative material in looking up some of the terms used by some of you that I have not fully understood in the past.

    Paul, loved the initial diagrams, thanks. And from a Newb with insatiable appetite for information and knowledge (because with this comes a level of safety), thanks to everyone's input, I know I appreciated it.

    NOW, what can we discuss next............ :-)

    (That is once we nail this term we are looking for
  • Job Chithalan
    by Job Chithalan 11 months ago
    Despite having been involved with powered hang gliders and trikes from the early days, I for one, found this topic very interesting and educational.
    Unlike any other forum that I know off, TPS is blessed with top designers, CFIs and experienced pilots who help a lot with their valuable contributions.
    Different points views are only natural and not to be taken to heart.
    Well done everyone and many thanks :-)
  • monty stone
    by monty stone 11 months ago
    i got it! the 'lost term' is auto-spatial-orientation-ability-enhancement-self-adjustability-regulation! it came to me on the toilet! ps'paul what's a 'brainic', should i be 'blushing' or reaching for my list of 'free contingency lawyers'?
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    One of the great things I love about flying are the people and trying to learn something every day. Everyone contributes something. Thanks to all.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Monty, You should be blushing.....
  • monty stone
    by monty stone 11 months ago
    now i'm 'REALLY' blushing.
  • Bill  Hilton
    by Bill Hilton 11 months ago
    Gents,
    Thanks for the great topic and the vast knowledge shared by the group. Yes i agree this topic required a little more critical thinking by most of us, however, its this sharing that makes this forum valuable to the triking community, as a whole. This forum continues to be an incredible tool for sharing our experiences and knowledge, as well as, advancing our communities safety.

    Thanks again guys for the topic and education.
  • wexford air
    by wexford air 11 months ago
    Of course the old "billow bloomers" as I used to call then on my favorite wing, the raven. I was actually referring to that earlier. The elastic between the top and bottom surface looked like a pair of yer grannies pants. The early versions of this wing by the way also had a tendency to lock into an aggressive side slip, sorted with an extra batten underneath.
  • Leo Iezzi
    by Leo Iezzi 11 months ago
    Paul, no offense taken whatsoever. My career is manufacturing engineering. If you need something made, I can tell you if the design is manufacturable, with which methods, associated costs, and design changes that would yield a faster better product in some cases. I'm not a design engineer ( although I do ok based on my experience since I can do the math, ask Larry :D ) , nor an aerodynamicist.

    Larry, so, with that said....I'm chomping at the bit! What Neil describes translates somewhat to airfoil shape ( even though the battens are responsible for that). If it's a flex wing term outside the norm, then I wouldn't know what you would call that.
  • Doug Boyle
    by Doug Boyle 11 months ago
    Elasticity of the undercamber....
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Leo, to clarify what Neil is talking about is given the upper surface has a set pattern and shape and the under surface has a set pattern as well, the further back you sew the under surface to the upper surface the more the triangulation you have making the under surface restrict billow. Some wings used Neoprene sections to allow "stretch" in the connection, we use bungee cords. There are two factors, the distance back caused by under surface area and then taking that same area and moving it back another 1/4" for example.

    Paul's hint was particularly good since a single surface wings do not have the "triangulation effect" caused by the under surface coming back so far.

    I thought, though not sure now, that the shape of the cut along the leading edge AND the position of the stitching in the undersurface to the upper surface was considered the LUFF CURVE, but now I'm not sure about the second one. In which case I've had everyone searching for a term that may not exist. :-0 in any case Neil described it perfectly.
  • Leo Iezzi
    by Leo Iezzi 11 months ago
    Haha Larry! Well, name or no name I can picture what Neil described and I see where it makes sense.
    Great post by the way! Lots of great information!
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    We use the term luff curve to describe the cut in the leading edge of the sail that allows for leading edge flex... but I don't know if that's the generally accepted meaning either. The term comes from the cut of a yacht sail and has been appropriated into hang gliding.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Personally, I do think it's worth all pilots having at least a broad-brush understanding of how pitch stability is derived, how dive recovery is done, how yaw stability is achieved, and what billow shift is about. Understanding angle of attack, particularly in turns, saves lives, my own included....

    I'm an ex 'freestyle' competition hang glider pilot, and fairly used to what is termed 'recovery from unusual attitudes'. My worst moment in triking has come from doing something I considered utterly routine and completely benign: a slipped turn. For the first time ever for me, the usual roll level didn't work because the wing's billow shift was being fed by drag from the outside wing, resulting in the roll continuing to increase. All over in a few seconds, after getting the billow shift neutral with a touch of pitch forward, but those few seconds were a pants-filler....

    I think this thread is great. Doug, Joe and others obviously have a good level of understanding and are looking for which correct answer is the one Larry is thinking of; and others are following and learning. I think this kind of Q&A approach is great. I don't want to intrude on the flow of this topic, which is valuable, but when people are ready to move on I'd like to suggest another question, as although on the face of the question is a separate topic from billow shift, it relates billow shift to coordination in turns:

    Where does bar pressure in pitch come from? Or, in other words, why is there a neutral position in the forwards and backwards movement of the bar?
  • Walt Baydo
    by Walt Baydo 11 months ago
    :-O Haha!
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Awe shoot... there goes another member.... ;-)
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Static pitch stability Bryan. Not to be confused with dynamic pitch stability.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Yeah, I should have said 'when in straight and level flight...'
  • Paul Dewhurst
    by Paul Dewhurst 11 months ago
    Posting the billow shift description of the manual reminds me of my pet hobby horse - it's been explained that way for decades but it wrong! ( or I believe it is - it's a brave man who states anything definite in this game..:)

    It all relates to what is assumed is happening to angle of attack. The traditional explanation says that due to billow shift the angle of attack on the down going wing is lower than the rising one. I say the opposite is true..:

    Imagine if our wing was rigid - if we try to roll it by weightshift, as it starts to move downwards due to our efforts the angle of the relative airflow changes and angle of attack increases, and the loading on that wing also increases. - we would feel this as resistance to our roll input and a low roll rate - high roll damping.

    What billow shift does is to respond to this increase in angle of attack and loading. As the downgoing wing experiences that increased loading it twists in response to that load - reducing its incidence. It still has a higher angle of attack than the upgoing wing - but the differential is not as great as it would be without billowshift. This results in less roll damping, and a faster roll rate for a given roll control effort.

    So quite different to the explanation in the WSC manual. And it also makes the illustration of the lift envelope whilst rolling quite wrong too.

    What do I know to dare to disagree with this old received wisdom? - well, I don't claim any cleverness, and I used to trot this out to my students too. Then Bill Brooks (P&M chief designer) gave a flexwing theory talk to the UK instructors seminar and explained it the other way. We were all shocked - he was bemused that we believed otherwise and pointed out the flaws in the reasoning. A few years later I asked Gilles Bru ( Aircreation wing designer) to explain to me Billow shift - and he explained the same mechanism as Bill Brooks - with no collusion. And the more I thought about it the more it made sense to me.

    So the book is all wrong..! - unless someone can convince me with a new explanation..!
  • wexford air
    by wexford air 11 months ago
    Its a chicken and egg thing i reckon Paul. which happens first? the billow shift or the dropping wing and I may be wrong but I think some wings vary. I think that the billow shift happens first in an XL anyway because when I move the bar I feel the trike sway first and then the wing starts to roll, what do you think?
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Many years ago, in an attempt to reduce drag on a hang glider, a real effort was made to remove the keel pocket on what was then a modern wing. We stuck a camera on the keel and recorded what happened during a turn in time-lapse. We did our best to quantify not just billow shift but AoA and timing, based on the weight shift. I've got those photos in a box somewhere, unfortunately not in digital. We started with what we thought we knew about AoA in turns: the driver of billow shift is the increasing AoA on the downgoing wing; the keel pocket allows billow shift to happen in response and reduce that AoA in a relative sense. Exactly as Paul Dewhurst says above. It was hard to give quantitive figures for the amounts we saw - lots of 'it looks about this much' - but timing was easy. Weight shift, then wing begins dropping, then billow shift in proportion to roll rate. So in essence we saw what Paul Dewhurst is suggesting. But I suspect the intention of the manual regarding AoA is also meant in a relative way... and maybe it doesn't ultimately matter to any but those interested whether the driver is load due to AoA or load due to weight. Understanding that billow shift aids roll by AoA changes a la airfoils is probably enough for most pilots.

    And for what it's worth, we couldn't get rid of the keel pocket without making roll rate, roll pressure, and roll/yaw coupling going to pot. It took ages before adding dihedral - the opposite of where we were heading - proved to be the key.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    My $0.02 on this discussion. Billow shift does not cause the wing to start to drop or its aoa to start to change. You dropping it causes the wing to drop start changing its angle of attack and billow shift then happens in aid of roll. Softer handling benefits from more billow shift otherwise roll is opposed more by aerodynamic damping naturally. However too much billow also means generally that you will lose efficiency. Nothing is for free and a balance is created in design to have acceptable parameters. Paul Dewhurst IMO has it right.
  • Doug Boyle
    by Doug Boyle 11 months ago
    While we all ponder Paul's proposition on this "shifty" subject (haven't wars been fought over the veracity of one book over another :)), I will "dynamically" alter the responses to Bryan's query on Pitch Stability by "statically" suggesting Center of Lift (CL) as a possible answer.
  • Joe Hockman
    by Joe Hockman 11 months ago
    There is a chance that I over reacted a bit yesterday, and if so I apologize for that. I guess my belief is that almost all (maybe all?) of the active posters on this recent thread are not NEWBIES and using that term and then say, well don't worry you don't need to understand these detail, I found disrespectful to others that had been posting. In any case, enough said, lets move on.

    Larry, thanks for your kind words. I was trying to add some of my thoughts to the discussion based on my understanding. And BTW, I do not claim to be an expert. I have never designed a modern flexwing from scratch. I will exclude my rather meager efforts in my very early days with bamboo and plastic as that was extremely primitive Rogallo stuff and not so successful. Also I do not have a degree in aerodynamics or engineering. My undergraduate and graduate degrees are in natural sciences but I did spend more than 2 decades within the applied statistics group within the engineering dept of DuPont. I have a fairly good grasp of engineering principles and yes I am an applied statistician consultant with years of empirical modeling expertise.

    Now I understand the "curve" you were talking about. Yes the triangulation of the lower (or pressure) surface to the upper surface does give some rigidity to the airfoil but since we are talking about fabric for both surfaces, the airfoil does change or distort some what. I think this is basically the elasticity that Doug referred to in the elasticity of undercamber. Clearly when we contrast a single surface vs double surface the airfoil thickness changes quite a bit but in both cases we are really talking about a relatively thin airfoil. There is a mean camber line (kind of the centerline between upper and lower surface) that has relevance in thin airfoil theory. For a perfectly symmetric airfoil (as may be used in a tail or wing tip winglet) that mean camber line falls directly on the chord line. Of course, this mean camber essentially falls right on the camber of upper surface for a single surface wing. I say essentially because at the leading edge there really is a lower and upper surface around leading edge tubing so mean camber would be intermediate line through this section. As an aside, single surface wings often do have a small percentage (say 20 to 35%) that is actually double surface and in this case forward portion of mean camber would follow a middle line between upper and lower fabric.

    So why do I bring this up? With the above as background, there are parameters such as aerodynamic center and center of pressure that apply to thin airfoil which are very relevant to washout. The more the mean camber line is cambered (as in the case of a single surface) the more the center of pressure moves aft of the aerodynamic center in that airfoil cross section. This tends to push up more on the trailing edge of the airfoil. Conversely, when the mean camber line has a much more moderate camber (case with mean camber line on 100% double surface airfoil), the center of pressure transitions further forward closer to the aerodynamic center. For reference, typical cambered airfoil has an aerodynamic center that is 1/4 of the chord behind the leading edge.

    So why might this be relevant. With a single surface wing the center of pressure tends to be more aft on the airfoils from root to tip as compared to double surface. Also many double surface wings are not 100% all the way to the tip. So in this case the center of pressure tends to transition from more of a mid chord position to an aft chord position as one moves from mid span to the tip. So from this perspective, as a roll progresses it is reasonable to expect billow (and billow shift) to develop on average more quickly with a single surface wing than with the double surface since much of the billow is located in the midspan region. This of course does not account for the triangulation that may impart some "rigidity" to the airfoil which I too believe is important.

    As an aside, there is a property of a vorticity distribution that is more accentuated with a mean camber that has a more pronounced camber, which helps to explain why single surface wings tend to generate more induced drag through more pronounced and well developed vortices rolling off the wing tip. Conversely well designed double surface flex wings tend to have a lower induced drag and hence are usually more efficient.

    I happen to agree with Paul D. hypothesis that angles of relative wind begin to deviate from being parallel to the root chord (when viewed from above or below) during the onset of a roll and I don't believe such deviations have been properly accounted for with causes for billow or with hypothetical lift distributions that are graphical drawn. Actual AoA changes due to this deviation and so we can not think of the AoA from our standard chord line on airfoils from parallel cross sections from root to tip.

    Lastly, I'd like to suggest there is a fair amount of complexity to flex wing aerodynamics simply because our airfoils are made of fabric and significant distortions occur as soon as we deviate from steady state S&L flight. Changing AoA, billow, billow shift, washout just in rolls probably smashes around our lift distribution in ways we may not think. Almost all the asymmetric lift distributions I have seen and drawn myself have some regularity about the asymmetry and I can easily imagine quite a bit of irregularity about the asymmetry as one progresses through a maneuver. Perhaps others have a different view on this. I would guess that real experts like Bill Brooks may have a good feel for the nature of the irregularities that occur. Maybe it is a bit of a black art or black box from an average trike pilots perspective. So as Paul H suggested there can be a fair amount of trial and error or trial and triumph as wing alterations are made. Perhaps unless one actually has well developed and validated mathematical models that do apply for a given flexwing where we are trying to change some property and we "know" that a particular wing modification will result in a "known" change to a model predicted property. My guess is it is so much easier to modify and engineer a fixed wing with a rigid airfoil than to do the same to a delta flexwing with a very flexible airfoil.
  • Joe Hockman
    by Joe Hockman 11 months ago
    Oh forgot to mention, I liked Bryan's new question on causes for pitch pressures. Paul H. brought up one topic that has peaked my interest for years. That is VG. Perhaps I will start a blog on VG some time but that will be for later. There is a broad frontier of possibilities that trike wing designers could pursue in this area. Yes I am very aware of the existing methods that have shown up on trike wings like the Corset, cranks to lift inboard trailing edge luft lines, etc and I do have a VG haul back tensioning system on my Solairus 16.5M along with electric trim. I had this customized for my needs. In the mean time if some one is inclined to start a blog on this topic that is fine.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Paul D, Abid and Joe, by your thought process a Cessna will have its low wing as it attempts to bank, increase AOA due to the relative wind from the wing dropping and stop rolling all together. Hopefully we can all agree that a little aileron can change the mean cord line of the wing and decrease AOA on the low wing. If we cannot agree on that we must agree the LIFT is greater on the ascending wing. If we can agree on that we can agree the lift is greater on the ascending wing of a Trike. Can't we? Then in that case we can replace the word AOA with AOI and agree the. AOI decreases on the descending wing from billow shift. Do we all still agree with that?

    I can roll my trike without weightshit!!!!! I can roll by billow shift only. No weight shift required. All I need to do is twist my nose wheel or twist my control bar or activate my trim tab and my keel shifts and my trike rolls for the same reason as a fixed wing. My AOI of my mean cord line reduces when my wing billows. My wing is high billow, I can roll 90 degrees per second my wing warps and twists from right to left as if I had barn door ailerons.

    And lastly I can go weightless to 0 Gs and induce a 0 G roll at the top of a parabolic arc.

    Everything is relative in this case the descending wing to the ascending wing is the relativity to 2 different angles of realative wind (agreed) and the only reason I didn't mention the ascending wing is for simplicity.

    So I am conceeding that the falling wing may have a higher AOA than it did in S&L , but the rising wing has an even higher AOA than the falling wing. So Bryan and others, HERE IS YOUR LEAD IN TO WHY PULLING BACK ON THE BAR TO ENTER A TURN IS AERODYNAMICALLY CORRECT.

    So Paul D, if we are on the same page great, if not tell me what you are not "buying" here. I can give examples and experiments you can try and prove that keel shift is primary.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Larry:
    Your explanation and theory is the standard one for flexwing turns.
    But during the dynamic phase of turn initiation versus the stabilized phase, things are different.

    Regarding your posting about adverse Yaw creating a helpful roll torque:
    Per Steve Siebel Experiments and his correction to his earlier thoughts:
    "Most modern flex-wing hang gliders actually exhibit a negative coupling between slip (yaw) and roll through most of the flight envelope. This mean that the slipping airflow created as an aircraft adverse-yaws will provide a helpful roll torque, not an unfavorable roll torque ..."

    So there is a coupling. One can cause the other. That does nothing to negate what Paul D. has said Bill Brooke's explanation of what happens during the dynamic phase of the turn initiated by pure weight-shift. Your video about the Blue Devil simply shows that you had a significant negative coupling between yaw and roll.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Thanks Joe for understanding I was talking to those who were following not those contributing. Again I should be more clear to make sure there is no misunderstanding. Common sense, experience and everyone looking at things in a different way, of which we all may not agree, stimulates learning. This has been a great blog for this.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    I do agree that as a wing drops and goes into a turn the angle of attack on the inside wing of the turn increases in trikes and airplanes similarly. The question is how much and does it really matter?
    Lets look at an example question:
    In a steady state start to finish, 50 MPH airspeed, 4 seconds to roll from level to 30 degree bank, with no billow shift (stiff wing), how many degrees is the angle of attack increased from the lowering wing with a 32 foot wingspan?

    a) 1 degree
    b) 1.5 degrees
    c) 2 degrees
    d) 3 degrees
    e) 5 degrees
    f) 10 degrees
    g) All of the above
    h) None of the above
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Lets look at the other side of this dilemma; In a turn, the inside wing is flying slower producing less lift than the outside wing. I hope we all agree with this. How much and does it really matter? So here is the question:

    If to are flying in a 30 degree bank, 50 MPH, how much faster is your outside wing flying than your inside with a 32 foot wingspan?
    a) 2%
    b) 5%
    c) 10%
    d) 20%
    e) 30%
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Abid the slip pushes back on the descending leading edge billowing the wing. The wing billows for WHAT EVER REASON without any weight shift and drops. This to me proves weight shift is not even needed. My theory agrees with what Steve Seibel said. Do you think the wing just rolls because...??? It rolls because lift is controlled By 2 things, AOA and speed. So adverse yaw would speed the wing, yet it drops. So obviously the only answer left is the AOA is heavily increased on the ascending wing compared to the descending wing.

    Keel shift happens first whether it's from a nose wheel trim tab or pilot pulling the keel over to turn (normal roll input). It is in fact asymmetric leading edge pressure that "helps" the pilot roll by billowing the wing so that the mAoA is different from right to left.

    Your theory, which Paul dewhurst describes would only work if your keel was locked. Then you would have a weight shift trike and the wing would only billow due to AOA or wing loading which wouldn't be much and controlled by leading edge flex only.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Excellent point Paul H. I haven't Done the math so my guess is A and A maybe B on one of them. Totally insignificant and countering effects at that as increased AOA and decreased speed cancel each other out.


    Abid and Paul D. Am I correct in assuming you are saying the turn is eantirely weight shift induced?

    Abid when you flew the RIVAL S (great example because it "torque steers" Fast. Do you remember you could roll faster by pulling in with one hand and pushing out with the other than moving the bar sideways. Sideways shifts weight and as a by product shifts the keel where as twisting the bar just shifts the keel. It rolls faster without weightshift! This is an extreme example to illustrate my point. I'm sure if we were turning something sluggish we may not have such a clear example.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    One more thing. Adverse yaw is caused by induced drag. Induced drag is drag caused by creating lift from the increased AOA. of the ascending wing. Proving my point again. The adverse yaw will generally only help the wing billow shift by a tiny bit, and unless it only takes a tiny bit to make the wing billow shift, then In that case it's like power steering and the pilot not only can flight the slight help and roll opposite (not recommended) but they can push out and coordinate and the wing will stop rolling or as you saw in the blue Devil video cause it to roll out of the turn again with no weight shift in roll what so ever. This makes the wing lock into a bank angle and removes the "roll assist" when you push forward.

    Getting back to the earlier topic, if a wing is heavy in roll it will generally take a huge slip to cause it turn into a turn on its own. And when that happens not only is the wing heavy to roll back out, but the slip is pushing hard on the descending wing which in combination can over power the pilot. So by coordinating the turn the pilot then can roll out easily.

    If we assume weight shift is what makes us roll and we don't believe it is billow shift making us roll, then none of what I'm writing can make sense. But what I wrote is fact and I have a video to prove it. Unless there is some other explaination for what I am describing. And that explanation cannot be a just a term. Tell me the reason the wing is rolling in the Bue devil video. Explain any dynamic that supports your argument. I told you exactly why it is. Now apply a different theory that could possibly explain it and I will listen intently.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Larry: Adverse yaw is caused by perpendicular lift vectors elements of left and right wings that are fore and aft instead of up and down when a rolling motion is in the picture.
    To put it more simply:
    You roll RIGHT
    RIGHT wing goes DOWN
    LEFT wing goes UP, its effective angle of attack is decreased (Airplane performance, stability and control. 1949. Wiley and Sons)
    A component of lift due to this ROLL to the right becomes:
    FORWARD on the RIGHT/DOWN going wing (resultant lift vector tilts forward)
    AFT/BACKWARDS on the LEFT/UP going wing (resultant lift vector tilts backwards)

    The ROLL caused these vector components to happen. Introduction of roll causes induced yaw via lift vector components fore and aft creating a torque around the yaw axis. This applies to airplane wings as well as trike wings.
    Its different in a rotary wing where the whole disc is tilted, its effect is felt 90 degrees off phase. Quite interesting.

    Starting the roll to the right does require for a very small period, higher lift on the left wing than the right. Lift has a squared relationship with speed. Slight increase in speed, creates squared increase in lift. Roll produces less drag on the LEFT wing by decreasing its AOA briefly, creating higher lift briefly and vice-versa on right wing. But this creates adverse yaw as I wrote above.
    Quickly though this imbalance of lift disappears to almost nothing and steady roll rate condition is achieved. So this is what I call the dynamic phase of roll till steady roll rate has established. Airplane guys sometimes try to use Frise ailerons and/or differential aileron deflection to reduce adverse yaw. At Searey, after ASTM compliance was done we developed Frise ailerons for it because its twin V struts on each side were specially prone to adverse yaw. Its a draggy beast so there wasn't a big penalty with Frise ailerons otherwise I would never put Frise ailerons on a deserving clean airplane.


    NOTE of Interest:
    Now if you develop on aerodynamic ideas of Horton and introduce twist in such a way to take lift distribution curve to zero at the tips in a precise manner, recent NASA research has shown that you can in fact without a tail cause "Proverse Yaw" which is opposite of adverse yaw and is more efficient. But we ain't there yet. Future jet liners might start to use this idea.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Additionally, a coupling by very definition would suggest that roll/yaw coupled (negatively) things would have a cause and effect both ways. When roll and yaw are coupled using effective dihedral or negatively with effective anhedral etc. etc., it just means that you roll and get adverse yaw and you create a yaw and get opposite roll. That is by very definition a coupling. Your Blue Devil video is doing that.

    Billow shift is a mechanism that flexwing uses in absence of ailerons that change camber and thus lift/drag on each wing to enhance change in lift on right versus left wing in a roll. Its an aid but not the cause.

    But its a darn rag wing with a tiny market. Whether you think the Chicken came first or the egg, who cares. They come so close together that even for design, its practically meaningless except for conceptual understanding.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Abid, you are saying adverse yaw on a fixed wing is Not from induced drag of the aileron? But admitting differential ailerons eliminate adverse yaw (agreed). If that's the case doesn't that prove the lift creates drag?

    As I said, WHAT is causing the wing to roll in the Blue Devil video? Is it Weight shift, billow shift or something else in your opinion. The post above touches on adverse yaw with an explaination on how adverse yaw is chieved, but then you name differential ailerons which supports drag caused by an aileron coming down below the wing. That's not lift vector, that's good old fashioned drag.

    The proverse yaw experiment seems to me as simple as taking differential ailerons to the next level. Because much like a trike, the flying wing with elevons needs reflex for pitch stability so when they turn one elevon reduces drag by cleaning up in a straight line while the other acts like a spoiler almost. We can do the same thing (differential aileron effect) by pulling back on our control bar as we turn so only the low wing reduces AOA and the high wing does not increase drag. And that works!

    Correct me if I am wrong. Did Paul Dewhurst say the wing is pulled down by weightshift and the billow was just a result of the increased AOA which made the wing not fight the pilot as much? And if so isn't that what you wrote you agreed with? And back to my main question which wasn't what causes adverse yaw (which you answered, but gave an example (differential ailerons) that not only supports what I said which is caused by induced drag from the aileron, but supports that there is an increased AOA on the high wing creating the induced drag). Moving the control bar side to side gives more adverse yaw than pulling in and rolling (like differential ailerons). We KNOW incread AOA creates drag.

    You are saying the low wing rolling has a higher AOA but lift pulls it forward???? Do you mean what powers a Gyro blade? That's where you are coming from right? So the book has it wrong for fixed wings as well? Induced drag is not responsible for the high wing of a Cessna getting pulled back when the aileron comes down? Or it is drag, but more like parasitic drag from the aileron coming down??? What makes the Blue Devil roll in the video? Weight shift? Can we agree it here was no weight shift in the video?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Also billow shift does not cause a change in camber, it changes AOI. And above you also wrote there is "an increase in lift briefly" on the ascending wing". That doesn't sound like what Paul d wrote. And your final answer is that roll yaw coupling make roll control yaw and yaw control roll (which I agree with) but that also supports everything I wrote as to the explaination as to why that's True. And I think Paul h's question will prove the change in AOA on the wing as it rolls. The nice thing about the Trike in that example is the wing changes AOA by twisting/billowing so if the twist creates a greater AOA change than the relative wind change of angle. And to me that is the crux of what I believe this latest discussion is about. Paul D, if I'm not mistaken, thinks the greater Angle change happens to the relative wind. I think the greater angle change happens in the wing.
  • Craig Dingwall
    by Craig Dingwall 11 months ago
    So........., once we have settled this differing of opinion I'd be really interested in hearing continued discussion on this..

    "HERE IS YOUR LEAD IN TO WHY PULLING BACK ON THE BAR TO ENTER A TURN IS AERODYNAMICALLY CORRECT."

    And this.......

    "Where does bar pressure in pitch come from? Or, in other words, why is there a neutral position in the forwards and backwards movement of the bar?"
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Craig,
    A couple of thoughts about pulling back on the control bar for entering a turn is first, it makes turning easier. Of course the steeper the turn the more it is helpful and the steeper required. The classic "J" turn was been taught for years. Pull in, move the control bar sideways and push out to coordinate the turn. The big question is why. First off the more you load up the trike the faster the stall speed AND trim speed increases. So get up to the higher anticipated trim speed before you start the turn. Additionally, that irritating increase in angle of attack becomes less as the wing rolls down the faster you go. Here are a couple of reasons of which I hope to hear more from others.

    Now the big question about pitch trim pressures. This is more complex and the best way to understand is in the FAA Weight-Shift Control Aircraft Flying Handbook starting on Page 2-13. It has diagrams with a simple and more complex explanation. This should answer your question on pitch pressures.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Hopefully my answer to Craig's question answers Bryans question "Where does bar pressure in pitch come from? Or, in other words, why is there a neutral position in the forwards and backwards movement of the bar?". Yes Doug's answer, I feel is correct, center of lift movement. Again the question is why and any additional explanations and/or comments are welcome. This is a tough one to answer but one of the unique aspects of the trike.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Another multiple guess I mean choice:
    When flying straight and level, you move the control bar to the side what first happens:
    a) the wing rotates above the aircraft initiating the turn
    b) the weight of the carriage shifts underneath the wing initiating the turn
    c) both a) and b)
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Larry I described what causes and starts the adverse yaw. The aileron deflections increase profile drag increases adverse yaw. Yes but it is not what starts the adverse yaw torque. Its too bad Billy Brooks isn't on here. He probably would not want to participate on a forum because people have think skins sometimes but next time we see him, ask him.

    In airplane the camber of the wings is changed by aileron deflection increasing lift on upgoing wing but in trikes its by tightening one wing increasing its lift.. And yes increasing lift also results in increase induced drag but its not the only thing or even the only kind of drag aiding in the yaw torque that is causing adverse yaw. Frise ailerons reverse the effect of profile drag and counter the induced drag in the opposite way causing a reduction in adverse yaw effect but they don't make it disappear. This tightening is aided by floating cross tube and billow shift and cause the a change in length and camber in wings just like ailerons do. BUT: You start roll with weight shift and that process is aided by tightening the up going wing or the roll would be opposed heavily. May be the problem is the timing of actions is being reversed. The start of adverse yaw precedes the billow shift and billow shift like aileron deflection adds to adverse yaw. In WSC aircraft additionally to this airplane like principle, we need to remember that weight shift divides the wing up unequally compared to CG of the aircraft, starting the lift difference right away and rolling. That is what I think is getting lost. The person who best understands this is probably Billy Brooks. He working with some Master's students has given certain thesis to universities in the UK that I had gotten my hands on. You know he is always after billow shift and even uses what he calls tweakers (??) to help shift the sail near the back of the keel. He knows it aids in roll while keeping things stable but he does not believe the generic instructor level explanation taught to everyone. I think I agree with him. I am surprised Paul Dewhurst has not tried to chime in. May be he will and put Bill Brooks explanation in words.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    https://vimeo.com/39838913
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Ok Abid, so you are not touching the weight Shift pulls the wing down. That is not what you are endorsing now? You are saying weight shift activates the billow. And what makes the wing roll primarily. I believe Paul D said weight shift is the primary cause of roll meaning Until The desired bank angle is achieved. and I am saying primarily reason for roll is billow shift. This tangent of adverse yaw in no way helps answer the question. I agree adverse yaw causes the wing to roll, it is my proof that weight shift is not needed to roll. I even agree with the term since we know positive roll yaw coupling means right rudder yaws right and banks right.

    So primary roll is asymmetric wing loading or asymmetric lift between the wings or something else?
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Larry: So I would say that your method of countering roll in flight using roll/yaw coupling method is using a secondary and not primary control effect. The primary control effect in weight shift is weight shift which by its introduction in steady flight makes the lengths of the wings compared to the CG of the aircraft unequal and thus a differential lift on each wing is introduced causing roll which starts adverse yaw by a twisted lift vector (down going wing's lift is tilted forward a bit and up going wing's lift is tilted backwards a bit).
    This is then further aided by change in airfoil shape. Like in airplanes by use of aileron, down aileron of the up going wing, increases the wing's lift co-efficient and up aileron on down going wing, decreases the wing's lift co-efficient OR in weight shift flex wings, its the billow shift that changes shape of the wings instead of ailerons.
    What you are doing is using the coupling in yaw and roll to induce adverse yaw to roll the wing the other way. This is an indirect method using a secondary effect. Primary effect would be to change the wing shape directly to increase downgoing wing's CL. I do not remember exactly but I remember a hang glider wing which had the trailing edge of the battens made from flexible composite rods which had an eye hook attached close to the end and a cable was attached to these eye hooks and the cable was routed to the downtubes on a knob. The pilot could turn the knob and actually change the shape of the wing directly (almost like a flap) in a certain area slightly, equalizing the lift. Not exactly a clean and crisp solution but acting directly on primary lift device. I guess the STARs system although I have made no effort to examine it in detail or understand it changes the shape of the wing as well changing its CL. Both methods work by approaching the issue from different sides.

    What differential ailerons do is make the rising aileron move more than the descending aileron, decreasing or even countering some of the adverse yaw. In extremes it is possible to create proverse yaw even but then other handling issues can arise. Spoilerons are inefficient but they can be used in roll to eliminate most of the adverse yaw and even cause proverse yaw. But what I was suggesting for proverse yaw in flying wings was based on lift distribution that makes a very efficient solution as researched by Al Bower at NASA extending the Prandtl and Horton concepts to their logical conclusions.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Here is the typical instructor fed explanation of weight shift wing's turning. Its ok and I would accept it in a practical test and its what is written in many manuals as well. I and Bill Brooks and it seems like Air Creation guys just think they are missing a whole section of why roll starts to happen at first.

    http://www.flight-mechanic.com/weight-shift-control-flexible-wing-aircraft-aerodynamics-part-two/
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Go back, way back and re read Paul Dewhurst's post. You said you agreed. You agreed the AOA is higher on the descending wing as the trike banks. It only billows as a result of increased AOA and billow shift does not work like ailerons on an airplane.

    Abid, do you agree with that or not?

    We can discuss further that weight shift is not required to initiate billow which I have nothing else to add to what I already said, but back to the question above. Do you agree with those statements Paul D made?
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Larry: So to be clear the primary effect is not billow shift. Billow shift comes in after the primary weight shift input. It starts first simply by lengthening one wing compared to the other in reference to the aircraft CG due to shift in CG.
    This immediately makes the downgoing wing to have an increase in its angle of attack. This happens both in airplanes and trikes. I cited a book written in 1949 if people want to understand why. This also immediately starts adverse yaw due to twist in lift vectors or conversely twist in relative wind vectors.
    However, these motions would all be opposed and what enhances them and makes them palatable is billow shift that comes in shortly after. Finally a steady roll rate is achieved and billow shift has taken place.
    This I think is fairly close to what Paul Dewhurst said. I have never talked to Bill Brooks directly about this but I believe he has similar ideas.
    The idea of down going wing having lower angle of attack due to billow shift and so on ... well ok, I'll accept it in an oral test. It is not important for pilots to get down to de-skin the cat so much. The end result is not affected practically so much that it matters.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Abid, either the pilot is pulling the wing down or the ascending wing is flying up making the wing roll to the desired bank angle. I am still unclear which you are supporting. I understand you are admitting weight shift is not required to billow shift and billow shifting another way is possible but not primary. That may be true depending on the wing design. Agreed. But again I am saying billow shift all by itself dynamically rolls the wing or at least does in the case of the Blue Devil and many other wing designs.

    Agree or disagree?
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Actually I found an explanation by Steve Siebel for why the explanation of downgoing wing having a lower angle of attack that is fed in all the flight instructor manuals is not correct. It worded more simply than my explanation because I immediately start thinking in terms of vectors etc. which may not be the best way to describe it for everyone. Here it is:

    "27) The "layman's" explanation of adverse yaw, and a look at aerodynamic damping in the roll axis

    We'll ease our way into a discussion of "aerodynamic damping in the roll axis" by giving some attention to the "layman's explanation" of adverse yaw, which holds that as an aircraft rolls into a turn, the rising wing creates more drag "because it is creating more lift". This explanation cannot be accurate. In the case of a "conventional" aircraft, once the roll rate becomes constant (no more acceleration in the roll axis), Newton's laws tell us that the net roll torque must be zero, which means that both wings must be creating an equal amount of lift. In this article we've examined many other sources of adverse yaw that do not involve the idea that the rising wing is "creating more lift".

    The reason that the roll rate is constant, and the lift created by each wing is equal, even though the ailerons are deflected, is that the increase in lift coefficient created by the lowered aileron on the rising wing is cancelled out by the decrease in lift coefficient created by the decrease in angle-of-attack experienced by the rising wing. This decrease in angle-of-attack is caused by the change in the direction of the relative wind that is caused by the rising wing's upward motion through the airmass, as we described at the beginning of our discussion of the causes of adverse yaw. Likewise, the decrease in lift coefficient created by the raised aileron on the descending wing is cancelled out by the increase in lift coefficient created by the increase in angle-of-attack experienced by the descending wing. In other words, the angles-of-attacks of the rising and falling wings are altered by virtue of their own relative motions through the airmass, which eventually equalizes the magnitude of their lift vectors and brings the net roll torque to zero, preventing any further increase in the roll rate. This phenomenon is called "aerodynamic damping" in the roll axis: a crude visualization would skip over the nuances of angle-of-attack and simply think of the rising wing as meeting "resistance" as it rises upward through the airmass in a paddle-like fashion, and similarly for the descending wing."

    I want people to understand though that this discussion is getting into details that as a pilot you want to understand if possible but it is not necessary if you do not. Stick with your instructor and learn to fly safely. At the end of the day in flight actions are not done by analysis but by instinct and those instincts are developed by training and understanding.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    A long time ago (and through tufts and cameras) it was seen that increasing separation flow around the midspan of the rising wing was A cause of adverse yaw in flexwings. At least for the duration that this was occurring, we have good clues as to the behaviour of a wing at the initiation of a turn. You can figure it out, but it supports the idea that billow shift and not roll rate is the leader of AoA.

    Craig, let's come back to pitch pressures when the above discussion has run its course, but a clue is to think of what happens to twist as a result of the wing being 'swept', or having a nose angle of less than 180 degrees...
  • Larry  Mednick
    by Larry Mednick 11 months ago
    That sheds light on where you are coming from. And going by that then in the case of the Blue Devil the wing was at a consistent AOA during its banking and in no case is the AOA higher or lower as the wing banks at the same rate.

    Ok I have a question...do you have any idea the amount of energy it takes to aileron roll an airplane and drive the wings against the relative wind? Things in motion stay in motion in a vacuum. That's like saying torque only happens as you increase RPM, and while that may be true, if you apply that idea to a Prop or a helicopter see what happens when the tail rotor fails. Not buying it...

    So it sounds like regardless of pilot weight shift or adverse yaw induced billow you are saying once the wing is rolling in a steady rate you believe the AOA is equal on both wings.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Larry, you wrote:
    "But again I am saying billow shift all by itself dynamically rolls the wing or at least does in the case of the Blue Devil and many other wing designs."

    Yes if you induce billow shift without weight shift input it will create a net roll torque effect. That is why you are able to induce adverse yaw and that starts to slightly billow shift and change airfoil in the wing of course.

    That is however, "not" what happens in pilot rolling a trike normally and there are some more caveats but I need to watch an episode of Magicians so later
  • Larry  Mednick
    by Larry Mednick 11 months ago
    So is AOA in my example is responsible for initiating the bank? But you think the AOA equalized after it is in a constant rate of bank? Vs Paul D thinks the AOA is higher on the descending wing in a constant rate of bank. Have I got it now?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    I just chewed opening the paragraph that Steve wrote that AOA is the same as the aircraft rolls. IF the wing is fully symmetrical and IF the plane is not opposing gravity. So in a 90 degree nose down dive the plane is aileron rolling not only is the AOA not equal to the relative wind, but if the AOA on one wing is 12 degrees, the other wing is -12 degrees. So I'm sorry to say I have to throw that theory right out the window. If the first statement is wrong, where does that leave everything else? Lift make drag. If not I don't need a motor...

    Paul H. How about those results you figured?
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Bryan, guess what happens when you INCREASE AOA on a wing? The Air flow arcs up higher and higher right until it stalls and can't make the arc back down towards the trailing edge. Your observation supports adverse yaw caused by the ascending wings increased AOA.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Larry, yes, that was the point I was making. To spell it out: at least for the duration that that form of adverse yaw occurring, we were seeing characteristics of increasing AoA (flow separation, and some degree of stall) on the upgoing wing. Conclusion: The primary cause of adverse yaw is drag from high AoA, and billow shift has a greater influence on AoA than roll while that phase of the turn was occurring.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Hi Larry:
    So why can you use the negative coupling at low angles of attack (faster speeds) in the Revo for yaw and roll to induce a roll by yawing the trike. Simply put because of airflow effects. Billow shift here again isn't the primary reason this roll happens. Its an aid to rolling and negates to a certain degree aerodynamic roll dampening. Some smart people have done science via experiments in great detail and taken painstaking notes to figure these things out. you using adverse yaw to cause a very slight change in relative wind on the carriage that shifts the carriage to one side in a way is weight shift as well. The faster you are going the better this effect will surface and in trikes we have a hard connection in yaw of the carriage to wing's keel so some of this force translates up and makes the relative wind hitting left and right wing at slight different angles as well. Again from Steve S.

    "Q: When the nose of a flex-wing hang glider is pointing in a slightly different direction than the glider is actually travelling through the airmass, does the resulting sideways (slipping) component in the airflow over the glider create a roll torque? If so, in which direction?

    A: These questions were explored through experiments with a controllable rudder, and also through experiments with a small wingtip-mounted drogue chute (don't try this at home without contacting me first!), that allowed me to make a yaw input on a flex-wing hang glider and then continue to fly the glider in a linear, non-turning slip, while noting what roll inputs were required to hold the bank angle constant. These questions were also explored by seeing which way the glider naturally wanted to bank after I deployed the yaw control device and refrained from making any roll inputs. In general all the gliders tended to roll in the opposite direction as the nose was yawed in relation to the airflow--a left yaw input, causing the nose to point to the left of the actual direction of the flight path and airflow, and causing a yaw string to deflect to the left, would create a right roll torque in most cases. For reasons that we'll explore in more detail below, we'll call this relationship a "negative coupling between slip (yaw) and roll". The results were strongly airspeed-dependent with higher airspeeds (lower angles-of-attack) correlating to a stronger negative coupling between slip (yaw) and roll, and low airspeeds (high angles-of-attack) correlating to a weaker negative coupling between slip (yaw) and roll. The results were also strongly dependent on VG setting: when the VG was tight this decreased the negative coupling between slip (yaw) and roll at low angles-of-attack (high airspeeds), and created a weak positive coupling between slip (yaw) and roll at high angles-of-attack (low airspeeds). "

    Note that IS possible to create a "positive" coupling between yaw and roll as well in a hang glider or trike. Its not preferred by designers because pilot has very reduced starting effect on roll torque in a trike to begin with, only enhanced with things like billow shift etc.
    BTW, trikes also have limited pitch control so as to not allow them to get to zero lift or negative angle of attacks easily. Airplanes by comparison have no such limitations because zero or negative G in them is still controllable.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    So billow shift is always something that comes in after a primary input. The primary input being pilot rolling the glider, the roll producing adverse yaw right away, also producing "higher" not lower angle of attack on the downgoing wing and lower angle of attack on the up going wing, billow shift thus kicking in due to air loads simply on the sail to act against aerodynamic roll dampening to make the roll better till steady state is achieved and roll rate stops increasing and lift on both wings is equal.

    In your Blue Devil, the primary effect to cause roll is simply sideways airflow and coupling between yaw and roll or slip and roll
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Here is Steve Siebel's explanations which I think are in line with my, Bill Brooks and Gilles Bru ideas. Yes things in his experiments and his deductions are very counter to all the hang glider manuals, Pagan's explanations and WSC manual by FAA but so what. The explanation of lift in FAA manuals is far simplified also. Its a lot more complex than that but it works to convey quickly an idea for pilots who need to simply pilot the aircraft.

    "28) Details of aerodynamic damping in the roll axis in weight-shift aircraft

    In the case of a weight-shift controlled flex-wing hang glider or trike, the same phenomena are taking place but we have to modify our description slightly. For convenience--it will simplify our description--let's define the "center" of our system in relation to the CG of the whole system (including the pilot's body), rather than in relation to the keel of the hang glider or trike. When the pilot shifts his body to one side--say to the left--the left wing becomes shorter and the right wing becomes longer. (Remember, we are defining the center of the system, which divides the "left" wing from the "right" wing, based on the CG of the whole system, not based on the keel of the aircraft). Since the right wing is longer than the left wing, the aircraft begins to roll toward the left. As the aircraft begins to roll, the descending left wing experiences an increase in angle-of-attack and an increase in lift coefficient and lift-per-unit area, due to the change in the direction of the relative wind caused by the fact that the left wing is descending through the airmass. (This is the "aerodynamic damping" effect that we explored above.) Similarly, the ascending right wing experiences a decrease in angle-of-attack and a decrease in lift coefficient and lift-per-unit-area, due to the change in the direction of the relative wind caused by the fact that the right wing is rising through the airmass. (This is another component of the "aerodynamic damping" effect that we explored above.) When the difference in lift-per-unit area has risen enough to exactly counteract the fact that right wing is bigger than the left wing, then both wings will be creating the same amount of lift and the net roll torque will become zero and the roll rate will stop increasing and become constant. (Astute readers will note that to keep the length of this article manageable, we're making one slight oversimplification--we're ignoring the fact that because the left and right wings have slightly different spans and therefore act at slightly different moment-arms from the CG, in order for the roll torques to be in balance, the longer, rising right wing must in fact be creating slightly less total lift than the shorter, descending left wing. If we were to take this into account, it would further reinforce our conclusion that when the pilot's body is shifted to the left and the roll rate is constant, the left wing is creating more lift-per-unit-area than the right wing.)

    29) Notes on the causes and consequences of sail billow shift

    Here's another key point in relation to flex-wing hang gliders and trikes: the "aerodynamic damping" effect that we've been discussing will be greatly reduced by the fact that the wing will undergo a "billow shift"--a physical change in the shape of the wing--that will partially offset the increase in lift coefficient and lift-per-unit-area experienced by the descending wing, and will also partially offset the decrease in lift coefficient and lift-per-unit-area experienced by the ascending wing. This will allow a much higher roll rate to be achieved by the limited roll torque available from the pilot's meager weight-shift input than if the wing were of a fixed, rigid shape. But it remains true that when the roll rate becomes constant, both wings must be generating the same amount of lift, which means that the smaller, descending left wing must be generating more lift-per-unit area than the larger, rising right wing. So clearly, although the billow-shift effect partially relieves the aerodynamic damping effect that we've been discussing, it cannot actually reverse the situation to the point where the rising right wing is actually experiencing more lift-per-unit-area than the descending lift wing.

    Let's consider the cause-and-effect relationships surrounding the billow-shift effect in more detail. A wing "feels" the wind. It does not "feel" gravity. The billow-shift phenomenon is not actually driven by the fact that the pilot's weight is shifted to one side. (To take an extreme case, in a vacuum, the pilot's weight-shift inputs would have no influence on the shape of the sail!) The billow-shift phenomenon is actually driven by the damping effect that we've been discussing--i.e. by the fact that the relative wind has more of an upward velocity and creates more lift-per-unit-area on the descending wing, and has more of a downward velocity (or less of an upward velocity) and creates less lift-per-unit-area on the rising wing. This disparity in lift, acting on the surface of the sail, is the actual physical force that makes the shape of the wing change. The billow shift is the glider's way of "venting" or "relieving" a large part--but not all--of the asymmetrical lift-per-unit-area load that the asymmetrical relative wind--not the asymmetrical distribution of the pilot's body weight--is imposing on the glider. Because of this fundamental relationship--i.e. because of the fact that the "twist" in the direction of the relative wind across the length of the wingspan, and the resulting asymmetry in lift-per-unit-area between the left and right wings, is the fundamental driving force behind the billow-shift effect--it can never be the case that the billow-shift effect is so pronounced that it causes the descending wing to actually experience less lift-per-unit-area than does the rising wing. In other words, the "passive" billow-shift effect is not really completely analogous to the action of ailerons or to "active", hard-wired, wing-warping controls such as were used by the Wright brothers. Instead, the "passive" shifting of the billowed surface of the wing is actually tangible evidence that the descending, smaller wing is experiencing more lift-per-unit-area than is the ascending, larger wing.

    In other words, when the pilot shifts his body to the left, and the left wing starts to drop, the change in the direction of the relative wind increases the left wing's lift-per-unit-area and lift coefficient. (These two terms are equivalent, for our purposes here.) Despite the fact that the sail billows in a way that decreases the efficiency of the left wing, the lift coefficient of the left wing remains higher than the lift coefficient of the right wing as long as the glider is rolling. (The sail billow will also remain shifted to the left as long as the glider is rolling.) These points would need to be kept in mind if we were to take this analysis of the adverse yaw to the next level and try to assess the changes in the drag coefficients in the left and right halves of the sail as the glider rolls and as the sail billows.

    In an aircraft controlled by an "active", hard-wired wing-warping system or by ailerons or spoilerons, we have the power to do something that we can't do in a standard weight-shift flex-wing hang glider: as we've already noted, we can actually cause the descending wing to initially experience less lift-per-unit-area than the ascending wing. Then as the roll rate increases and the "aerodynamic damping" effect became stronger and the roll rate stabilizes at a maximum value, the lift-per-unit-area generated by each wing again become equal, at least if the pilot's body is located on the aircraft centerline. If the spoilerons or ailerons are connected to a weight-shift system as per modern rigid-wing hang gliders, then when the roll rate stabilizes at a maximum value, the slightly shorter descending wing is once again generating more slightly more lift-per-unit-area than is the slightly longer ascending wing.

    We'll close this article with just a few more thoughts on sail billow. (Admittedly we've now strayed rather far from the subject of adverse yaw!) We've made the point that when a flex-wing hang glider pilot makes a roll input (let's say to the left) to initiate a rolling motion during wings-level flight, the sail won't change its shape until after the glider starts rolling, which creates a difference in the relative wind as experienced by the left and right wings, which forces the change in the sail shape. Once the roll rate stops increasing and becomes constant, we can think of the glider as having reached sort of a "steady state" where the pilot's uneven weight distribution, in combination with the lack of further increase in roll rate, is serving as evidence that the airflow is forcing the left wing to "work harder" than the right wing. This same airflow is also responsible the shifted sail billow. It might therefore be reasonable to expect to see very roughly the same amount of billow shift in a case where the pilot's weight is shifted the same amount to the left in a constant-banked turn. (Depending on the high-siding or low-siding characteristics of the glider, this turn might be toward the right or toward the left.) In either case, the pilot's uneven weight distribution, in combination with the lack of change in roll rate (which in this case is fixed at zero, at least if we overlook the slight rolling-in rotation about the glider's roll axis that accompanies a constant-banked descending turn), is once again serving as evidence that the airflow is forcing the left wing to "work harder" than the right wing. As noted previously, we'll explore the balance of roll torques for a circling flex-wing hang glider in more detail elsewhere on the Aeroexperiments website in the future."
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    But after all this, the answer to the question what makes an aircraft turn is still, a net force acting towards the center of the turn curvature. Without that aircraft would not turn
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    All,
    An incredibly convincing perspective from Steve Siebel. What he said is true except he is missing the importance of the billow shift/washout/twist change. Why do I say this?

    Long ago and far away in the distant space of hang gliders, they had evolved from advanced kites to flexible wings, I got to fly the wings before they floating crossbar was developed to aid in billow shift/washout/twist change. Huge jump in performance but they were almost uncontrollable. Especially for me weighing not much. Not fun and very scary.

    Roy Haggard put the floating crossbar and on the Comet hang glider and with UP they changed the course of flex wing control. All the manufactures copied it. All of a sudden the wings were easy to turn. My experience was about four times easier to turn with the floating cross bar upgrade creating the billow shift/washout/twist change.

    So if someone is trying to say the floating cross bar billow shift/washout/twist change is a byproduct of the WS being able to turn, from my experience this is simply not true. Billow shift/washout/twist change is a very large factor in rolling. Yes as Steve Siebel said convincingly well, that the weight divides the wing and moving it provides different lift areas on both sides with all the aero dampening, but sorry, as you shift the weight from the wings center the keel moves over and billow shift/washout/twist change provides a rolling moment. Simple laws of physics. You are simply pulling the keel over as you move the bar side to side. The billow shift/washout/twist change simply has more effect on roll moment as claimed by Steve.

    And as far as who actually said what from Air Creation Gilles Bru and P&M Bill Brooks, I would have to see something they wrote rather than get it second hand through a third party (Anybody PLEASE do not take offence to this is just how I work, directly with the source). It is hard to base absolute credibility in this he said/she said. As we all know, the story usually changes as it is passed from one to another. ( I am still not buying Air Creation Gilles Bru story about strutted wings bad verses wired wings better....)

    I had plenty of correspondence directly with Wills Wing and Airborne to develop the basics presented in the WSC handbook. Perhaps I should have included the weigh shift moment additionally creating a moment but that is pretty much common sense and a smaller factor to the actual billow shift/washout/twist change aerodynamic rolling moment. Many concepts were edited out to present the most relevant concepts and keep it as short and sweet as possible.

    Abid, you are saying the billow shift/washout/twist change happens after the roll is started. This is pretty vague. One mili second, one second, 1 degree of bank, 10 degrees bank. What is after????

    I always want to be open to understand the real story so please, ANYONE do not think I am on any side. Always want to hear all sides and allot of open issues here. We will get there. For me roll basics first then YAW.

    So in summary, both camps are generally right, it is only to the degree of weight shift moment verses billow shift/washout/twist change moment which rolls the trike wing. It is not all billow shift/washout/twist change moment and it is not all weight shift moment and the degree each contributes will be different for different designs. One size does not fit all. Many times the truth is in the middle of two valid arguments....
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Yes I hope everyone agrees with this:
    by Abid Farooqui 2 hours ago

    But after all this, the answer to the question what makes an aircraft turn is still, a net force acting towards the center of the turn curvature. Without that aircraft would not turn
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    For the questions, only Larry answered one. Like to get feelings/guesses from others before releasing answers/details. How do pilots feel or think about these questions? These are simple mathematical answers but what counts is what the pilots think/feel are the correct answers....



    I do agree that as a wing drops and goes into a turn the angle of attack on the inside wing of the turn increases in trikes and airplanes similarly. The question is how much and does it really matter?
    Lets look at an example question:
    In a steady state start to finish, 50 MPH airspeed, 4 seconds to roll from level to 30 degree bank, with no billow shift (stiff wing), how many degrees is the angle of attack increased from the lowering wing with a 32 foot wingspan?

    a) 1 degree
    b) 1.5 degrees
    c) 2 degrees
    d) 3 degrees
    e) 5 degrees
    f) 10 degrees
    g) All of the above
    h) None of the above

    Edit | Delete
    .


    Paul Hamilton

    by Paul Hamilton 1 day ago

    Lets look at the other side of this dilemma; In a turn, the inside wing is flying slower producing less lift than the outside wing. I hope we all agree with this. How much and does it really matter? So here is the question:

    If to are flying in a 30 degree bank, 50 MPH, how much faster is your outside wing flying than your inside with a 32 foot wingspan?
    a) 2%
    b) 5%
    c) 10%
    d) 20%
    e) 30%
  • monty stone
    by monty stone 11 months ago
    as einstein's wife said, " i know your'e smart, but you sure need a haircut". 10% ?
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Paul. Steve is saying something that makes complete sense to me and it seems to Bill Brooks and Gilles Bru.
    I do not think you are completely capturing what he is saying or you would agree.
    What Steve is saying just like all of us is that billow shift occurs "after" roll has started and is something that is a result of aerodynamic roll damping not weight shift. How long after you ask. That is a loaded question. Right after. But it is after. Even after adverse yaw that starts right after rolling starts. Usually if you hang a light weight under a glider, weight shift would be over come by aerodynamic roll dampening and that is why it was hard without billow shift and floating cross tube. But there is a sequence and there is something that happens to AOA that is generally taught backwards to most pilots. Billow thus is not responsible to start the roll. It is responsible to overcome roll dampening to a great extent to make roll more easy. That is exactly what you are saying. However, if you think that billow shift starts the roll in a glider than there is a difference between what he is saying and what you are saying.

    Also what he is saying is that down going wing has increase in AOA and up going wing has decrease in AOA which is also something Paul Dewhurst said he has come to understand after being explained by Billy Brooks and also after talking to Gilles Bru. I agree with this. I have never discussed it with either of them directly though. But this is also true in airplanes as well and is documented in books as well, one of which I cited. This change in AOA creates adverse yaw at the start and then other factors come in to either add to or counter this adverse yaw depending on certain things. But what is important to realize IMO is that the rolling motion starts everything, then AOA changes happen, adverse yaw torque comes in and then billow shift happens and overcomes aerodynamic roll dampening which without billow shift would make the glider very hard to turn and then steady state is achieved and roll rate is steady and wings reach the same lift (almost) on both sides.

    The issue is this is counter to what is taught as an explanation of roll generally to instructors. I agree you should directly ask Bill Brooks and Gilles Bru about this but what Paul D wrote about Bill saying, I agree with.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Abid,
    I highly respect you and I talk directly with you. You are one of the best Airplane/LSA/trike engineers on the planet. Do not try and BS me. I asked a specific question about and I will repeat myself AGAIN as I asked above:

    Abid, you are saying the billow shift/washout/twist change happens after the roll is started. This is pretty vague. One mili second, one second, 1 degree of bank, 10 degrees bank. What is after????

    Please do not avoid the question....

    To all, Abid is only one of very few I could be this direct without hurting his feelings and him/her getting all sensitive and taking it personally so.... .

    You got to know him to love him....
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Got dinner and a date. Will talk tomorrow
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Haha ... In Larry's wings 0.15 seconds. In normal wings 0.3 seconds, in old slow wings 1 second. My best guess. I have never measured these things but I am quite sure that roll does not start with billow shift, it is the third thing that happens after rolling starts, adverse yaw starts, aerodynamic roll dampening starts to happen and almost in concert with that and as a reaction to roll dampening the glider billow-shifts to counter this dampening or you would have a hard time keeping the roll going. Roll rate stabilizes and becomes constant. Lift equalizes.
  • Craig Dingwall
    by Craig Dingwall 11 months ago
    Bryan, good call, I'm loaded and ready for that one :-)

    Paul, how can you possibly leave this discussion for dinner and a date :-) perhaps you should have cancelled, where's your commitment
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Guys good discussion. I think Steve Siebel's explanation is better than I could myself put it in words. His website is a maze but its quite good except a couple of things he has got wrong but he updates and post corrections as he figures them out. His data is based on careful experimenting in a very scientific engineering way.
    Bill Brooks is one of the few who understands flexwing in the world. His answers are usually quite convincing to me.
    I may be busy tomorrow depending on weather here which seems to be getting worse and cold. Have another ex-triker from California coming in to order a gyroplane.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Abid, Actually According to Paul dewhursts post I believe he said that Giles Bru and bill brooks both explained the exact same thing which does NOT support his beliefs. Also I would like to point out that Paul Dewhursts post does NOT agree that AOA is equal during the bank (correct me if I'm mostaken). As I said earlier I can buy the idea that weight shift primarily rolls the trike if the keel Is locked. And what initiates the billow shift can be multiple things depending on what the pilot does and the design of the wing. (More on that another time) however that is skirting the actual debate which is that AOA is higher on the descending wing. During the duration of the roll.
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Here is Paul D's post about bill and Giles NOT agreeing:

    Then Bill Brooks (P&M chief designer) gave a flexwing theory talk to the UK instructors seminar and explained it the other way. We were all shocked - he was bemused that we believed otherwise and pointed out the flaws in the reasoning. A few years later I asked Gilles Bru ( Aircreation wing designer) to explain to me Billow shift - and he explained the same mechanism as Bill Brooks - with no collusion.

    I have a tremendous amount of respect for all 3 so I am really torn. But 60 years of design experience vs. possibly no experience in flex wing design and I think the experience is a bit weighted against Steve.

    What I cannot for the life of me understand is HOW in the world can equal lift allow an aircraft to roll without decelerating roll
    Rate.? Yes in a vacuume sure. But I cannot get past that. It seems to lack the most basic understanding of physics. Obviously I'm missing something... someone please help explain this one thing.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    I don't think that Larry. I think Paul D. was saying that he used to believe that AOA on downgoing wing is less like WSC manual says but he believes now that its not after Bill Brooks gave him and other instructors an explanation. We would have to let him tell us I guess
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Larry you are mixing acceleration and steady roll rate. Roll rate is accelerating at the very beginning but roll dampening is dominant force both in airplanes and definitely in trikes. That is why billow shift aid is needed to make it practical. But roll rate stabilizes very quickly. That is why roll initiation in very simple terms is because you shoved the bar sideways. End of story.

    The angle of attack being greater on down going wing and smaller on high wing is quite established and talked about in many places. If you don't find that book published by Wiley and Sons, you can find this description elsewhere
    https://www.av8n.com/how/htm/vdamp.html#sec-roll-damping
  • Joe Hockman
    by Joe Hockman 11 months ago
    Gee you guys are still at it, but nonetheless I find many of the new contributions to be additive and thought provoking. Parts of this discussion are a real drag. To be more precise, parts of this discussion focus on real drag. So pun was intended. More on that later. So here are the parts I am inclined to believe.

    Steve Siebel's explanations make total sense to me. Thanks Abid for adding in his comments and your perspective. Typically a roll is initiated by a WS induced force. As the turn begins the adverse yaw coupling kicks in and relative wind begins to change and this in turn generates the billow on lower wing. As adverse yaw kicks in the upper wing is lengthened and lower wing is shortened (ie aft end of keel is shifting toward lower wing tip). During dynamic state (ie roll rate continues) lift from upper wing must be higher than lower. Part of this may be due to AoA changes, both in upper and lower wing, and part of it may be due to increased speed of upper wing and fact that upper wing in longer. Once a steady state or static bank angle is reached (assisted by roll torque damping and an equalization of lift and drag forces) then the lower and upper wings are generating the same amount of lift. For that to be true, the lower wing must have an average AoA greater than the upper wing due longer upper wing (origin or center of asymmetric lift distribution shifted toward lower tip) and fact that lower wing is traveling slower than upper wing.

    We seem to get bogged down in statements like AoA increases or decreases here or is greater or lower here. Fact is that in a static turn we have a considerable variation in AoA on lower wing just due to the effects of billow and washout. Parts of lower wing may actually have a lower AoA (such is near top of billow arc) that most of the upper wing. Since upper wing airfoil is flattened, the AoA probably does not vary as much as for the lower wing. So we really should think in terms of "average" or "net" AoA across the upper or lower wing. The average AoA for lower wing must be greater than upper wing due to considerations mentioned above.

    Now, Larry you have clearly shown in your blue devil video that a turn can be initiated without WS. So I think we have all agreed that when you twist your control bar you are directly affecting yaw and due to the adverse yaw/roll coupling, a roll is initiated. Almost simultaneously relative wind direction shifts, keel moves and billow is generated. When turn is initiated by turning front tire or changing trim on wheel spat, you are also generating a yaw force, and maybe some asymmetric parasitic drag helps as well. In any case, you indeed can make a turn with a simple yaw force in the blue devil configuration. I will say, however, that with some trike / wing combinations the yaw force generated by, for example turning the front tire, is not sufficient to initiate a turn. Perhaps a twist on the control bar may work on most trikes.

    There was mention of ease of inducing a turn with a yaw force that depends on speed. This too seems quite logical to me. If you have an electric speed trim that only move hang block fore and aft (and not coupled with a roll trim that affects anhedral), moving the hang block changes the relative position of the CL vs the CG and I think that change in relative position either strengthens or weakens the negative roll/yaw coupling although other variables such as sensitivity to minor changes in AoA at different speeds could be involved as well in determining the strength of that relationship.

    Some additional food for thought. We often talk about and focus lift and lift distribution. So what about drag and drag distribution. Various kinds of drag have been mentioned: profile drag, induced drag, parasitic drag. My hypothesis is that some different kinds of asymmetric drag are involved that influence performance in rolls. With adverse yaw and relative wind angle changes we are also changing the nature of the parasitic drag distribution around the trike which could also affect roll response rates, and even yaw, etc.

    Lastly, I strongly believe that the kinds of experiments that Steve is conducting is perhaps the best way to understand influence of variables on flexwings. Due to my background as a scientist and statistician I know one can readily uncover true cause and effect relationships through experimentation. My statistics specialty is in experimental design, and I have been involved in many discoveries (at DuPont and in academia) that led to new products or materials or enhanced understanding through strategic experimentation. Also have a number of patents because of it.

    So I would advocate being creative in trying some experiments and then assessing performance impact. Changes in batten tension or amount of washout. Easy to experiment with haul back cable tension or adding same size wing tip shims to flatten airfoil. If you have a control bar that is an inch or so longer than your standard one you can experiment with reduced anhedral. Has been considerable discussion on role of keel movement due to floating cross bars. Has anybody ever thought to try to limit cross bar movement. Maybe straps around keel and cross bar junction will help to limit the amount of float. Now be vary cautious with some types of changes and it may be prudent to only make one kind of change before some test flights. Clearly one should expect a major change in wing roll behavior if you are able to limit the cross bar floating as the amount that upper wing lengthens and lower wing shortens and amount of billow should change so don't do any radical roll maneuvers until you have a good feel the change. Larry I would be curious as to how well a roll can be induced without WS if floating is limited. If you do some simple experiments, please be careful. Some changes may be very benign and have very little impact whereas others could have more of an impact than you might expect.
  • Mikael Markow
    by Mikael Markow 11 months ago
    Paul, to your two questions my answers is:
    b) 1.5 degrees (at the wing tip, zero at the root)
    b) 5%
  • Larry  Mednick
    by Larry Mednick 11 months ago
    Joe, some really good thoughts there. Especially regarding high twist low twist AOA that need to be called a net AOA. That's for sure...

    As I said much earlier if the keel cannot shift (which can be controlled by the cross tube strap length. We use a long one on the RIVAL S but a short one on our other wings) OR (saying this for the first time) if you have a giant keel Pocket the pilot cannot geometrically alter the sail shape with out the help of wing loading (which supports Steve). BUT if the keel pocket is fairly tight at the back and the cross tube has a large strap and the triangulation effect of the rear wires has been relieved either through looseness or a pigtail or mechanism THEN the pilot can alter the shape of his wing effortlessly to where it can be used as a primary roll control. When I fly most of my maneuvering is done pulling my hand back to my hip and I can roll faster than most. If I pull back to my hip and push forward with the other hand I can turn faster than you can imagine.

    So billow shift Can initiate the roll although I realize most pilots and most wings do require weightshift to activate billow shift. So no argument there. My point was the power of billow shift.


    Also during the bank where the trike is in a solid statecoordinates turn I agree there is a balance of lift as Steve describes is happening while it is banking. After sleeping on his article I also understand the wings are making the same amount lift while it's rolling based on the roll creating a new relative wind. And therefore if you think of it that way, IF the pilot has to pull on the control bar to continue the roll weight shift in fact is causing the roll. As I think we all agree I can demonstrate weight shift not required.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Yes Mikael that is correct 1.5 degrees.

    Here is the why...

    Ok good let's look one of the answer to this question:
    • In a steady state start to finish, 50 MPH airspeed, 4 seconds to roll from level to 30 degree bank, with no billow shift (stiff wing), how many degrees is the angle of attack increased from the lowering wing with a 32 foot wingspan?

    a) 1 degree
    b) 1.5 degrees
    c) 2 degrees
    d) 3 degrees
    e) 5 degrees
    f) 10 degrees
    g) All of the above
    h) None of the above

    Yes b) 1.5 degrees is closest.
    note that you have to have speed of the trike wing AND speed of the roll rate to figure this additional angle of attack on the tip as the wing rolls.

    Note that for the BNC (bar-napkin calks) presented here I round things off to keep things simple. No sense doing detailed calks for WAG's (Wild As Guesses) since there are so many more factors/variables to get the CLOSEST answer to the actual situation.

    Ok you are flying at 50 MPH and lets use 70 FPS (feet per second) for this. First we need to figure out how fast your tip is going down.
    Your tip is 15 feet out and with a 30 degree bank it travels about 9 feet (8 in an arc) in an arc as it drops. So it is dropping at about 2 foot/sec.
    So as it drops 2 FPS into 70 FPS air the change in angle is about 1.5 degrees. Note this does not have any billow shift/washout/twist change. Stiff wing.

    Now with Larry and Abid both CLAIMING :-) twice that speed, 100 MPH, note twice the speed would be half the angle (less than 1 degree) at the same roll rate. Twice the roll rate double the angle at the base speed. Twice the speed, twice the roll rate same angle. Not much magic or theory here.....

    And the question becomes, which is why I posted this question (which would typically be of great debate), is how much does that really matter? Is it worth making a big deal about? Especially at the tip where there is not that much lift? So let's say 1 degree of twist change at the tip with a 70 MPH turn and there is no increase in angle. 2 degrees in twist means LESS angle of attack at the tip for roll.

    Here we now have a NUMBER. Is it exact? NO, but pretty close for a BRN and WAG since there are not that many variables.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Yes Bryan and Craig. Extenuating circumstances.

    However, I did had have a long and detailed conversation with RB who wants to remain anonymous at this time. He said everyone is looking at this weight shift ROLL the wrong way. More on his thoughts soon.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Thanks Abid for some numbers. I do think that this billow shift/washout/twist change is initially activated from the basic physics of pulling your weight and moving the keel creating the billow shift/washout/twist change initially in combination with weight shift torque moment but so be it. This is great to get different perspectives.


    This detailed balance is why the flying wing developed by Northrop had so much trouble succeeding because it is so much easier to slap a tail in the back of a wing and have an airplane.
  • Bryan Tuffnell
    by Bryan Tuffnell 11 months ago
    Paul, I have some quantified data for a Moyes GTR. Rates depend on pilot input, and on that wing are - or can be - a little faster than Abid's guess for Larry's wing. And as I've previously said, roll leads billow shift and adverse yaw; billow shift dominates roll. What goes in in pitch when roll is initiated (another, and important, story, which some may guess at from this thread) has a profound effect on rates.

    But we were of course trying to eliminate a keel pocket at the time. Bear in mind that although the intent of a keel pocket was to aid roll rate and reduce roll pressure (and the two are not the same) it has secondary and tertiary effects (sigh, as mentioned earlier) of resisting yaw and feeding roll/yaw coupling. Larry, why don't you try the same experiment on your wing? Throw a camera on the keel, stick some tufts on the wing and go for it. I'm perfectly confident on our result, but we had a very different wing. You may well get a different result.

    Paul, as an aside, I'm a great admirer of what Roy Haggard achieved with the Comet, but he wasn't the first to use floating crossbars. I think that was Gerard Thevenot, on his Atlas, but it may have been earlier still. There was absolutely nothing new in the Comet - crossbars, keel pockets and double surface where all well established - but Roy was unquestionably the first to put them together successfully. The Comet was an amazing breakthrough and showed how profound the role of subtle changes to dihedral and airfoil section were in getting handling sorted.

    Finally, an interesting case in hang gliding is a spin. Here the pilot is pitching up while weighted to the outside of the turn to keep the billow shift on the high wing, in order to keep the low wing stalled. The centre of rotation is inboard of the lower tip (!), and any relaxing of pitch or weight causes the wing to recover. The whole manoeuvre is right on the ragged edge of a tumble.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Bryan,
    Yes with most new technologies/innovations sometimes it is hard to determine exactly who invented it. It would not surprise me that Gerard did. He is a great wing designer so I changed my Roy statement to be more true.

    Roy Haggard put the floating crossbar and on the Comet hang glider and with UP they changed the course of flex wing control.

    I have found that in researching hang glider history, everyone thinks they invented it and it was generally changes from all.
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Note here: I created a new blog on a different way to look at this roll so as not to hijack the information presented here with a different subject.
  • Abid Farooqui
    by Abid Farooqui 11 months ago
    Paul: You wanted to hear directly from Bill Brooks or Gilles Bru about what they believe happens to angle of attack on the down going wing or up going wing and how the adverse yaw starts to happen due to lift vectors tilting
    Well I found it. I think Gilles wrote a letter mostly pointed to Revo and Larry's videos that was posted somewhere and someone (possibly Larry) forwarded me a pdf of it. I did not pay a lot of attention to that letter at the time. I had gotten out of Evolution trikes for 2 years by that point and really did not care to get in between a pissing match between the two.
    But that very letter actually says something about what he thinks is happening with AOA of each wing during a turn and how adverse yaw is starting with lift vectors twisting. Its exactly as I had stated here. I have not talked to Gilles Bru beyond saying hi and bye a few times. But I think he agrees with me on this independently. I'll post screen shots from that letter with highlighted text. I am going to suggest that Paul D was saying that Bill Brooks is also of the same opinion as Gilles Bru in this and so am I.
    Look for those pics
  • Paul Hamilton
    by Paul Hamilton 11 months ago
    Thanks Abid. Is this the same letter where he said that strutted wings were unsafe or a new one? With a position such as that about strutted wings I had a hard time having confidence in the perspectives of the document. If you have the PDF pleas e mail it to me...
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Abid, Again, please send me the complete document e mail which you have Paul@SportAviationCenter.com so I can look at it in detail. Having a hard time understanding the new concept of weight shift without billow/washout/twist change initially. Perhaps making this public to all would be helpful to all.

    Like to put together the pieces here......
  • wexford air
    by wexford air 10 months ago
    Just do a Google search Paul for:gilles bru air creation forums, lies and videos. Is the first answer that comes up from alltrikes
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Wexford thanks got it. Will read/analyze in ITS ENTIRITY.
  • Larry  Mednick
    by Larry Mednick 10 months ago
    The Anhedral story:

    Back in the day when I was doing the 3 year R+D on the "Blue Devil" wing we never put into production, I decided to go from a 54" control bar to a 55" one calm morning before a fly in 20 miles away. All seemed good and roll rate seemed to be improved so I left it on and headed out with friends to the breakfast. I commented on the way there that the air was getting really bad, and one of the new pilots said she didn't think it was to bad. Well that should have been my clue THERE WAS A PROBLEM! As we descended into the bumps I was fighting for my life to keep the wing up right. I got it on the ground and realized the wing was flat out dangerous.

    So the larger control bar reduced anhedral and flattened my wingtips to the point where it went from perfectly safe to dangerous.

    The moral of the story is when it comes to flex-wings there are no absolute truths. Reducing anhedral does not in all cases increase roll dampening the same as leaning out your carburetor doesn't always make more power. A wise man once said "today I can tell you why everything works one way in a flex wing and tomorrow I can tell you why the complete opposite is true.". (Or something like that)
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Ok I will try to put it together.

    I will start out with a quote from Air Creation Gilles Bru (from "the letter") which I completely agree with:

    Start of quote
    "· So how does a weightshift aircraft be safely controlled without any aerodynamic
    control/deflective surfaces?
    In fact, a completely rigid wing can not be controlled by shifting the center of gravity !
    The increased angle of attack (AOA) and thus lift on the wing in descent, and the decrease
    in AOA on wing in climb, creates a torque opposite to the desired movement. These are
    basically the deformations of the structure and the sail during roll movements which, by
    limiting the impact of angle of attack variations allow the roll movement. There is an
    increase in the billow and washout of the wing in descent and a flattening of the billow and
    reduction of washout of the wing in climb. These changes in the shape of the sail are
    obtained either by deformation of leading edge tubes and fabric, or through the transfer of
    billow between each half-wing, made possible by systems of floating keel and/or keel
    pocket, often both."
    End of quote.

    This clearly states that the roll is allowed to happen with billow/washout/twist change. What TYPICALLY causes the billow/washout/twist change? Weight shift. As you move your weight over it SIMOTANIOUSLY pulls the keel over and creates the billow/washout/twist change. Additionally, as the weight is moved over to one side, it loads that side up more assisting in the billow/washout/twist change. Yes as you shift your weight over it creates a moment to roll the wing because one wing is shorter that the other, but this is MUCH smaller that the effect billow/washout/twist change has in the roll.

    If the weight shift was the main contributing factor to start the roll, all similar size/shape/loading wings would start to roll the same. This is not the case.

    So to summarize, most collective wisdom as published in the Trike book, myself, Gilles Bru (as quoted above) feel that the PRIMARY reason the trike wing rolls is the billow/washout/twist change caused/created by weight shift.
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Paul you have got one thing terribly wrong and its not getting through.
    First some clarifications
    1) Angle of attack on downgoing wing is increased and remains increased through the whole roll turn, otherwise the downgoing wing would continue to keep rolling down and never achieve a steady state.
    2) The roll is started by wing tilt and weightshift, NOT by billow shift
    3) Billow shift comes in to overcome aerodynamic roll damping to relieve the opposing force of downgoing wing having a greater angle of attack and up going wing having a smaller angle of attack and makes it EASIER to roll or it would be very difficult. Billowshift ABSOLUTELY does NOT CAUSE the roll. It is in fact an aid to roll and an important aid for sure but it is not the cause. It can never be that billow shift overcomes the roll damping completely or you will have an unstable roll effect. You can almost see this in some of Larry's videos when he yanks on the wing hard in roll and to stop it, he has to actually put in opposite input very quickly or push the bar out significantly to increase AOA and have more dihedral effect to stabilize roll. This is what Gilles said in that letter was undesirable. I told the same thing to Larry when I flew it that I like it, experienced people would like it but its probably is not a wing I would suggest selling to beginners or low time pilots. Larry then went on to work and further refined the wing and also added the dampener that some have started using.
    4) Billow shift does not happen in direct response to weight shift. It happens in response to aerodynamics or air loads. Period. You have got this point 100 percent backwards IMO. Steve Siebel gave a very detailed explanation of this which I have posted previously in this very thread
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Abid perhaps we are saying the same thing. Yes of course the turn starts to bank buy moving the bar to the side, shifting the weight, tilting the wing, or what ever you wan to call it. It then rolls mainly because of billow shift/washout/Twist change As Air Creation expert Gilles Bru wrote in his paper AND I QUOTED ABOVE again which you originally said he did not say. Which one is it?,

    Gilles Bru said:

    "So how does a weightshift aircraft be safely controlled without any aerodynamic
    control/deflective surfaces?
    In fact, a completely rigid wing can not be controlled by shifting the center of gravity !"
    End quote

    Basic collective knowledge which I guess you disagree. That is fine. Yes of course billow shift/washout/twist change relieves roll dampening, that is the primary rolling moment.

    Perhaps it would be better said that weight shift/wing tilt/bar movement creates billow shift/washout/twist change which is the primary control resulting in roll.

    I have made the point very clear above and not going to repeat my self and debate the BASICS of billow shift/washout/twist change putting the trike into a turn.
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Sorry if I was somewhat responsible for a long and protracted debate..!

    But reading all the posts after mine I conclude that I have heard nothing to convince me to change my views - indeed they have been reinforced - particularly the extract of mr Siebels - which describes the mechanism in detail - and makes a big point about the angle of attack always being higher on the wing being rolled downwards. Billow shift means that it is not as large an increase as it would have been without it - reducing damping and the effort we feel on the bar.

    Look at it another way - if lift were equal on both wings and remained so whilst rolling we would feel no resistance to our roll input - the fact that we do feel resistance is because the act of rollling causes an increase of lift on the downgoing wing and reduction on the upgoing wing.

    I don't agree with all Gilles Bru says - for instance I know that a rigid wing will roll due to weightshift - I have that demonstrated every time I fly a side by side fixedwing microlight solo - it constantly wants to roll towards the side I am sitting - and I get bored having to apply constant aileron deflection to counter it..!

    Paul
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Hi Paul H:
    No in fact weightshift in a rigid or rotary wing can apply roll torque on the aircraft. This is why side by side helicopters and gyroplanes have to either apply ballast on one side in the nose (like water) or have another way to roll trim. The roll is just not very effective and is very slow. I do not know if Gilles meant that rigid wing aircraft simply cannot roll at all with weight shift or the weight shift control is just not a practical solution to effectively control roll. I agree with the latter sentiment. We can ask him. If he thinks the former, I disagree with him and can demonstrate it if I felt it necessary.
    In fact weight shift even has a pitch stabilizing effect in airplanes. High wing versus low wing airplanes. Cessna high wing has a slight stability advantage in pitch compared to a low wing Arrow. But that's a tangent.

    I should have also made another point in my above post about adverse yaw starting to come in as soon as roll started due to change in angle of attacks of down going wing (greater AOA) and up going wing (Smaller AOA) which creates a twist in relative wind and lift vectors as I have explained elsewhere before in this thread and then continues through billow shift and deformations. Even the adverse yaw is not started by billow shift, its started simply due to change in AOA right as roll starts and because of most of today's wings having a slight negative coupling between yaw and roll aids in roll further.
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Paul D:
    Oh finally you come in after throwing in the lamp in the camp and doing a disappearing act. Hahaha
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Honestly, there are a lot of things that could be explored or played with in this genre to make at least interesting studies that could possibly develop future products. It took me many many years and actually studying airplanes to somewhat understand trikes because honestly there are no books on it and the ones there are that even touch design including Pagan's I believe are off base.
    Alas, the trike market is so puny that its hard to justify me doing a lot of work developing some concepts further. Its not a great business model.
  • Larry  Mednick
    by Larry Mednick 10 months ago
    One thing I wanted to add about the video where you see the rapid roll input, the wing banks so fast there is a whipping effect of the carriage, meaning by the time the wing reaches 60 degrees, the carriage is only at maybe 15 degrees. Then the wing locks into the 60 degree bank and when the carriage swings out the controls then appear to go opposite way more than any movement applied by me to stop the roll. The real trick is to coordinate your turns by pushing out. That locks in the bank. There are 2 types of pilots. Those that coordinate their turns and those that do not. Those that do not probably will have problems with a wing like the Blue Devil. Barry Maggio flew that wing from Massachusetts to Tampa (1000 miles) in one day and said he never used more than hi finger and thumb on the PTT switch to fly the trike the whole way. So we talk about what's "good" it's all preference and what you are trained on. If you ask me, training on a wing with extremely high roll dampening is bad for teaching how to fly properly. IMO it creates bad habits such as jabbing the controls sideways and not coordinating any turns. Then You jump in a soaring trike or a Wizzard or something sensitive and you can't fly it properly.
  • wexford air
    by wexford air 10 months ago
    Right! Now that that's all sorted, who's up for turning the trike around and sticking the engine at the front?
  • Bryan Tuffnell
    by Bryan Tuffnell 10 months ago
    Hey, Wexford, you're onto it! Of course, we'll need to turn the seats around... :-)

    If you're ever in New Zealand, Wexford, I'll buy you a beer for that one!
  • Lucian Bartosik
    by Lucian Bartosik 10 months ago
    I stand in the corner with Paul Dewhurst and Billy Brooks, that's all I have time for!
  • wexford air
    by wexford air 10 months ago
    SEATS! Where were going Bryan, We wont need seats! But I do think it would take more than one beer to get "into the right frame of mind" to give this the proper consideration! Maybe you will be up this way for the rugby world cup!
  • Larry  Mednick
    by Larry Mednick 10 months ago
    To simplify the key point that was not clear to me to make all of this make sense is that both wings are said to be making EQUAL LIFT as it banks after initial acceleration in roll. The "disconnect" for those using the school of thought of: if one wing is climbing and one wing is descending the climbing wing is making more lift. And if you can't get past the last statement being correct then none of it is possible to understand. Having said that many turns can be accelerating and descellerating with no in between which then means lift is NOT equal during the banking process.
  • wexford air
    by wexford air 10 months ago
    roll is initiated by moving the weight to one side, or by grabbing the bar, connected to the strut/sidewire, connected to halfway out on the wing. A larger portion of the weight(gravity) is now acting on the connection point of the sidewires to the wing. Even if they are making equal lift, one has more gravity acting on it. Go back to the 4 principles of flight
  • Gilles Bru
    by Gilles Bru 10 months ago
    Hi Abid and Paul D.
    About rigid wing and weightshift : The word « safely » is missing. As it was in the preceding sentence, I found it implicit.
  • Larry  Mednick
    by Larry Mednick 10 months ago
    Wexford, so going back to my earliest comment. I can continue a bank (even start one) without weight shift. Although Abid pointed out the keel shifts the hang point shifting the CG. But obviously if you are pulling and shifting weight during the turn as is the normal case, what you say makes perfect sense.
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    The equal lift thing is not correct for a trike I reckon . It is for aerodynamic controls - when roll reaches its steady state at X degrees / sec then by definition all the aerodynamic forces are in balance. For a trike all the forces are in balance in that steady state too - but that includes the torque provided by the control force. so in a steady state rolling movement in a trike the lift must be higher on the dropping wing than the rising wing - with balance being provided by the control force torque. Very simple basic force mechanics that again show the basic theory in the WSC manual must be wrong...?!

    Larry - of course it's possible to roll without weightshift - by secondary effect of yaw - yaw generated by nosewheel movement. But is it billow shift as the driver? The direction of roll reaction depends on dihedral / anhedral effect - and that changes according to wing design and speed - my Chaser exhibited dihedral effect at low speed / high angle of attack and would roll left with a left yaw. At higher speed it had an anhedral effect - rolling right from a left yaw. So billow shift from drag definitely not the mechanism causing the roll at low speed.. The book Tailless aircraft in theory and practice' by Nickel and Wohlfahrt has a lot of explanation why swept wings exhibit this effect and cross over of yaw / roll couple from high to low angle of attack. Also a good related explanation of high speed rollover which afflicted some early CFX performance HG,s.

    Paul
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Hi Gilles!

    Yes not so safe - if I want to roll right by weightshift in my Skyranger I have to undo my seat belt and climb into the other seat...;)

    For a trike is it just safety due to roll rate being too slow for safe control? - could it be OK fur a very small wing - or are there other problems that billowshift prevents like tip stalling when rolling?

    Paul

    PS Hi Lucian too - many decades no see!
  • Larry  Mednick
    by Larry Mednick 10 months ago
    Paul, I noticed your earlier response saying the wings were not creating equal lift. Am I mistaken or did Steve Seibels article say the wings WERE making equal lift (and the descending wing is at a higher AOA)?
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Again if this is looked at in detail, there are no corners to sit in, we are all saying generally the same thing. Weight shift/tilting the wing/moving the control bar creates the roll. Not sure why anyone thinks there are different corners.

    I guess the different corners might be "is it the chicken or the egg?" In one corner we have weight, in the other corner we have washout. Perhaps the real question is what causes billow shift/washout/twist change.........

    Do not really see any controversy here.

    I agree with Paul D and Bill B. I understand this completely. Weight shift/tilting the wing/moving the control bar creates the roll by loading up that wing allowing/contributing it to billow shift/washout/twist change which rolls the aircraft. I agree with this.

    Similarly and simultaneously, weight shift/tilting the wing/moving the control bar pulls the keel over ALSO allowing/contributing to billow shift/washout/twist change to initially roll the wings. Simply, after the wing is tilted the weight pulls the keel over assisting in the to billow shift/washout/twist change. Cannot argue with gravity.

    Weight shift/tilting the wing/moving the control bar creates the roll. No corners or differences of opinion for me here. One in the same. Each of these contribute, perhaps the real question is when and how much. In reality, EACH WING IS DIFFERENT. I have not picked one side or another. Both are correct and contribute. I guess the "corners" or sides of the story help us describe the situation so we can look at it cliser.

    I will clarify one statement I made above. I do not completely agree with Gilles Btu statement and I will add his word "safely" to clarify (thanks Gilles for the clarification):

    "So how does a weightshift aircraft be safely controlled without any aerodynamic
    control/deflective surfaces?
    In fact, a completely rigid wing cannot SAFELY be controlled by shifting the center of gravity !"
    End quote

    Actually I do agree with the statement when it includes safety. Why? because shifting the weight with a rigid wing will allow it to turn BUT NOT VERY FAST OR FAST ENOUGH TO FLY SAFETLY. I would guess a rigid wing would turn maybe 5 to 40% as fast as the same wing that allows billow shift/washout/twist change. We need billow shift/washout/twist change to safely roll/maneuver.

    I could actually say that a rigid wing WILL NOT turn with WEIGHT SHIFT alone. Why? Because I did an experiment once. It happened to me the first time I ever flew a Fledge Wing. This is a hang glider TRUE RIGID wing that turns by actuating rudders on the tips. An ACTUAL RIGID wing. It could easily be put on a trike and flown and it has been. It was one of the models where you had to hook in the tip rudders with cables (not twist grips) onto your body so as you would weight shift, just like a hang glider or trike, it would actuate the tip rudders and you would turn. My first flight I did not hook up the tip rudders, big
    mistake. I took off and started into a slight right turn. I weight shifted to the left to counter the turn and no response. I moved every pound of my weight to the left as far as possible and no response. SIMPLY, weight shift HAD ABSOLUTLY NO RESPONSE THAT I COULD FEEL TO TURNING A RIGID WING. Been there done that. In horror that slight constant turn continued as I had all my weight opposing the turn and I crashed into the side of the hill. Luckily it was a bunch of big bushes and I only got slightly beat up. So when someone tells me that weight shift alone will roll a RIGID wing, It does NOT, or not enough to maneuver/control safely. Would the weight of the trike have been enough to turn it? Maybe. Weight matters.

    Going back the beginning FAA WSC handbook page 2-13, it presents both corners (weight verses washout).

    Here it is saying exactly that:

    To turn, shift the weight to the side in the direction of the turn, INCRTEASING THE WEIGHT ON THAT SIDE. This increases the twist on that side while decreasing the twist on the other side, similar to actuating the ailerons on an airplane. The increased twist on the side with the increased weight reduces the AOA on the tip, reducing the lift on that side and dropping the wing into a bank. The other wing, away from which the weight has been shifted, decreases twist. The AOA increases, increasing the lift on that wing and thereby raising it. Thus, SHIFTING THE WEIGHT TO ONE SIDE WARPS THE WING (changes the twist) to drop one wing and raise the other, rolling the WSC aircraft about the longitudinal axis.



    Hopefully this clears things up that we are all saying the same thing, WEIGHT SHIFT/TILTING THE WING/MOVING THE CONTROL BAR ROLLS THE WING BY LOADING UP THAT SIDE UP AND PULLING THE KEEL OVER BOTHCREATING BILLOW SHIFT/WASHOUT/TWIST CHANGE that rolls the aircraft. That is the magic of the modern wing. We have both tools helping us.
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Paul - if you scroll up to the top of this page there is a drawing from the handbook that shows the lift envelope of the wing as a turn is initiated. It shows the lift envelope changed such that there is more lift on the rising wing. i maintain that it is fundamentally wrong. The envelope actually shifts in the opposite direction. The act of rolling changes relative airflow so that the lowering wing has the greater lift. What billowshift does is to relieve a large percentage of that which results in lower roll control loads for a given roll rate / or faster roll rate for a given effort.

    Larry - I disagree with Steve on that point - as explained in my last post. If the lift balanced out as equal then why do we still feel roll effort on the bar? - that roll effort is part of the equation of the balance of forces - so there must be an opppsing residual force from lift inbalance?
  • Lucian Bartosik
    by Lucian Bartosik 10 months ago
    Thinking back to the early 70s with our big baggy hang glider wings full of billow and fixed keel bolted to the cross tubes, it was an effort on side ways control bar pressure to get that wing to roll, compared to later years when we went to a floating cross tube.

    Paul, we hope to pop back home to Wales for a holiday this late spring/early summer to show everyone my little boy Luke who just turned two, we went with Anna two years ago, so maybe we can get together while I'm there. Send me some contact info.
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Paul D you are free to disagree with the diagram and I welcome your opinions. Note that almost every diagram is exaggerated to intentionally drive a point home to the learning student pilot. Also note is is not an engineering manual for someone to design a wing.

    So to understand your disagreement with this specific diagram, Figure 2-24, do you feel this diagram misrepresents the initial movement, the movement a millisecond or second (some time period) after the control bar is moved? Do you feel this diagram is never the case under any circumstances?

    You must feel that the weight moment is always greater than the aerodynamic billow shift/washout/twist moment out on the tips.

    Do you have any numbers or proof of this theory?

    Do you and everyone else agree with my numbers above (and shown again below) about the roll dampening (increased angle of attach as the wing lowers in a turn) as we will call it above for a rigid wing. They are presented here again for simplicity:

    Ok good let's look one of the answer to this question:
    • In a steady state start to finish, 50 MPH airspeed, 4 seconds to roll from level to 30 degree bank, with no billow shift (stiff wing), how many degrees is the angle of attack increased from the lowering wing with a 32 foot wingspan?

    a) 1 degree
    b) 1.5 degrees
    c) 2 degrees
    d) 3 degrees
    e) 5 degrees
    f) 10 degrees
    g) All of the above
    h) None of the above

    Yes b) 1.5 degrees is closest.
    note that you have to have speed of the trike wing AND speed of the roll rate to figure this additional angle of attack on the tip as the wing rolls.

    Note that for the BNC (bar-napkin calks) presented here I round things off to keep things simple. No sense doing detailed calks for WAG's (Wild As Guesses) since there are so many more factors/variables to get the CLOSEST answer to the actual situation.

    Ok you are flying at 50 MPH and lets use 70 FPS (feet per second) for this. First we need to figure out how fast your tip is going down.
    Your tip is 15 feet out and with a 30 degree bank it travels about 9 feet (8 in an arc) in an arc as it drops. So it is dropping at about 2 foot/sec.
    So as it drops 2 FPS into 70 FPS air the change in angle is about 1.5 degrees. Note this does not have any billow shift/washout/twist change. Stiff wing.

    Now with Larry and Abid both CLAIMING :-) twice that speed, 100 MPH, note twice the speed would be half the angle (less than 1 degree) at the same roll rate. Twice the roll rate double the angle at the base speed. Twice the speed, twice the roll rate same angle. Not much magic or theory here.....

    And the question becomes, which is why I posted this question (which would typically be of great debate), is how much does that really matter? Is it worth making a big deal about? Especially at the tip where there is not that much lift? So let's say 1 degree of twist change at the tip with a 70 MPH turn and there is no increase in angle. 2 degrees in twist means LESS angle of attack at the tip for roll.

    Here we now have a NUMBER. Is it exact? NO, but pretty close for a BRN and WAG since there are not that many variables.
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Lucian,
    Do you still agree with the roll basics of your great book "Trikes - The flex Wing Flyers - Beginning and Intermediate Trike Flight"
  • Larry  Mednick
    by Larry Mednick 10 months ago
    Paul D. What you say makes much more sense than Steve Seidel on that point t. ( I realize you agree with the rest) and with a wing where I am hanging on the bar to get it to turn I am convinced you are right and I am changing my original though and answer to agree with that. But I still contest my wing works like an aileron in the acceleration phase of roll with an increase in lift. That's my final position on the subject. Thanks for bringing up the subject. It was enlightening to say the least and Abid thanks for the explaination and references.
  • wexford air
    by wexford air 10 months ago
    Inducing a turn with the nosewheel: here's my thought on it. If I turn the nosewheel to the left I get a turn to the right. I think it's because I'm twisting the trike and making it point to the left. This causes a slip to the left creating more lift on that wing thus raising the left wing. Anyone agree/disagree??
    Sorry for the drift (pun intended) Paul H.
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Wexford, It is a simple rudder in front of the CG rather than in back so everything is backwards. Think about a nose wheel as a rudder but it in front rather than in back. Same Thing. Long XC flights in calm air let you figure this out from practical experience.
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Paul Dewhurst and Larry:
    There is no need to disagree with Siebel on this. You have to pay close attention to the whole article:
    " (Astute readers will note that to keep the length of this article manageable, we're making one slight oversimplification--we're ignoring the fact that because the left and right wings have slightly different spans and therefore act at slightly different moment-arms from the CG, in order for the roll torques to be in balance, the longer, rising right wing must in fact be creating slightly less total lift than the shorter, descending left wing. If we were to take this into account, it would further reinforce our conclusion that when the pilot's body is shifted to the left and the roll rate is constant, the left wing is creating more lift-per-unit-area than the right wing.)"

    He states there clearly that the down wing (left in his example) actually continues to produce slightly higher lift. So Paul no disagreement there with what you think. He simplified in the first part to make a point because so many manuals are completely opposite on this.

    Now if someone can start thinking about what happens in a climbing turn to all this ... Nah, too late for that but things do become interesting.
  • wexford air
    by wexford air 10 months ago
    Yes Paul H. But if it were as simple as that then surely the left turned nosewheel would produce a left turn? Mine ends up in a net right turn.
    Abid, that's where lots of this hang glider research ends, when you add power to the mix
  • Joe Hockman
    by Joe Hockman 10 months ago
    Wexford, when you turn the nosewheel you are introducing a yaw moment, so Paul H is right that this is acting like a rudder. Don't forget the negative yaw/roll coupling. BTW, you might check out that tendency to roll right by flying at different speeds. There may be a possibility that your yaw/roll correlation is weakened at slower speeds.
  • wexford air
    by wexford air 10 months ago
    Hi Joe, yes it's much less at slower speeds but what your saying then is that a yaw to the left produces a roll to the right. Can you explain the process a bit more
  • Joe Hockman
    by Joe Hockman 10 months ago
    Hi Wexford, actually I think many contributions on this whole blog shed quite a bit of light on the yaw/roll coupling. In particular, I think Abid probably did the best job in describing the negative roll/yaw coupling.

    If you really want to get into the details (weeds) on this topic I suggest you look at Steve Seibel's website. This particular link (http://www.aeroexperiments.org/questrolltorque.shtml) is part of a Q&A on impact of this coupling and as it relates to roll torque. The more I read on Steve's site the more I realize this guys thinking process is very similar to mine. His home page is at http://aeroexperiments.org/. He is a private pilot (airplanes and sailplanes) and hang glider pilot, with a background in physics, biology, and oceanography. He admits he has no formal degree in aerodynamics, but he clearly has an insatiable interest in understanding the physics associated with flight particularly for flexwings. But what distinguishes this individual from perhaps many on this site is that he has proactively pursued many, many kinds of aeroexperiments to develop an understanding of many theoretical concepts that apply to flexwings. This is exactly what I have been advocating for decades. Rather than sit back and cogitate in a reflective manner on what some aerodynamic theory may be true that might apply to the wing I fly, he has gone out and devised scores of ways to experiment and test hypotheses. I too do not have an aerodynamics degree but I do have a creative curiosity and also have conducted numerous experiments to really understand what is going on. In any case, I do strongly recommend you spend some time on Steve's website IF you are the kind of person that really wants to understand some rather technical phenomena related to flexwings. Although all his experimentation is primarily focused on HG wings, almost all of the concepts also apply to trike flexwings. Also I am not admitting that I agree with every thing he says as I have discovered a few differences in views or conclusions. Although his writing is a little on the verbose side, it is very understandable.
  • wexford air
    by wexford air 10 months ago
    Not sure Joe if that site would have anything specific to the point here as there is a power element. But it is a huge site and something I would love to get lost in if I had the time and I wouldn't be doing any justice to the hard work gone into it by skimming over it. Has he done experiments using trikes?
    Wonder if a floating keel has an effect here?
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Larry - I guess it all comes down to what is causing the control pressure we feel when rolling. When we are rolling it takes substantial pressure on the bar to do it. Why is that? It feels an awful lot like the wing is resisting being rolled - why would that be? - it can only be because the act of rolling is causing the lowering wing to increase angle of attack and the rising wing to reduce angle of attack. That provides the resistance we feel on the bar. If we have great billow shift that nearly cancels out the angle of attack change then the resistance is less. If billowshift could just maintain equal angle of attack on each wing wouldn't then we have the case where roll effort would be zero? Or is the force we feel on the bar being generated by something else?

    Paul H - not quite sure what you feel is proven by putting a number on the angle of attack Change at the tip due to rolling?. I think you are saying the number is so small it would be an insignificant force? - but if so why would we need billow shift...? Another way to look at it is to consider it as a percentage. The cruising average angle of attack may be quite a low number 5 degrees or so and will be less at the tip. So a one degree change is a big deal percentage wise - and it's plus on the descending tip and minus on the rising one - so two levers - and quite long ones, dont take much force at the tip to translate into big pressure differences on your arms.

    Interestingly your analysis of the angle of attack change due to rolling being greatest at low speed and reducing at high speeds explains perfectly why roll is heavier at low speeds than high speeds - Which I believe is I accordance with the argument that when rolling the downgoing wing must have a higher angle of attack and greater lift than the rising wing...

    Wexford - not all trikes and not even the same trike will roll left with a right yaw. It depends on effective dihedral at the time. Most trikes have some geometric anhedral built in to calm down the natural dihedral effect of sweepback, and exploit adverse yaw to improve roll response . The dihedral effect of sweep back (yaw making the into relative wind wing longer and therefore produce more lift) reduces as angle of attack increases. Geometric anhedral increase in its effect as angle of attack is reduced. So at low speed in such a wing you might get positive dihedral effect - yaw left / roll left, but at high speed the anhedral wins and the couple reverses and yaw left causes roll right. It also alters whether you add a climb or descent in the mix - in a climb the wing experiences an overbank tendency ( higher angle of attack on the outer wing)which exacerbates any anhedral effect, and in a descent it has an increased lateral stability ( higher angle of attack on the lower wing)which enhances dihedral effect. A lot to get your head around perhaps - but a small taste of why trike aerodynamics are so fascinating. And trike aerodynamics are a relatively undocumented science - why always makes for interesting discussions - even the really experienced designers can have different theories to explain observed phenomena!
  • wexford air
    by wexford air 10 months ago
    I'm going to print your answer Paul and stick it on the back of my helmet. That will stop passengers asking that question! I'm going to have to draw pictures for myself to digest it.
    Is there a typo in there about anhedral and dihedral decreasing as aoa decreases?
    It's great to talk about this stuff and it may even help designers come up with the next great idea.
    But your right it is fascinating and nebulous, and the fact that the goalposts are always moving I suppose makes development slower but much more interesting and fun than 3-axis
    And I think your saying that I'm at least almost right in my assumption ☺
  • Bryan Tuffnell
    by Bryan Tuffnell 10 months ago
    And Wexford, about that next rugby world cup... got any theories? I'm gonna be mighty thirsty by then ;-)
  • wexford air
    by wexford air 10 months ago
    It's getting longer and longer to scroll all the way down here to answer a post!
    Are you suggesting something like mounting the top of the a-frame to the cross tube instead of the keel? Then you would have to move the keel pocket perhaps. Of course it wouldn't be an a-frame then maybe "straight up" frame.
    I think you should become an ambassador fur rugby and come over here to sample some of our pubs and then report to the top brass how they should give us the gig
  • Larry  Mednick
    by Larry Mednick 10 months ago
    Paul D, the carriage naturally wants to hang perpendicular to the wing. Anytime the pilot moves the bar sideways they feel the weight of the carriage as it's moved from the vector of lift. Secondly the wing wants to be symmetrical so the keel wants to center itself. So applying roll control upsets both of these forces. You can reduce these forces to 0 as shown in the Blue Devil video. I hear exactly what you are saying and I think your question was rhetorical, but I have an answer to it. So as much as your "proof statement" is convincing, I can attribute roll pressure to 2 other things. I also fly by twisting the bar, this is one of my "proof statements". I do believe a wing that rolls 20 degrees per second is weight shift primary. A wing that rolls 90 degrees per second cannot be... it is "driven" aerodynamically driven like an aileron in effect. This is like 2 people riding a tandem bicycle, you say the guy in the backs legs are turning because the pedals are turning. I'm saying the guy in the back is driving the pedals. Depends on the effort of the guy on the back.

    Roll pressure is lighter at slow speed on the RIVAL S than at high speed. How did I do that? Keel triangulation of the rear cables. So does a big keel pocket help the sail shift independent of the keel and a small pocket restrict its movement or does a small keel pocket force the wing to billow due to a large keel shift and a large keel pocket take away the effect of the keel driving the sail?

    This is my theory on what's possible, I feel I have demonstrated this with the Blue Devil video.
  • Bryan Tuffnell
    by Bryan Tuffnell 10 months ago
    Wexy, you sound like trouble. The Monster and I will therefore be over in a flash.

    No, it's all a matter of perspective. A floating crossbar lets the crossbar move while the keel stays still... or the keel can move while the crossbars stay still. If you, Wexy, UK Rugby Ambassador and Keeper Of Knowledge Of English Pubs, are flying along on a trike that has little in the way of a keel pocket and you shift your weight to the right, imagine that instead of rotating your arse in an arc around the hang block you remain hanging vertically beneath the keel and you pull the keel sideways. If the leading edges have stayed put, you 1) haven't lifted your weight in an upwards arc against gravity, therefore your arms are fresher at the post-match function, 2) by moving the keel you've directly moved the centre of the sail sideways, making billow shift, which 3) rolls the wing nicely because the wing is now trying to make the angle of attack the same on each wing half. Lots of posters will wince at the above as being a simplification, but that's roughly it...

    Put on a tall keel pocket and what happens? You move sideways, the keel moves sideways, the keel pocket leans like drunk English rugby supporter heading home after losing the world cup, and there's no billow shift. Eventually your weight causes the right wing to drop, its angle of attack increases, and finally billow shift comes to your aid.

    That was why we were trying to get rid of the keel pocket: less drag, faster roll rate through driving roll by billow shift first and not weight shift first. Making it work in practice was... problematic.
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Paul D,
    I am trying to establish some baseline to go from. I learned a long time ago to open your mind to all possibilities. I also learned to not base a FINAL decision on emotion, legend, WAG, tribal stories, or something that one told another and you heard it from his friend OR HIS GIRLGRIENDS BROTHER. Trying to ignore numbers and facts based on emotion clouds our ability to ULTIMATLY find the truth.

    I am activity seeking to figure out the diagrams validity in the above blog 2-24 based on your claim. No one is more concerned than me to get to the bottom line and find the truth. The truth comes out over time so I would rather find out now than later.

    So back to the basic question which you avoided answering specifically. Do you agree with the simple scenario of ROLL DAMPENING numbers as presented??????

    Please do not BS me and others a simple Yes or No is all I ASK. Please take emotion out. This comes down to your basic claim dispute:

    a) If the twist produces an angle of attack at the tip less than 1.5 degrees you are correct, the figure is WRONG,

    b) If the twist produces an angle of attack of 1.5 degrees, well this is middle ground so let's ignore this for now.

    c) If the twist produces an angle of attack of greater than 1.5 degrees the diagram is correct.

    Again, do you agree with the basic scenario/numbers presented as a starting point?????

    YES OR NO????????
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Wexford:
    Just because trikes have an engine does not negate anything about how a wing works.
    The yaw/roll coupling in any aircraft (regardless of power or no power) has to do mainly with Net Dihedral Effect or Net Anhedral Effect.

    Sweep (swept back wings) are kind of thing that increase net dihedral effect
    Billow and Washout combined contribute to a Net Anhedral Effect
    Winglets usually add to Net Dihedral Effect

    so on.

    If your wing has a Net Anhedral Effect at a given angle of attack, if you yaw the nose of your trike left, it will roll right
    If your wing has a Net Dihedral Effect at a given angle of attack, if you yaw the nose of your trike left, it will roll left

    Usually a higher angle of attack will change and add net dihedral effect and increase positive yaw/roll coupling tendency
    A lower angle of attack will add to net anhedral effect and increase negative yaw/roll coupling tendency

    You can have a single trike like Pegasus Chaser that at slow speeds (high angle of attacks) has positive coupling to yaw and at higher speeds (low angle of attack) has slightly negative coupling to yaw/roll.
    Bill Brooks generally designs his wings to have a slight net negative coupling to aid in roll.
    I like slightly net coupling in yaw/roll as well. I use winglets when I want slightly more dihedral effect and more straight as an arrow (XC) performance which also handles performance better. I try and make the wing so if I took off the winglets, it still behaves well and is controllable throughout the speed range but now the net dihedral effect is reduced and roll/yaw coupling is more negative and roll pressures are reduced. Bill Brooks had some issues with QuikR and earlier wings to be able to keep them behaving without the winglets. I think I figured out why he did later on. I think he has figured it out also.

    Larry's wings have a lot of billow and a lot more washout and are much more negatively coupled in yaw/roll.

    Very simple.
    Engine power effects it only in as much as due to PTE (Propeller Torque Effect) and P-factor the roll torque is favored in one direction and opposed in another. P-factpr may yaw your nose in one direction or another. If significant, that may also induce a roll as above due to that nose yaw. I am going to avoid confusing other propeller effects for now because these two are primarily effecting the trike. The others effect airplanes and gyroplanes much more.
  • Craig Dingwall
    by Craig Dingwall 10 months ago
    Paul D:
    "When we are rolling it takes substantial pressure on the bar to do it. Why is that? It feels an awful lot like the wing is resisting being rolled - why would that be?"

    Bear with me but could this not just be that with the air flowing around the wings it creates an inherent stability (similar to the way a dart flies through the air) and more to point could it not go back to the whole lower pressure over the top of the wing and more importantly airflow impacting the lower surface of the wing and therefore you are pushing the wing down against the airflow.
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Craig Dingwall:
    Air flowing around the wings is exactly it. That flow defines "relative" wind vector on the whole which determines "angle of attack"
  • Bryan Tuffnell
    by Bryan Tuffnell 10 months ago
    DInga, your question could be rephrased as 'does air have inertia?'
  • Bryan Tuffnell
    by Bryan Tuffnell 10 months ago
    Larry's comment above came in while I was typing mine, but we're saying the same thing. The difference appears to be that Larry had considerably greater success at getting the result we were after in the mid eighties.
  • Larry  Mednick
    by Larry Mednick 10 months ago
    Bryan's post is spot on about the keel pocket.

    Abid, net dihedral/anhedral is very real on a rigid wing but you MUST consider billow shift in your statement. It is in fact lack of side slip (adverse yaw) the winglets change causing the relative wind to act on the leading edge. The same effect can be see with respect to the carriage. Put a larger wind screen on it and you get the same effect. Nothing to do with anhedral. The effect has more to do with ease of billow shift vs. anhedral. Again which is primary in this case I say again wing design dictates which is primary (anhedral or billow shift making the negative roll yaw coupling) perhaps another endless debate? ;-)
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Larry: Billow and washout increases net anhedral effect. If you have a lot of billow shift, you have more net anhedral effect in action. Putting more area on the carriage in front of hang point moves its CP forward, that is increasing anhedral effect on the aircraft. You have to think of the whole system. You are starting to induce yaw effects that with negative coupled yaw/roll in the wing produce undesirable stuff.
    Billow shift helps a lot, Paul D. me, Gilles, Bill Brooks I believe all agree with that but billow shift is not what causes your roll to start happening. Its a huge aid in making it happen effectively. Anhedral effect and billow shift are not exclusively separate. Conceptually to be clear, its anhedral effect that couples roll and yaw. Billow and washout increase anhedral effect which is not just simple geometric anhedral (actually technically negative dihedral) built into the leading edge structure.
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Larry:
    You are stuck on dihedral angle I think.
    In aeronautical engineering, in aircraft stability concepts, Dihedral effect, represented usually by Cle is the rolling moment due to sideslip put simply.
    I hope this starts to clear up why your Blue Devil experiment was able to roll. It was due to side slip due to negative yaw/roll coupling and thus Dihedral effect. Billow shift is just a mechanism. You are executing the negative dihedral effect concept and just because you happen to be doing it on a trike it uses billow shift to aid roll.

    From Wikipedia:
    "Dihedral angle (or anhedral angle) has a strong influence on dihedral effect, which is named after it. Dihedral effect is the amount of roll moment produced per degree (or radian) of sideslip. Dihedral effect is a critical factor in the stability of an aircraft about the roll axis (the spiral mode). It is also pertinent to the nature of an aircraft's Dutch roll oscillation and to maneuverability about the roll axis."

    Blue Devil has this instability in roll by the horns. An experienced pilot would enjoy its maneuverability, an average rookie might get in trouble.

    Dihedral effect is effected by everything from sweep angle (adds dihedral effect), dihedral angle, billow, washout, billow and washout together (M shape from the back usually contributes net negative dihedral effect), vertical center of gravity of the aircraft/trike (lower creates more positive dihedral effect, higher vice-versa), pendulum effect aka keel effect (trike has tons of it and this contributes positive dihedral effect) and most importantly for your comment about larger windshield --- the height and size of anything on an aircraft that changes its sidewards force as sideslip changes.
    So there you go.
  • Larry  Mednick
    by Larry Mednick 10 months ago
    Thanks Abid, I stand corrected. I did not realize billow shift and keel shift were encompassed in the definition. that's simple enough. Yes on same page now.
  • white eagle
    by white eagle 10 months ago
    Abid has sure been showing he knows his stuff lately .
    Gotta hand it to him larry!
    I think i need more tannic acid in my diet?
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Hi Paul H - I see what you are asking now. The answer to your question - from my understanding, is the very kernel of my first post - the twist change (AKA) billow shift must be less than the angle of relative wind change induced by rolling, because the fundamental theory I was espousing is that the relative wind change causes assymetric loading which is what activates the billow shift. It is only activated by that loading change so can only exist whilst the loading remains assymetic and therefore can never completely remove the angle of attack change. Very effective billow shift can only reduce the amount of change and hence reduce roll damping to pleasant light control forces.

    That's the theory - as explained to me by BB and GB separately- and also ( I have learnt from this thread) its in Steve Siebels stuff. And I haven't seen any evidence to disprove it - but then we lack hard 'evidence' either way - at present it's which theory makes the most logical sense and fits with what we feel and observe!

    Paul
  • wexford air
    by wexford air 10 months ago
    That's good reading Abid and Paul D. And Larry of course! (Gets easier to grasp on the 5th read) it's great for us fair weather flyers to get answers like this from the people who live it and breathe it day by day
  • wexford air
    by wexford air 10 months ago
    Although there is one famous quote that I love “ If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls. ”
    But that guy never knew trikes!
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Larry - yes a pedulumn held out of plumb requires some force - but that force in our case is used as torque to roll the wing. If billow shift was doing all the work - why would there be any noticeable torque after roll initiation if billow shift was the driver for roll? - wouldn't it then need opposite control pressure to stop the roll? Or is it all down to the keel wanting to self centre? The Chaser keel is detached from the sail entirely and sits in a very wide lateral slot in the sail - yet it still needs bar pressure to maintain a roll rate.
    And same for an XL with super deep keel pocket.
    So it keeps dragging me back to the theory that seems to fit for all these design variations - that billow shift can only ever ease lift inbalance due to rolling ( roll damping) and not swap the lift round and be the primary driver. But heck I am willing to be convinced otherwise - and thus thread has made me think long and hard and I am not quite as fully secure in my belief now - but it's still winning just now:)

    I am in Aeros in Kiev today - and chief wing designer Sergei shrugs and grins at such questions.. :) one of our reasons for being here scheming up a worlds first radical wing - another collaboration between Flylight and Aeros - watch this space....!

    Paul
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Wexford - the amusing thing is that the designers can't all agree on just how trikes work - yet they do! Design seems to be largely to try things and note down what happens, and scheme a theory to fit later..:) Sometimes very unexpected things happen - quite contrary to what was anticipated !

    I have been working with a team on new syllabus and exams for UKNPPL. We have questioned the relavence of a lot of the classic principles of flight questions. The deeper you dig the more you find theories are contencious - so what is the benefit of teaching and learning them? - so we have concentrated more on things that are directly relevant to flight safety. It's surprising what was in there unquestioned for years that have no practical use at all or were just plain wrong - like the stupid 2/3 -1/3 lift split between upper and lower surface - an ancient beloved exam question. Clearly nonsense when a flat bottomed aerofoil will generate lift at a small negative angle of attack..!
  • wexford air
    by wexford air 10 months ago
    ya. I didnt hear bernoulli mentioned at all in this thread either. On the point you made earlier, I notice that once you start making a turn and co-ordinate a bit, your billow shift returns to neutral as the trike and keel are centred (almost) again...
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Well Paul D I agree with allot what you said about except one very important force: GRAVITY. Have included a diagram that shows the wing tilt concept and the 350 pounds in a 20 degree banked turn that pull the keel over to be a significant part of billow shift/washout/twist change. Perhaps a picture/diagram will help explain this important principle.

    Yes true Wexford we are covering how we get into/initiate a turn. Also to clear something up, we have aerodynamic and mass/weight forces to over come. If you want to simulate the mass/weight forces get a 100 pound 30 foot pole and try and rotate it. Same as we do with the trike wing. It takes a force. The faster you want it to rotate the more force you need to put into it. This should help understand the mass/weight forces.
  • Larry  Mednick
    by Larry Mednick 10 months ago
    Paul D, but I DID design a wing that would keep rolling once initiated. I assume you saw the video. If not here it is again. https://youtu.be/RM6nBybJfCA

    Btw the billow shifts so easily, any adverse yaw activates the billow. You can see pushing out to coordinate the turn makes it bank the other way. Saying it. Simply saying it has negative dihedral effect is absolutely correct, but quite a generalized statement.

    A disconnected keel or a wing that won't billow much is the only way I can conceive a flex-wing can have positive roll yaw coupling.
  • Craig Dingwall
    by Craig Dingwall 10 months ago
    Paul D: "I have been working with a team on new syllabus and exams"

    what a breath of fresh air to have some new fresh training documentation based on current understanding and incorporating the latest learnings.
  • Larry  Mednick
    by Larry Mednick 10 months ago
    So Wexford, the more camber you have in the last couple of battens out at the tip, the faster the wing tends to roll. How is that for Bernoulli effect with regard to billow shift?
  • Paul Dewhurst
    by Paul Dewhurst 10 months ago
    Paul H - great video! Brilliant to try and put some hard evidence into theory land. Sat in a hotel room and can't do hard analysis and I think it will need some freeze frame and attempt st narrowing in on some number accuracy and also defining roll rate - looks like 2 secs or less to reverse what you are narrating as 45 to 45.

    If it does show a greater angle of attack change due to billow shift than change due to rolling then it's interesting to work out exactly why. We have some mechanical aids now to assist - forced keel movement and stars that in effect force wing warping which is a different mechanism to basic billow shift from load variation. So maybe we have a number of effects using different mechanisms.

    Good stuff - well done!
  • wexford air
    by wexford air 10 months ago
    Ok you got me Larry! It was a bit tongue in cheek
  • Abid Farooqui
    by Abid Farooqui 10 months ago
    Billow shift is according to Siebel due to air. To make that point he states that in a vacuum there would be no billow shift. If you think about it, he is right. Weight shift and tilt can create sail shift but not billow. Billow only results because of the reaction of air for the sail's trailing edge to act against creating an upside down catenary shape.
    It is correct that new things like STARs etc. can change the equation but my guess would be they do not flip it. That would be interesting to figure out for sure though.
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Abid, do not forget about gravity pulling the keel.
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    Craig Dingwall,
    That is great, are you doing this for the Australian Government or for your self and/or a school? What are you using now for syllabus and training materials/books/DVD/CD's etc.

    I am also converting my paper system http://www.pilot-stores.com/sport-pilot-student-course-trike/ to online, not as easy as I thought it would be. I am continually updating/revising/upgrading my system. Just did an updated printing. In my last syllabus update I made a recommendation based on opinions expressed here about transitioning to a higher speed trike "Important Safety notice" to the speed endorsement:

    Flight Lesson 16 -- Airspeed >(above) 87 knots CAS.
    Ground lesson 16 needs to be completed before this flight lesson.
    Note: Trikes do NOT need a speed endorsement if they are below 87 knots Vh. For Vh above 87 knots this endorsement should have been given before the checkride for the aircraft that the student was trained in. This lesson module would be for adding an above 87 knots VH for a pilot upgrading to a faster trike.

    Important safety notice: although the FAA has a regulation that requires an additional speed endorsement above 87 knots/100 MPH, it has been found that transition training for higher speed trikes (above 70 MPH/60 knots Vh trikes as an example) to deal with the additional speed and trim requirements is highly recommended for safety.


    Are you doing it paper or on line. What significant things will you be changing to the existing system? Is it specific to trikes or are you combining it with airplane/gyros?

    Always trying to update/improve my system. Thanks in advance for your reply.

    If you feel more comfortable feel free to e mail me a response. paul@SportAviationCenter.com

    Best,
    Paul Hamilton



    to online
  • Paul Hamilton
    by Paul Hamilton 10 months ago
    This has gotten pretty long so I have created new blog to simplify and describe the five main forces/moments to initially roll a trike.
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