Jan 20th

.....the wing's the thing!

By monty stone

a trike,  is  basically a wing, and a bunch of 'other stuff' hung under it. during flight at altitude, to me it is an object of 'adulation' and awe! (with a tinge of 'don't fail me now!). however many air mollecules hit it they are ignored!, but, UV will attack it. we in washington state run screaming for our 'moms' when the sun DOES come out. we don't 'TAN' we 'RUST', one month it came out TWICE! we all stayed indoors both days!  until recently when the trike wing 'gurus' finally began to blurt out the TRUTH about THE  WING! we sure had to squeeze them! , we, average trike drivers were abyssmally ignorant of the 'true nature of 'flex wing flight, now we are ALL eggspurts, more or less, and each of us could convert a stack of tubing and a roll of bedsheet into a wing, that somebody else should be willing to test-fly! !. we are completely at peace with them little engine pishkins hurrying to and fro 80 times a SECOND, just behind our heads. no problem, but that wing is our personal magic carpet, enableing us to enjoy the SECOND most pleasureable experience known to mankind, next only  , of course to sugar coated cream-filled do-nuts. the alluminum parts start to die as soon as they are born, though slowly. the plated fasteners pretty gold stuff starts to 'go somewhere else ' in a few months, but doesn't leave the fitting degraded, only 'unprotected' till the ensuing rust film will then protect from corrosion  . the wire bits get 'longer' but, unless kinked or cut, would probably outlast the other stuff, though mfrs, and wire co's reccommend replacement periodically. the 'jesus' bolt, the most feared and respected bolt in history, is also replaced, periodically, though a 'used' bolt has already been 'tested' and works, whereas a 'new' bolt is an 'unknown' quantity, to each his own. if replacing a 7$ bolt takes your mind off a 'looming' mega $$ wing fabric replacent cost then it works! so, how can we extend the life of our wing. don't ding the leading edge, (or any other wing tubeing), ONLY fly at night, i tried this, using a hand held flash light to help with the landings, and though fun, it has it's limitations.  but if you INSIST on flying in daylight then, realizing the insideos nature of UV degradation we cover, when possible, though it often 'aint possible all the time. i seem to remember some magic 'potion-lotion' that mega$$ yacht sails use to repel uv rays?? i paint my leading edges and outer two panels with a secret latex paint i get from my local hardware store, i tell 'em it's to paint my dogs kennel, i lied, i don't even have a dog, if i told them it was for my wing they would double the price and make me sign a 12page 'waiver', plus a seven day 'waiting' period, credit ck and homeland insecurity interview  . this 'secret' paint seems to accept being frequently rolled up by not cracking or peeling. this is on my 'lowly' northwing, my chronos repels it and looks like a 'molting python'. the french 'trilam' is slightly shiney and doesn't like being painted. when 'observers' point to the 'flakes' i put my finger to my lips and whisper ' prototype boundery layer test'. i recently sent a test patch from my morthwing to the factory and they said i had 10% of life left.  i took this as a  'kind' gesture, but at 82 they are being 'optimistic!,  for the wing also! . kamron said they usually 'retire' the wing fabric at 50% but the frame lives on! so, anyone out there in 'flexwingflyingthingamyland' got any ideas to prolong my life, i meant my wing's life let's have it,.............freazier nutszoff


Jan 20th

What initially rolls the trike wing and what creates billow shift/washout/twist change. Five fundamental forces/moments

By Paul Hamilton


There is plenty of speculation about what "initially" rolls the wing and creates billow shift/washout/twist change. We tend to focus on only a few and there are five fundamental forces. Three that help and two that do not. Note there are many more but here are the five  that are the greatest contributors.  We will look at these FORCES individually to get a basic understanding.  We are going to ignore the anhedral/dihedral and roll coupling because it complicates matters and we are looking at the FORCES and resultant MOMENTS that initially roll the wing.


 Forces helping us to roll:


1. As the weight is shifted to one side that weight shift moves the center of gravity to one side creating a moment rolling the wing. Gravity pulling down on the weight of the carriage and lift pulling up on the wing. As an example we will say the bar is moved 6 inches with a 1000 pound carriage to get 6000 inch pound rolling moment on the wing simply from weight shift. Let's call this "1 WEIGHT MOMENT" for short. This is the most straight forward and easiest to understand.


2. As the weight is moved over, it loads up the wing which creates uneven loading on the heavier wing side and the flexibility of the wing of the loaded side creates more billow shift/washout/twist change. Let's call this "2 WING LOADING ROLL" for short.


3. As we rotate the wing at an angle the weight can be broken down into two components, a component perpendicular to the wing and a side component . This can most easily be seen in this diagram. Let's call this "3 KEEL PULL" for short.




Note for this example the side load is 350 pounds pulling the keel to the side from basic gravity creating billow shift/washout/twist change. Note this force starts with any movement of the bar/shifting of the weight. One degree is 17 pounds and 30 degrees is 577 pounds. This is a huge force pulling that keel to the side starting billow shift/washout/twist change initially with the movement of the bar starting a turn.


We have all these factors helping us turn. The big question is how much does each of these three specific forces effect the turn. We will cover this later but they are different for every wing.


We have two specific forces working against us for rolling.  Both work against airplanes the same as trikes. We will number them consecutively since we are forces


Factors hurting is from rolling


4 MASS. Newton's laws of motion. Back to the basics for review:


Newton’s Basic Laws of Motion


Newton’s First Law: “Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.” This means that nothing starts or stops moving until some outside force causes it to do so. An aircraft at rest on the ramp remains at rest unless a force strong enough to overcome its inertia is applied. Once it is moving, its inertia keeps it moving, subject to the various other forces acting on it.


Newton’s Second Law: “Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration.” When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force. This takes into account the factors involved in overcoming Newton’s First Law. It covers both changes in direction and speed, including starting up from rest (positive acceleration) and coming to a stop (negative acceleration or deceleration).


Newton’s Third Law: “For every action, there is an equal and opposite reaction.”


In triking, we have this 100 pound (more or less), 32 feet (more or less) wing above us that must be moved. This is a pretty formidable force which many ignore. Take a 32 foot long pole weighing 100 pounds and try to roll it 30 degrees in 4 seconds. Takes allot of force/effort, more than you would think.




Here is a new one for some. As we roll, the wing going down sees a greater angle of attack than the side going up which creates  a lower angle of attack, thus this slows down the rolling. However, the billow shift/washout/twist change relieves this roll dampening by reducing the angle of the airfoil on the down going tip and increasing the angle on the up going tip. Again how much billow shift/washout/twist change is a question which we will cover later. First an example to look at the actual roll dampening angle of attack increase for a rigid wing or trike with no billow shift/washout/twist change :


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?


Lets use 50 MPH (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. Again note this does not have any billow shift/washout/twist change. Stiff wing.


With a stiff wing and no billow shift/washout/twist change how much force out on that tip is that?


If we have a rigid wing with no twist and the desired roll rate adds 1.5 degrees of angle of attack on the wing about 15 feet out on the tip what is the force of 1.5 degrees additional angle of attack? Some basic math produces a force of 75 pounds. Calculating a moment for this we can compare to the weight shift moment we calculated above: 12 feet out for a moment of 10800 inch pounds. Note this is significantly more than the 6000 foot moment for the "1 weight moment" alone.


So we know that with no billow shift/washout/twist change, considering ONLY weight shift and roll dampening ONLY,  it is going to be much longer than 4 seconds. Almost twice as long. Note this is ONLY 2 of the 5 total forces for initial rolling. Add force "4 MASS" makes it harder/longer and "2 WING LOADING ROLL" and 3 KEEL PULL" make it easier with billow shift/washout/twist change.


To summarize,  we now have 3 forces helping is roll 1 WEIGHT MOMENT, 2 WING LOADING ROLL and 3 KEEL PULL. Note that 2 WING LOADING ROLL and 3 KEEL PULL BOTH create billow shift/washout/twist change.  We have both 1 MASS and 2 ROLL DAMPENING slowing/not helping roll.


Quite the mix of forces/factors. All of these forces come into play as the bar is moved to initiate a turn. Yes some believe that as the wing starts to drop and the rushing up air  helps billow shift/washout/twist change however this may or may not be is a secondary effect which is a result of the primary forces.


Which ones are more influential. Two of the four are pretty much the same for all similar wings, the 1 WEIGHT MOMENT help,  and the 4 MASS moment hurt. All others 2 WING LOADING ROLL, 3 KEEL PULL and 5 ROLL DAMPENING are effected by  billow shift/washout/twist change. It should be noted that the force from the 2 WING LOADING and 3 KEEL pull is the same but the effect with billow change/washout/twist change is different.


Why do some wings roll faster than others? Because the 2 WING LOADING ROLL , 3 KEEL PULL and "EFFECTIVE" 5 ROLL DAMPENING are effected by  billow shift/washout/twist change.


Faster rolling wings have more billow shift/washout/twist change. Slower rolling wings have less.


A rigid wing will not roll fast enough to be flown safely with 1 WEIGHT MOMENT force alone so billow shift/washout/twist change is needed. Again how much billow shift/washout/twist change is accomplished when the weight is shifted depends on the specific wing. Each wing is different.


So how much does the billow shift/washout/twist change in a turn, the following video shows plus and minus 6 degrees at the tip with one of the fastest rolling wings in the world.




So we know we only need 1.5 degrees to overcome the roll dampening to at least get the lift on each wing tip equal. We are getting 6 degrees in the above video for the fastest turning wing abruptly turning 45 to 45 degree turns. From this measured look at twist change for this wing that probable was one of the highest billow shift/washout/twist change, a good approximation for maximum twist change is 6 degrees plus and 6 degrees minus. Most other wings/situations will be probably be less. We can also see that it does not take much billow shift/washout/twist change to overcome the 1.5 of 6 degrees   "5 ROLL DAMPENING" force.


I would assume most wings are able to overcome this roll dampening force to provide less lift on the down going wing to roll fast enough to fly safely under the pilots control.  This is the magic of the FLEX wing.


This is a diagram showing the lift distribution of the wing in a turn with the twist greater than the 1.5 degrees needed to overcome the roll dampening










Jan 19th

good news/ badnews

By monty stone

           doctor speaking to patient.  "well i have good news, and bad news"                                                                                                                                                          patient.."tell me the good news doc"                                                                                                                                                                                    doctor " the good news is you have thirty days to live"                                                                                                                                                                          patient.." doc, what's the bad news ?"                                                                                                                                                                                                             "the bad news is i should have told you three weeks ago"

Jan 19th

Brain teasers for those that believe down wind turns are

By Joe Hockman

If you believe that downwind turns are "different" from upwind turns, or if you think that a pilot can "feel" the direction of the wind, or that an aircraft tends to "weathervane" to point into the external, meteorological wind, then you might enjoy these brain teasers. Primary context is hang gliding but also applies to flying trikes.

Brain teaser #1:

1.) You are flying indoors.  In an immense, enclosed room.  The walls and floor and ceiling are black. You've launched off a platform near the ceiling and are practicing turns, stalls, stalls from turns, etc.  There is no evidence of any air movement in the room. Does your aircraft behave differently when flying in any particular direction?

2.)  Sunrise. You realize that what you thought were black walls, are clear glass panels. The room is actually the enclosed gondola of an enormous balloon.  As you look down at the newly visible earth, you see that the ground is passing by very swiftly far below. The balloon is in a stiff south wind, and is being blown northward over the land.  Now does your aircraft fly differently in any particular direction, within the closed room? Is it now more dangerous to turn downwind (toward the north) than upwind (toward the south)?  Just because the sun came up and now you can now see the ground? What if you close your eyes? Can you "feel" the wind by the way the aircraft responds when flying in different directions?

3.)  You notice that each of the transparent walls of this enormous, enclosed room has several large windows.  Someone comes and opens all these windows. But no air blows in through them.  Likewise the flags that decorate the outside of the gondola hang limp. Anyone who has ever been in a balloon will recognize this to be true, and the explanation for this is simple: the balloon is moving freely with the airmass without resistance, and so the balloon's velocity is constant, and so acceleration is zero, and so net force also must be zero: the wind cannot be "pushing" on the balloon in any way.  Since the windows are now open the airmass in the room is now the same as the airmass outside. Now does your aircraft fly differently in any particular direction? Is it more dangerous to turn downwind (to the north) than upwind (to the south)?

4.) The balloon is too heavy and needs to shed some weight.  Someone hits a button and all of the walls get jettisoned. The floor, ceiling, and corner pillars are all that is left of the "room".  Again, no air is blowing through the "room". Now is a downwind turn (to the north) somehow "different" than an upwind turn (toward the south)?

5.) You fly out of one of the missing walls and into the clear blue sky.  Now is a downwind turn any "different" than an upwind turn? Is it easier to stall when turning downwind than when turning upwind?

(P.S. Part 3 of brain teaser #1 brings to mind another old puzzle: if a fly takes wing within an enclosed aircraft, do the wings of the aircraft no longer need to support his weight?  What if a window in the cabin is open?  What if the fly is buzzing around the cockpit of an old open-cockpit biplane?  What if the fly flies out of the open window (or out over the side of the open cockpit) and then flies along in formation with the aircraft?  What if he positions himself directly over one of the wings?  At what point as the fly approached the window (if any) did the aircraft stop "feeling" the weight of the fly?)


Brain teaser #2:

We are flying in still air over the San Andreas fault. Suddenly the block on the west side of the fault starts sliding rapidly northward.  (Devastation is breaking out below).  As we fly from across the fault from east to west in the still, uniform, airmass, we suddenly find ourselves flying in a north wind in relation to the land immediately below.  Does this affect the way the aircraft flies?  When we are on the west side of the fault line, are we in more danger of stalling during a "downwind" turn (toward the north) than during an "upwind" turn (toward the south)? 


Brain teaser #3:

Aliens arrive.  After consulting with Art Bell, they decide to use their advanced engineering prowess to abruptly halt the earth's rotation.  You are piloting an airliner at 30,000' over the equator, and the effects of this little disturbance have not yet propagated to your altitude--the layer of the atmosphere surrounding your aircraft is still rotating at a normal rate.  From your perspective, the ground has suddenly started moving toward the west at 1,038 mph.  Relative to the ground, you are now flying in a 1,038 mph west wind.  Does this have any affect on the way that the plane flies?  Are "downwind" turns (toward the east) now different than "upwind" turns (toward the west)?


Brain teaser #4:

You are in still air. Looking straight down, you see a train driving south at 60 mph.  You decide that the train constitutes the "surface" of the earth for the few seconds that you are overflying it. As you overfly the train, you are in a 60mph south wind, in relation to the "surface".  Does this affect the way your aircraft flies?  If you close your eyes and fly in circles over the train, will the "feel" of the aircraft tell you which direction the wind is blowing, i.e. which direction the train is travelling?  Is there a greater danger of stalling when you are flying "downwind" (flying toward the north), or when you are performing a "downwind" turn (flying toward the north), than when you are flying "upwind" (flying toward the south), or when you are performing an "upwind" turn (turning toward the south)?

(Extra credit for hang glider pilots: do you have to "flare" your glider differently when landing on top of the southbound train with the nose of your glider pointing south, than when you land on top of the southbound train with your nose pointing north?  Obviously answer is "yes"--landing with a 60mph tailwind would be disastrous--but why?  Does it have to do with the behavior of your glider in relation to the air?  Or does it only relate to the fact that you are trying to minimize your glider's groundspeed at the instant that your feet touch the ground?  If you were practicing flares at high altitude, aiming for a given profile in the airspeed and sink rate with no concern for ground track and groundspeed, could you tell when you were over the train by the way the glider felt when it flared?)


Brain teaser #5:

This one also applies to those who believe that an aircraft flies differently in "lift" (rising air) than in "sink" (descending air).

Let's ignore the earth's surface, and take the sun as our reference point. In relation to the sun, the earth's atmosphere (as well as the rest of the earth) is moving at 66,674 mph.  If we are near the equator, the direction of motion of the atmosphere (as well as the rest of the earth) is (roughly speaking) toward the west at noon, toward the east at midnight, straight up at sunrise, and straight down at sunset. So we have an east wind at noon, a west wind at midnight, an updraft at sunrise, and a downdraft at sunset.  (Don't confuse yourself by factoring in the earth's rotation around its axis, which is a mere 1,038 mph at the equator).  Bearing this incredible wind velocity in mind, does an aircraft fly differently when turning to the west at noon, then when turning to the west at midnight? Does an aircraft fly differently in the sunrise updraft than in the sunset downdraft?

Jan 18th

Motorkite Dreaming (Cool Video Link)

By Rizwan Bukhari

Hi All,


Here is a cool link for you to enjoy some trike flying fun video. I own the Motorkite Dreaming dvd (which is different than this, some parts are the same). Fun video for all trike pilots :D


I hope you enjoy it





Jan 16th

Sun N Fun or Oshkosh or just the Flyins?

By Rizwan Bukhari

Hi all,


I live in Boise, ID and have never been to Sun N Fun or Oshkosh and was wondering for someone who has prime interest in Sports/Recreational flying which event (Sun N Fun or Oshkosh) is better.


I might visit one of these events this year and was wondering if anyone could give me some tips about their experience to either of the events and what did they like, what were some of the challenges of either of the event.


Or, is it just better to attend the local flyins?





Jan 15th

Thoughts on safety.

By Bryan Tuffnell

Why are so many trike pilots dying? We've heard lots of answers to that, most of which I don't buy. God isn't lurking behind a cloud with a Lee Browning taking potshots at unlicensed pilots; if engine failures had to be fatal there wouldn't be a whole lot of hang gliding going on; if higher performing trikes were dangerous how come so many of us are clocking thousands of hours in them?

The root cause of the majority of triking accidents is surely that the pilot lost control of the aircraft, for whatever reason. And yet trikes must be about the easiest aircraft to control. What's going on?

I don't see many stall related accidents; nor is tumbling much of a feature. You've got to be trying to get into trouble through pure pitch. Trike pilots have little direct, independent control of yaw. Trikes don't spin. I'll bet dollars to donuts that most accidents happen because either the pilot can't roll fast enough, or far more commonly, because they can't remove bank - they are locked out of a turn.

This is topical, with all the discussion about roll that's been happening. It's also a pet subject of mine (hold me down).

Every three axis and rotary wing pilot knows how to coordinate a turn. Every trike pilot should know to pull in to initiate a change of bank, and to push out to turn a roll into a constant rate turn. Yet I believe that this lack of what should be a fundamental skill is killing pilots.

This is where the fuss about spiral dives and slipped turns and Arrow wings come from. What's the solution? I see three possibilities:

1 Manufacturers dumb down wings to cater for inadequate skills.

2 An upping of the standard of instruction, somehow.

3 A rating system for matching pilots with trikes.

Higher performing trikes are not harder to fly. There aren't killer wings. There are some trikes that ask their pilots to have a basic comprehension of the roles of pitch and throttle in banked flight, nothing more. I think having instruction that includes Turns 101 could save lives, and is the answer. I don't know how to make that happen.


What do you good folks think?

Jan 8th

How a trike rolls presented with a totally new perspective 2

By Paul Hamilton



This presents a completely different mindset  to look  at the trike rolling into a turn.  It was presented by RB so I will do my best to convey the message/concept. Historically, we have been looking at shifting the weight under the trike wing creating the turn. Forget all that for a minute and open your mind to a different perspective, a new way of thinking about how a trike rolls into a turn.  Simple. We are not shifting our weight under the wing to roll it, WE ARE  TILTING THE WING ABOVE THE TRIKE carriage which starts the turn.




This can be easily seen in this video. Note that the bar is moved, the wing tilts above the trike undercarriage the trike under carriage (with the camera attached) initially do not move much. Then after the wing tilts you can see the trike undercarriage/camera get into the turn AFTER the wing is tilted.  You can see this most clearly flying straight going into a steep turn and also coming out of  a steep turn. This can be clearly seen in this video.







Look at the simple physics. As an example: you have a 1000 pound trike undercarriage and a 100 pound wing. You have 1000 pounds verses 100 pounds, TEN TIMES the mass trying to oppose each other. Which one is going to move more, simple: the lighter weight wing. Yes it could be 5 to 10 times based on the specific weights but we will use 10 here to make the math simple.




So with 10 times the difference in mass, basic physics provides us an easy way to quantify this. You tilt the wing 40 degrees and the undercarriage moves 4 degrees.  Exactly as shown in the video.




So we rotate the wing, the lift vector goes to the side so we start turning. Hopefully we all remember this horizontal component of lift. The problem is that our momentum 1100 pounds at 70 MPH (or what ever speed/weight) wants to keeps us going straight based on Newton's First Law of motion.




So now we have a trike initially being pulled to the side from the changed lift vector, turning but pointed straight. Kind of a mess to start. It appears we are flying sideways to the relative wind. That pesky sideways adverse yaw we know happens but debate exactly how.  Well over time, however many fraction or seconds it takes, the trike yaw stabilizer (nose angle/billow/wheel spats/tip rudders or what ever you want to call it), the trike stabilizes in yaw track the trike into the turn. That pesky adverse yaw goes away. At about the same time (before, during or after which can be debated) the undercarriage swings out from the centrifugal force and we are in a coordinated turn.




Make no mistake, we are shifting our weight, changing our CG under the wing which helps roll the aircraft, but start thinking of it in a new way/perspective and perhaps it will make more sense.


 You can see from the video clearly that for a trike the wing tilts more than the carriage moves to start the turn.




I know all this perspective is very hard for anyone to swallow after we have been taught what we are shifting our weight to initiate a turn. Yes, again, this weight shift is correct but based on physics we are tilting the wing MORE than shifting our weight under the wing to turn it.




How are hang glider turning dynamics fundamentally different from the Trike? Weight ratio.




Look at the hang glider 70 pounds and the pilot at 170 pounds. Only two and a half difference verses 10 for a trike. So the pilot verses wing ratio is significantly different tilting the wing less and bringing the pilot underneath the wing more. Should we continue to base all our highest levels or roll based on a different animal?




Why is everyone's perspective of the weight shift trike turn initiation, weight shift rather than tilting the wing? We have all been brainwashed from the hang glider designers from day one. I am totally guilty of this myself. It started as we were infant pilots and grew. It goes to the fundamental principles of learning for humans: Primacy- we learned it first creating a strong almost unshakable impression, Readiness - we want to learn to challenge and keep us safe, Exercise - it has been repeated so much it is continually reinforced, Intensity - we practice it and imagine it during flight plus we are passionate and emotional about it as we debate it.




With all these fundamental principles of learning engrained it will be hard for many to embrace this new concept.



There you have a completely new perspective of looking at roll for the trike. We have a long way to go to develop, understand and evolve our sport so perhaps this new perspective will be helpful.


Jan 2nd

Firesleeve or No Firesleeve, that is the question

By Rizwan Bukhari

Hi all,


I was reading up on Rotax 912 engines and realized that same 912s come with Firesleeve and  some without Firesleeve. Which got me thinking why some engines on trikes have Firesleeves and some do NOT? Is this a manufacturer's discretion and some order engines with Firesleeves installed and some without it? and is this an important safety concern?


Following is the picture of an engine with firesleeve on it's fuel lines. (The firesleeve is orange in color in this picture).


My limited understading is that the purpose of firesleeve is to sustain an engine fire for 5- 15 minutes. Hopefully enough time for you to take necessary steps for your safety.


Now my question is

1) Howcome in the above picture, there is no firesleeve on the fuel lines running to the carbs? Are there only few critical areas where a firesleeve is needed?

2) Howcome some trike engines come with them and some don't?

3) Are they important safety item for trike pilots?

4) And the most important question is that can a Firesleeve prevent a Fuel Line Vapor Lock? Or does the Firesleeve NOT play any role in preventing a Fuel Line Vapor Lock?


Thanks for your help.









Jan 2nd

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

By Paul Hamilton




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:






[bil-oh] /ˈbɪl oʊ/




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...