Feb 16th

Weight-Shift Control Trike Aerodynamics- Wing tip angle of attack (AOA) in turns test demonstration Part 2

By Paul Hamilton

 

There has been a question about the basics of angle of attack of the tip in a turn for the weight-shift control trike wing considering wing twist and roll dampening. Here is a simple test with airflow and angle of attack clearly shown for a turn.

 

We saw in the last video that the twist in the wing tip could vary as much as 6 to 9 degrees up and down from neutral in extreme turns side to side in the Revo Rival S trike. This was a simple test, but the measurements were simple, reliable and repeatable with error bands providing a reliable 6 to 8 degrees twist change. We calculated the roll dampening factor. Here we are actually able to look and see it.

 

Here, with this visualization of actual airflow and wing twist in relation to the airflow, we are able to see the angle of attack of the tips for the phases for a turn. We went 60 degrees to 60 degrees bank to be able to detect the airflow and angle of attack of the wing. Any smaller is just too hard to see a significant enough change to provide reliable conclusions.

 

http://www.trikepilot.com/videos/view/_25207

 

So based on our visualization of this video, we will break the side to side (60 degrees left to 60 degrees right) turn down into six distinct phases. We have not considered the adverse yaw which is a completely different topic. Phases of the turn:

 

Phase 1 Initial weight shift/billow shift, washout/twist change.

 

This is where the weight is shifted, side pressure applied and the tip twists to reduce lift to start the roll.

 

Phase 2 Start of the roll.

 

This is where the wing starts dropping and starting the rolling momentum of the heavy wing above. The wing is just starting to accelerate down. Side control pressure is present to provide maximum twist in the wing to continue to roll/accelerate the wing down. Here we have a significant reduction in angle of attack of the wing from the billow shift/washout/twist change to provide enough tip roll moments to roll the wing.

 

Phase 3 Mid roll acceleration.

 

This is where the wing has gained some roll momentum, shown here as the “wing level” rolling side to side, and still gaining roll momentum. Here we see the angle of attack on the wing start to increase as the roll dampening (change of airflow to the wing as the wing drops increasing the angle of attack of the wing). Here we see the roll dampening start to increase. Here roll pressure is still applied to provide as low as angle of attack on the tip as possible to continue the roll.

 

 

 

Phase 4 Max roll acceleration.

 

This is where the roll momentum has built and the wing is dropping at the maximum roll velocity. Here the roll dampening is at its greatest and the angle of attack on the tip is the greatest because of the roll dampening. Here we are past level and the turn is initiating in the other direction. This is where adverse yaw is probable the greatest and trying to catch up with the turn.

 

Phase 5 Roll deceleration.

 

Now we have some centrifugal force and we can release the control pressure to let the centrifugal force bring the undercarriage out directly under the wing and stabilize the turn in the opposite direction.

 

Phase 6 Stabilized turn.

 

Once the control pressure is released we are in a stabilized turn and the angle of attack on the tips is equal as in level flight except we have greater g loading in the higher banked turn.

 

It should be noted that in a WSC roll the weight of the undercarriage provides substantial rolling moment. However, the percentage of actual weight shift verses aerodynamic billow shift/washout/twist change  rolling moments is largely dependent on the specific trike design and the ability of it to provide the twist change from the weight shift, wristed keel or roll assist (which is used with the P&M STARS system). Based on calculations, I estimate the percentage of weight shift rolling moment to twist change aerodynamic rolling moments to range between 25 to 75 percent of the total rolling moment for the trike depending on the specific design.

 

 We have a large range of trike designs out there with different design characteristics…

 

Feb 6th

Cross Country (Long Distance) Flying Prep & Gear

By Rizwan Bukhari

Hi all,

I wanted to start a blog about cross country flying. For new pilots who are exploring long cross country flying, what considerations should they take?

Jeff trike made an awesome recommendation "Delorme Inreach" a PLB (Personal Locator Beacon) GPS tracker that allows two way text messaging.

How about a tent, tie downs, thermal blanket, knife etc?

 

How do you prepare for a cross country trip? What would you take to your cross country trip? Any brand recommendations that have worked well for you?

 

Thanks,

 

Rizzy

 

 

 

 

Jan 31st

....cont.... the sound of silence, second bit.....

By monty stone

back at the hanger the engine, a 377 rotax, sc, si, was disected, the engine, had been fitted with a 'rebuilt ' crank, but the main bearings had steel ball cages, instead of delrin, and one cage had broken up, and had locked up the crank. on reflection my choice of a field was obviously very wrong, i don't think i panic'd, but certainly 'target fixed' on that field. what has all this to do with trikes? well, the older ultralights and 'unsophistacated ' trikes' both feel similar when flying, you 'fly' the wing, how you 'get there' is a little different , but speeds, climb rate, landing speeds are similar, (naked, older trikes!). the main difference, apart from the 'obvious' reversal' is the rudder. the rudder  is VERY dominant in ul's, neglect it and you'll be slipping and sliding all over with adverse yaw that'll keep you awake at night! we trikers are SO spoilled, due to some 'fancy wing design features, that we ' wingliterates' have been eagerly absorbing of late,  we ALMOST don't have to ko-ordinate our turns! (i did say almost!). and 'we don't need no stinking rudder! but we do need to make better 'landing field choices.' in my 'defence' this was my first emergency landing, though not my last!                                                               no damage, scratches only so i'll pretend i planned it.                                                                                                                                                                                                                                                                                               ps, this plane had a second chance chute, but it never entered my head to use it, i still had control and a triangular field in view! plus the 'boss' woulda 'reamed me a new arsehole over the repack cost!                                                 pps, to newbies only, when that big fan stops during a glide at idle  you will pick up a bit of speed and distance  due to getting rid of that big 'disc umbrella'.                                                                                                             ppps. 'cross-control' to increase drag doesn't have much effect with a 3inch boom tube.                                                                                                                                                                                                                                                                                                                      ..................freazier nutszoff

Jan 31st

the sound of silence part one!

By monty stone

the flight was routine. i was flying one of  five beaver ul's that our flying club had. it had a recent engine overhaul and i flew it, to check it out, for free, avoiding the $20 per hour rental. i was over our local training area we refered to as the 'valley of death', we named it due to several ' failed arrivals'. the asi started to slow, so i gave it more gas whereupon it seized,  and my world got 'silent'.   i set up a decent glide angle, i was at about 900ft with abundant grassy fields, so, ignoring the REALLY suitable field directly under me i 'fixated' on a triangular field, WAY smaller, and had numerous obstructions around it, the right side had a long row of tall poplars and buildings, the left side had a barbed wire fence with many black and white cows munching away on the other side. the 'base' side had power lines and a small river with houses and a road. we had always tried to 'arrange' unexpected landings close to a road to facilitate retreval. by the time i had second thoughts about my choice it was too late to 'start again'. most things get bigger, the closer you get, this triangle shrunk! i cleared the power lines, my chief concern, and started my 'round out' at about thirty feet, prior to flareing but as soon as i got in ground effect the plane would'nt sink, it just floated and floated! the 'sharp'corner' of my 'trigon' was getting closer, quickly. i figured at this rate i'd be in the 'tidal swamp' that waited for me at the end of my 'field', though at the time i didn't know  what was there. any how, i decided to 'wheelbarrow' it in, trying to break the nose-gear off and hope the 'jagged bits' would stop me. well, i hit on the nose wheel very hard, but it only bounced me up. by now i was running out of everything, altitude, airspeed and ability! this particular planes nose wheel was fixed, didn't steer, so was REALLY rudder dependant, i had full left rudder and swerved into the barbed wire, and stopped. i got out and walked the six feet to the end of the field. there was a five foot drop into 'ooze' that flooded twice a day with the tide. i turned my attention to the engine. the prop was now free, a common feature of two stroke seizures, the float bowl was full of clean gas, no water or debris, WTF! i saw a barn in the cow field and saw phone lines going to it (this was before cell-phones, a world without them!), i explained to the farmer i had a 'emergency' landing and could i use his phone to call the 'boss' of the club. while waiting for help to come the farmer told me that a few years before 18 skydivers had perished in his field when their plane nosed in. the boss and his mechanic arrived, we held on to the wing while the 'boss' fire-walled it, till it seized again. his mechanic took one of the other engines off another beaver and fitted it, we held on to the wing while the boss rev'd it, when he nodded we let go and he took off, clearing the power lines by a few feet! cont......

Jan 29th

Trike flying is so much fun

By Rizwan Bukhari

In the recent past, we have discussed a lot about accidents, In the end if you train well, take care of your aircraft and fly in safe conditions then there is nothing like trike flying :D Aviation is only as safe as you make it.

 

Here is a beautiful video of pure joy of trike flying.

 

 

 

 

Jan 25th

Experimentation to develop understanding of aerodynamic phenomena.

By Joe Hockman

I am going to start this blog by saying something that might ruffle a few feathers.  If you think about where we have come since Francis Rogallo days we can reflect on a rather consistent evolution and many may argue we have come a long way.  I can easily defend an argument that states we are still in the dinosaur era.  One factor at a time experimentation, tweaking and copying another manufacturers design improvements have led us to the slow evolution and state where we are today.  But if I think of where we could be today given the time span, availability of new materials, etc and using more efficient methods to extract fundamental understanding of flex wing phenomena, quantified understanding of what specific design alterations and materials yield, gee we could be light years ahead of where we are right now. Ok so I may have caught your attention. If so read on.  If not then feel free to exit now from this blog and move on.

I keep hearing this oft repeated phrase, through "trial and error" improvements are made.  That my friends is the most often used form of experimentation to discover or understand complex systems because there is very little fundamental understanding on certain attributes or phenomena. But that is the most inefficient method of developing needed understanding and design improvements.  That is why I occasionally toss in the idea of controlled experiments.  But so far no one seems to have picked up on that.

So if you don't mind I will jump on my soap box for a moment.  In my 25+ years working in R&D at DuPont and previously many years in academia I have had the wonderful opportunity of working closely with many brilliant minds.  Chemists, chemical engineers, engineers with many other specialties, etc almost all with Phd degrees and some with 2.  I taught experimental design for more than 2 decades to these highly educated colleagues.  Almost without fail, all believed the only way to develop understanding of a "black box" system is to do 1 factor at a time experimentation.  This has been drilled into their heads through course and lab work in academia at both the undergraduate and graduate level.  Then I teach them statistical methods of experimental design which can be described as objective driven experimentation within an experimental framework that manipulates multiple variables simultaneously yet is able to quantify individual variable effects and variable interaction effects in a mathematical model.  OK off soap box.

I am not suggesting that any wing designer (P&M, AC, Airborne, NW, Evolution, etc) should go out and willy nilly throw together a bunch of experiments that involve changes to many wing variables without giving very serious thought to many implications including safety, feasibility, etc. To the contrary, all aspects should be considered.  With regard to flexwing design and developing quantitative understanding we could initially take some baby steps focusing in on one variable in a way that not only enables development of a mathematical model on how that variable adjusted at various levels impacts any aerodynamic or other performance criteria that can be quantitatively measured, but also develop good estimates of uncertainty about that model. Only through careful and disciplined experimentation can true cause and effect relationships be established.

A very important word on measurement systems. Only through the use of capable measurement systems can one reliably establish true cause and effect relationships.  A capable measurement system must be both accurate and precise, otherwise either biased and/or uncertain results are obtained.  Additionally, a measurement system should be both repeatable and reproducible.  Repeatability implies replicated tests or trials will give consistent results with low error.  Reproducibility relates to different operators (in our case pilots) being able to reproduce the results of other operators. To tie this concept into recent discussion, using thick reference lines and pieces of 2x4s to give approximate guesses on amount of twist is a very crude measurement system.  It is slightly better than a WAG but it does not have the required accuracy or precision to be a useful measurement system. It is also likely not adequately repeatable or reproducible.  For this type of investigation one would want to use the best available yet practical measurement technique. Abid suggested the use of AoA sensors mounted in selected locations.  If such sensors provide accurate and precise measurements and would yield both repeatable and reproducible results then this would be by far the preferred measurement system if it is indeed relatively simple and safe to implement.  Only by using capable measurement systems can we be confident that the results and models developed truly reflect reality.  Most every one has likely heard the term "garbage in garbage out" which is extremely important any time experimentation is conducted. I am not suggesting the crude measurement Paul H used to assess twist AoA was garbage as I already indicated my view is it was better than a WAG.  In a modeling context, you can have the best most sophisticated and beautiful empirical model in the world but if all the Xs and Ys are noisy and based on questionable measurement systems then the model is likely not useful.

The experimental methods I am advocating have proven to be effective, efficient and do accelerate the discovery and developmental cycle times of any program where experimentation is needed to develop true cause and effect relationships.  I am 100% convinced that if we have specific "improvement" goals for flexwings (and for what hangs below the HB for that matter) then a simple statistically designed experimental approach will help achieve such goals quickly. Through intently observing many discussions of a technical nature on this and other forums I am amazed at what is actually "known" or "unknown" about how flexwings work or perform under certain scenarios.  I contend there is a significant in the unknown category that should actually be in the known category. I also believe that when more fundamental understanding is available to designers, instructors and advanced pilots the more we can advance the sport for the benefit of our community and make the participants and equipment safer for all.

Lastly, even though I am not a designer or manufacturer of wings, I would certainly contemplate developing the needed relationship with one (or maybe more than one) to help them pursue their innovation and improvement goals in a very efficient manner.  I have already thought about approaching Kamron at NW on this.

Feel free to add your thoughts related to both experimental methods and potential goals that specific designers or the community could pursue.

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.

 

5 ROLL DAMPENING

 

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.

 http://www.trikepilot.com/videos/view/washout-twist-change-test-verification_25106.html

 

 

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

 

 

 

 

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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?