Aerodynamic Surfaces - Flex Wing Gliders

A discussion restricted to the topic of hang gliding.
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Aeroexperiments
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rudder / drogue chute

Post by Aeroexperiments »

Dayhead wrote:The idea that your rudder may have moved the keel relative to the glider is interesting.
It must have moved it some, but my videos suggest it only moved it a little. When I yawed the glider by deploying a small drogue chute from one wingtip (which wouldn't create any keel movement), the results were almost identical.

Steve
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ACLaversa
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Post by ACLaversa »

I have been tracing the evolution of the flex wing through as many sources as I can, and discussing revolutionary ideas with may father for about two months now. We have pondered some very outside the box ideas…a dynamic leading edge (variable angle of attack) through a manual rotation of the D tube, or a new VG system that incorporates a joint about 15% of the way down the leading edge with a spring system that would allow the wings to sweep farther back (with a pocket for the sail to roll into on the keel), or control surfaces added in many different ways/configurations. The issue that we always run into is that when the fun of a having a new idea is gone your left asking yourself a few questions…

1) What are the gains, and are they worth it?
2) Does it still have the feel of a Class 1 glider, or has it moved to far away from the beauty/simplicity of a Class 1?
3) Do you care that your new glider will most likely fall into a different Class?
4) How will it perform verses the advancements in that other Class should you choose to use it in comps?

Personally, if I build an experimental HG, it is to be revolutionary, and hopefully bring new aspects to the sport…but alas it does have its downsides. Not everyone is going to want to make a big leap…think of cars, they have evolved in shape/design because all the cars that were viewed as a "leap ahead" rarely sold units…most people did not like that much change that fast.
Forever in the shadow of a true pilot, C.A. Laversa 1950 -
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Rhoenadler
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Post by Rhoenadler »

In the late 80s, the "Tropi" , a flexi from a small german producer, was using spoilers..

[/img]

The first hangglider with:
L/D - brake
steering - spoiler
thermal - brake

As far as I remember, the spoilers were actuated via rotating sleeves on the basetube … It was no commercial success…

Manfred
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ACLaversa
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Post by ACLaversa »

Thanks for the pictures, love to have the reference!
Forever in the shadow of a true pilot, C.A. Laversa 1950 -
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Aeroexperiments
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spoilerons

Post by Aeroexperiments »

Rhoenadler wrote:In the late 80s, the "Tropi" , a flexi from a small german producer, was using spoilers..
Interesting. One would need to take care that the spoileron didn't deflect the sail downward, making a reverse roll effect. In the photos John showed me, his spoilerons were located much further forward-- the hinges were sewn into the sail right along the upper seam of the leading-edge mylar. The hingeline of each spoileron was parallel to the swept leading edge. It seems to me a spoileron in this more forward location wouldn't flex the sail downward.

Also John's spoilerons were located much further inboard. I'm having trouble understanding why spoiling the lift way out near the tips as shown on the "Tropi" photos, wouldn't pitch the nose up.

Also I'm wondering if at very high airspeeds (low angles-of-attack) when the tips are generating minimal lift, the spoilerons on the Tropi would have much roll effect at all?

Steve
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CloudHopper
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Post by CloudHopper »

One would need to take care that the spoileron didn't deflect the sail downward, making a reverse roll effect.
Looks like your standard "up only" spoiler out there, with a bungee return. Thus, no downward deflection to worry about.
I'm having trouble understanding why spoiling the lift way out near the tips as shown on the "Tropi" photos, wouldn't pitch the nose up.
Hint: They are "spoilers" which not only spoil the lift but also add more drag. Any influence on pitching moment is minor compared to the yawing/rolling moment influence. Think of the spoilers more like little drag chutes being deployed out there than lift killers. Can you say Power Steering? Minor weight shifting can easily override any pitching moment effects.
Also I'm wondering if at very high airspeeds (low angles-of-attack) when the tips are generating minimal lift, the spoilerons on the Tropi would have much roll effect at all?
Again, if you think of them as "spoilers" more than ailerons, and consider the yaw/roll moment arm distance verses the pitching moment arm distance, that question is easily answered. At high speeds, the roll effect is great, with very little deflection required.
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Aeroexperiments
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musings on spoilerons

Post by Aeroexperiments »

CloudHopper wrote: Looks like your standard "up only" spoiler out there, with a bungee return. Thus, no downward deflection to worry about.
No, I meant that if you raise the spoiler (say to a 45 degree angle), the airflow pushes down on it, which pushes down on the sail. If the sail is free to flex down relative to the rest of the glider structure, then this will increase the angle-of-attack of the sail near the spoileron-- not what you want. This would generate a roll torque in the wrong direction. Perhaps on the "Tropi", a downward force at the location of the spoileron didn't flex the sail downward much. At any rate this would be an obvious concern that a designer would need to consider, and the further forward the spoileron, the less of a concern this will be, because the structure is more rigid further forward.
CloudHopper wrote:Hint: They are "spoilers" which not only spoil the lift but also add more drag. Any influence on pitching moment is minor compared to the yawing/rolling moment influence.
If they kill the lift at a location behind the CG, they will tend to pitch the nose up. John Coyne described to me how he had to fine-tune the location of his spoilerons so as not to pitch the nose up or down when they were opened.

Remember that a hang glider has negative "effective dihedral" at most angles-of-attack / airspeeds, especially at low angles-of-attack / high airspeeds. This means that yawing the nose to the left will create a roll torque to the right. If a spoileron had only a small lift-killing effect compared its drag effect, then deploying a spoileron from the left wing would tend to roll the glider to the right, especially at high airspeeds. Just like deploying a drogue chute from the left wingtip rolled the glider to the right, in my experiments in several different hang gliders. Especially at low angles-of-attack (high airspeeds).

John Coyne and others obtained good results from spoilerons with no wrong-way roll effects--even at high airspeeds-- which strongly suggests the lift-killing effect of their spoilerons was large compared to their drag effect.

I don't have first-hand experience that John's spoilerons created no wrong-way roll torque in the most extreme corner of the flight envelope where the glider's "effective dihedral" was most strongly negative (VG loose, bar fully stuffed), but when I asked John he said his spoilerons always were effective, at all airspeeds.

It does seem likely that a spoileron located further aft on the airfoil (like on the "Tropi" photo) would cause slightly less loss of lift, in relation to the drag it created, than a spoileron located as far forward as John's were.

In a "tip-dragger" that had no lift-spoiling effect, the pitching moment would indeed be minor compared to the yawing/ rolling moment. A tip-dragger would produce a "wrong-way" roll torque on a conventionally shaped hang glider, due to the glider's negative "effective dihedral" at most angles-of-attack, especially low angles-of-attack (high airspeeds).
aeroexperiments wrote:Also I'm wondering if at very high airspeeds (low angles-of-attack) when the tips are generating minimal lift, the spoilerons on the Tropi would have much roll effect at all?
CloudHopper wrote:Again, if you think of them as "spoilers" more than ailerons, and consider the yaw/roll moment arm distance verses the pitching moment arm distance, that question is easily answered. At high speeds, the roll effect is great, with very little deflection required.
If you mean think of them as generating drag rather than killing lift, then at high speeds they should be very ineffective. Or more precisely, they should generate a powerful roll torque in the "wrong" direction. Just as my wingtip drogue chutes did. After all, at high speeds (low angles-of-attack) is precisely where the "effective dihedral" of a flex-wing hang glider is most strongly negative. If the spoilerons continued to be effective at high airspeeds (as John Coyne's did), I would take this as a sign that 1) the spoilerons were mainly killing lift, not generating drag; and 2) the part of the wing where the spoilerons are located is still generating upward lift even with the bar well pulled-in. I would guess that the wing of the "Tropi" had relatively little twist?

PS I should add that without more photos of the wing, it's not obvious that the "Tropi" had negative "effective dihedral". But I suspect it did--especially at low angles-of-attack/ high airspeeds-- unless the designers decided to build a distinct amount of positive dihedral into the geometry of the leading edges relative to the keel. See my previous post on this thread on how even a glider like a Falcon or Spectrum with very little "airframe anhedral" still has an overall anhedral geometry, due to the way that sail billow creates an anhedral geometry. When you combine this anhedral geometry with a swept wing, you get negative "effective dihedral' at low angles-of-attack / high airspeeds, and the "effective dihedral" gets closer to zero as you approach min. sink. My experiments suggested that at min. sink the "effective dihedral" of many gliders is still slightly negative, but in some hang gliders it becomes slightly positive.

At any rate, from as little as we can see in the "Tropi" photos, it looks like a conventionally shaped hang glider.

Steve
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CloudHopper
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You're Right

Post by CloudHopper »

You're right Steve. I was oversimplifying the complexity of flexwing dynamics. That billow shift might have major influence on roll.

You are also correct in that if the spoiler is too far aft on the wing, it might act more as a trim tab and actually increase the AoA of the wing. The return bungee might help that some.

These problems are best solved by careful experimentation, as you and others have done.

Once worked out, I believe a good application might be on a tandem glider, so that the instructor would have more authority when needed without having to fight against the student for corrective weight shift actions.

- Brad
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Aeroexperiments
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a little more…

Post by Aeroexperiments »

In the interest of keeping this conversation going, I'll keep this conversation going. It's actually not a billow shift that I'm talking about. Rather, the billowed shape of a hang glider creates dihedral inboard and anhedral outboard, and since the outboard parts are further away from the CG, they tend to dominate over the inboard parts, as far as roll torque is concerned. So billow contributes an anhedral effect, overall. If we reached flying speed and then sprayed the sail with superglue or whatever so that there was no further flex, this wouldn't change the basic geometry that I'm trying to describe here, and the aerodynamic effects that result from it.

The change in "effective dihedral" as the angle-of-attack changes is not due to a change in the physical shape of the sail. Rather, it is due to the fact that the "effective dihedral" of a hang glider is determined by the competing effects of sweep (which contributes a positive dihedral effect) and the anhedral geometry of the billowed sail (which creates a negative dihedral effect). Since the dihedral-like effects of sweep are highly dependent on angle-of-attack, and become smaller as the angle-of-attack decreases, you end up with strongly negative "effective dihedral" at low angles-of-attack, and closer to neutral "effective dihedral" at higher angles-of-attack, e.g. near min. sink..

If you are having trouble understanding why the dihedral-like effect of sweep is so dependent on angle-of-attack, then consider the following: would a swept, flat wing create any dihedral-like effect-- i.e. any roll torque during a sideslip-- if it were flying at the zero-lift angle-of-attack, like in a vertical dive?

I saw the same relationships-- the "effective dihedral" shifting to become less positive or more negative as I decreased the angle-of-attack-- in my modified Zagi (with anhedral) which, of course, had no flex! At low angles-of-attack, left rudder made the Zagi roll right, whereas at high angles-of-attack, left rudder made the Zagi roll left.

Interesting stuff to talk about on a cold winter day. Actually, it was soarable here today, so what is my excuse?


Steve
CloudHopper wrote:You're right Steve. I was oversimplifying the complexity of flexwing dynamics. That billow shift might have major influence on roll.
Last edited by Aeroexperiments on Fri, Feb 04 2011, 09:49:34 pm, edited 1 time in total.
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Dayhead
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Post by Dayhead »

First, I'd like to express my gratitude to Steve for doing all those "aeroexperiments" that have increased our knowledge of flex-wing HG aerodynamics. Good on ya, Steve.

This discussion reminds me of things I read many moons ago about some early experiences with swept-aft wings.

Boeing decided to build a large swept wing bomber, the B-47. It was probably the largest wingspan swept wing aircraft built up to that time. And we know that the longer the span and the higher the aspect ratio, the more difficult it becomes to obtain torsional rigidity in a wing.

In early testing, the airplane exhibited 'control reversal' in roll. The ailerons demonstrated a definite trim tab, or servo tab if you prefer, action: They twisted the wing so much that right aileron produced a left roll.


It should be noted here that the B-47 and B-52 had anhedral on the ground only, wing flexibility resulted in positive dihedral in the air. They both had plenty of yaw stability, with large vertical fins placed way back there. There was a famous photo of a B-52 that had lost most of it's fin and all of it's rudder in turbulence. It reportedly flew just fine like that, with the pilot making an uneventful landing.

Meanwhile back at the ranch, it seems that our HG's, with some anhedral, whether geometric or 'effective aerodynamic', would best be served by devices that reduce drag AND lift at the same time. The alternative would be to greatly increase yaw stability with a large enough fin, or to rig the glider with a positive dihedral, like some of the rigid wings have.

One nice thing about having anhedral is the ease of ground handling in crosswinds. So I like that, but it wouldn't bother me if the glider assumed a positive dihedral angle when loaded, as it is in flight. I recently got a flight on an Atos 'B' model, which has a wide nose angle and several degrees of dihedral. It's stability is interesting, in that you don't just 'bump' it to make a slight course change, like we can with a flex. You have to give it some spoiler, wait until you're just about on heading then let it roll back. It seemed to want to roll out on it's own from small bank angles, but once you got it up to I guess 25 or 30 degrees it would stay in the turn, with controls neutral.

A couple posts ago I brought up the idea of an outer sleeve that could rotate on the keel, that would transmit pilot movement to the sail at the root trailing edge. Seems like that would amplify billow shift. Any comments? I'm always thinking what is the simplest possible way of augmenting roll, that would involve minimal change to an existing glider.

On another note, I keep thinking about making some extra long battens that would extend a foot or so behind the trailing edge, at about mid span, and small wings mounted on them. These would raise the trailing edge of the sail when angled to produce lift, and of course would create some drag at the same time. If they produced an adverse pitching moment, then an Atos-like tail could be coupled in to cancel it. But if sail tension wasn't too great, I think they might be an easy way to augment billow shift.

It's cold and blowing down here.
My real name is Steve Corbin, and I approve this message, for now anyway