Aerodynamic Surfaces - Flex Wing Gliders

A discussion restricted to the topic of hang gliding.
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VaughnEntwistle
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Aerodynamic controls on flexwing

Post by VaughnEntwistle »

Here's a scan of a subsequent Press Release. Does anyone know any of the people named in the release?
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Aeroexperiments
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Re: Aerodynamic controls on flexwing

Post by Aeroexperiments »

VaughnEntwistle wrote:Okay, I was finally able to get my scanner working. Here is a scan of the Sierra Whitehawk glider from the 1980s. Unfortunately, the original photo was small and low quality and was printed on Whole Air's low quality paper.

Hopefully the photo still provides an idea of the elevon/spoiler/divebrake position and the cool little winglets.
While I'm guessing that those spoilers worked mainly by killing lift (and also by deflecting air upward like an aileron), I don't mean to be too bombastic about the "negative effective dihedral" thing. If the designers made the sail very tight --minimizing billow-- and also didn't include any "airframe anhedral" (droop of LE's relative to keel), its characteristics might have been a lot different from the flex wings I did my experiments on, which all had either substantial sail billow or substantial leading-edge-tube droop relative to keel (airframe anhedral). Due to these specific differences it's not out of the question that a glider like the Whitehawk might have had positive "effective dihedral" over a large part of the normal flight envelope…

Interesting stuff…

PS obviously in those older gliders where we have stories of pilots flying hands-off for extended periods while changing batteries in varios, etc-- the "effective dihedral" must have been distinctly positive at trim. Negative "effective dihedral" is not an absolute requirement for having good control responsiveness in a flex-wing hang glider, but as the wings have evolved toward higher aspect ratio, negative "effective dihedral" helps, because it puts adverse yaw to work for you rather than against you.

Even the "trainer" type gliders I've flown like WW Skyhawk, WW Falcon, and WW Raven (which I was fairly heavy on, it was a very small version of the Raven) didn't have hands-off roll stability, to the best of my recollection, and nearly all those showed "wrong-way" responses to rudder or drogue chutes at trim, with the exception being the larger Falcon 170 at my relatively light hook-in weight.

Steve S.
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ACLaversa
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Post by ACLaversa »

Dayhead wrote:
The idea started out with contemplating how the entire leading edge tube could be rotated. Way back when, I flew a glider that had a turn in it, that would come and go. When I did a sail-off inspection, I found a bad bushing in one LE where it attaches to the nose plate. I replaced it, and the intermittent turn went away. It's my belief that the LE would sometimes twist in the nose plate, causing the turn, but for some unknown reason would find a different position that made the glider symmetrical and fly straight. The amount the LE could twist in the nose plate was small, but the turn produced by it was significant.
This actually goes back to an idea I mad mention of earlier...if you dig back:
ACLaversa wrote: 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...
What if you could reproduce the twist of that LE and cause a reaction via the rotation of one or both LEs. By doing so you add a different form of VG...your changing the whole shape of the airfoil...but most importantly is that it would effect the point of separation...they way I see it...this would effect lift, and drag, but it would be an interesting experiment.

1) Twist one (L or R)
2) Twist both (same)
3) Twist both (op.)

What effects do you believe this would have if the rest of the HG was unaltered?
Forever in the shadow of a true pilot, C.A. Laversa 1950 -
Ridgerodent
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Post by Ridgerodent »

Spoilers work well because they create drag , kill lift and push down. If the wing is twisted by some mechanical means the resulting adverse yaw would need to be countered by something . A rudder ?

Hang gliders have a small amount of damping about the vertical axis , anything that creates more adverse yaw would have to be dealt with.


Edit: actually damping is the wrong word ,( positive dynamic stability) is what I should have said.
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Aeroexperiments
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Post by Aeroexperiments »

Ridgerat wrote:Spoilers work well because they create drag , kill lift and push down. If the wing is twisted by some mechanical means the resulting adverse yaw would need to be countered by something . A rudder ?

Hang gliders have a small amount of damping about the vertical axis , anything that creates more adverse yaw would have to be dealt with.


Edit: actually damping is the wrong word ,( positive dynamic stability) is what I should have said.
Don't forget that even in a conventional hang glider, we twist the wing by mechanical means every time we weight-shift. We pull the keel sideways with our hang strap which allows one trailing edge to rise and pulls the other trailing edge down. This does create adverse yaw. If we watch yaw strings, we can see this much more clearly than if we just notice the initial "wrong-way" swing of the nose. I would say our positive static stability in yaw is pretty substantial-- it sure takes a lot of yaw torque on the control frame to yaw the nose of a hang glider very far out of line with the wind, when you are standing on the ground with the glider in substantial (say 15 mph) wind. Without this stabilizing "weathervane effect", we would see much more adverse yaw.

Other things to keep in mind about adverse yaw-- since the side area of a hang glider is small, adverse yaw probably creates only a small drag penalty, and very little degradation of the turn rate (since very little aerodynamic sideforce is created). Very different from what we see in more "conventional" aircraft. And in modern hang gliders adverse yaw generates a favorable roll torque-- certainly when the bar is pulled in, and in many cases even at trim. (The negative "effective dihedral" thing again.)

Steve
Last edited by Aeroexperiments on Mon, Feb 07 2011, 11:05:18 pm, edited 2 times in total.
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Dayhead
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Post by Dayhead »

Twisting the wing, by rotating the LE tubes, or by raising/lowering the trailing edge, will create an adverse yaw.

In conventional aviation, adverse yaw is seen as something to be corrected. An adverse yaw condition implies that
there will be a side-slip, and airplanes with a fuselage will suffer a drag penalty when slipping or skidding.

Our HG's are "pure" flying wings, a slip/skid angle should produce little or no extra drag.
If the effective dihedral angle is zero or negative, we should get a rolling moment from the twisted wings.

The tailfin on my Sensor seems to have a pretty good effect on yaw stability. I think that my glider has the widest nose angle,
or the least sweep, of any flex wing. It tracks most excellently throughout the speed (AoA) range, with no tendency to PIO.
If I get slow in a wingover and start slipping, the fin gets the nose pointed down very quickly.
The point is that if a glider is modified to have some form of augmented wing twist, and the adverse yaw is considered to be excessive, I believe that it wouldn't take much in the form of a fin to limit the yaw to an acceptable value.
My fin only takes about a minute to rig, weighs little and probably produces an insignificant amount of drag.

IMO, the trick is to find a method that augments roll authority but is very simple, has few if any chances of malfunctioning,
adds little or no time to set-up/breakdown, and is easy to pre-flight. I can think of several ways to warp a wing, but finding a way that doesn't involve a bunch of pullies or levers and cable runs is the catch in all this. I love the simplicity I have now, and
don't want to sacrifice any more than is absolutely necessary.

It may not be possible to do. But I don't believe that.
My real name is Steve Corbin, and I approve this message, for now anyway
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ACLaversa
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Post by ACLaversa »

Dayhead wrote:Twisting the wing, by rotating the LE tubes, or by raising/lowering the trailing edge, will create an adverse yaw.

In conventional aviation, adverse yaw is seen as something to be corrected. An adverse yaw condition implies that
there will be a side-slip, and airplanes with a fuselage will suffer a drag penalty when slipping or skidding.

Our HG's are "pure" flying wings, a slip/skid angle should produce little or no extra drag.
If the effective dihedral angle is zero or negative, we should get a rolling moment from the twisted wings.

The tailfin on my Sensor seems to have a pretty good effect on yaw stability. I think that my glider has the widest nose angle,
or the least sweep, of any flex wing. It tracks most excellently throughout the speed (AoA) range, with no tendency to PIO.
If I get slow in a wingover and start slipping, the fin gets the nose pointed down very quickly.
The point is that if a glider is modified to have some form of augmented wing twist, and the adverse yaw is considered to be excessive, I believe that it wouldn't take much in the form of a fin to limit the yaw to an acceptable value.
My fin only takes about a minute to rig, weighs little and probably produces an insignificant amount of drag.

IMO, the trick is to find a method that augments roll authority but is very simple, has few if any chances of malfunctioning,
adds little or no time to set-up/breakdown, and is easy to pre-flight. I can think of several ways to warp a wing, but finding a way that doesn't involve a bunch of pullies or levers and cable runs is the catch in all this. I love the simplicity I have now, and
don't want to sacrifice any more than is absolutely necessary.

It may not be possible to do. But I don't believe that.
I agree, I feel that there are simple ways yet unseen or envisioned by us as a community that could very well "change the game". I am interested in pursuing these and if it takes me a while...then so be it.

As for my rotating LE, I felt it had merit, but rigging it and controlling it were not considered as we never dug that deep into the idea...just had it and discussed effects as was done here.

If I could find a way to make it just as safe from malfunction as current designs, what can we gain from the adverse yaw it has the ability to create...and do you feel it would be beneficial enough to continue to give thought to?

PS - A tail fin could most def. be incorporated if the adverse yaw made it necessary... tail fins have allowed for some great leaps in control and I do not doubt they will be pivotal in our future attempt to "leap" forward.
Forever in the shadow of a true pilot, C.A. Laversa 1950 -
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ACLaversa
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Post by ACLaversa »

Vaughn why the posted article? Because of the whitehawks?
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Chris Bolfing
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Post by Chris Bolfing »

Interesting graph on high speed CL max and sweep. Anyone have info on low speed CL max and sweep and say 130 degrees vs. 150 degrees?

Chris
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Aeroexperiments
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graph

Post by Aeroexperiments »

Chris Bolfing wrote:Interesting graph on high speed CL max and sweep. Anyone have info on low speed CL max and sweep and say 130 degrees vs. 150 degrees?

Chris
The title of the graph is actually "effect of sweepback on low speed lift curve"; note the fall in the curves at the far right, showing that we are approaching or reaching the stall angle-of-attack. The header at the top saying "high-speed aerodynamics" is a reference to the chapter or section title; it's not surprising that someone writing from a military rather than a hang-gliding point of view would look at sweep mainly as a tool to deal with transonic drag rise rather than a tool to create pitch and yaw stability. Still the graph is completely relevant to how sweep contributes a dihedral-like roll torque in hang gliders, and why this is strongest at high angles-of-attack (low airspeeds)

But I can't refer you to anything more specific or quantitative as far as a specific sweep angle, etc. Would probably depend on other things too like aspect ratio, etc…

Steve S.