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Re: 50' span PRANDTL 4

Postby Frank Colver » Sun Mar 11, 2018 12:26 pm

I also never flew a single surface flex wing hang glider with a noticeable adverse yaw. When a sail billows more on the side the pilot's weight shifts to, it tightens on the other side, where adverse yaw would be initiated by increased drag. The tightening of the sail on the outboard wing should reduce the induced drag there and reduce the AY tendency.

My ridged wing "Skysail" probably would have had adverse yaw except by using tip drag rudders, for roll initiation, I would have been automatically overpowering it.

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Re: 50' span PRANDTL 4

Postby wingspan33 » Sun Mar 11, 2018 1:37 pm

Merlin wrote:Before buying my first Comet in the 80's I test flew two different 165's in longish soaring flights. Both handled very nicely. I bought a used one (which I had not flown) and found that it had significant and consistent adverse yaw. That is, I would initiate a turn, then the adverse yaw would happen, then the roll would start. So the yaw and roll felt decoupled. No doubt this occurs in a small way in most gliders, but this was dramatic. Actually it was the only glider I ever flew with any noticeable adverse yaw.


This is interesting. My best guess as to the difference between the 3 Comets was possibly that the one you bought had longer side wires. Comets were designed with loose side wires (to supposedly help with handling) so, without measuring them, the one you bought may have had a bit longer side wires (perhaps an earlier model?) . The possibly longer side wires translates into more dihedral. More dihedral means more adverse yaw.
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Re: 50' span PRANDTL 4

Postby wingspan33 » Sun Mar 11, 2018 1:49 pm

Frank Colver wrote:I also never flew a single surface flex wing hang glider with a noticeable adverse yaw. When a sail billows more on the side the pilot's weight shifts to, it tightens on the other side, where adverse yaw would be initiated by increased drag. The tightening of the sail on the outboard wing should reduce the induced drag there and reduce the AY tendency. . .

Frank


Frank,

That's my observation as well, even on double surface "comp" wings. Maybe that because I have no "feel" for what's happening with my wing. But after 43 years I think I would have noticed the consistent effect.

I will add that however the Prandtl 4 creates proverse yaw it seems obvious that real life testing has showed that it does. For such a high aspect ratio (rigid) wing that's pretty amazing. So I suspect that Mr. Bowers is correct in at least some, if not most, of his calculations.

I think it would be amazing to have the ability to have a modern full size flex wing tested dynamically (i.e. with an active pilot attached) in a sophisticated low speed wind tunnel. Might proverse yaw be found in evidence?
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Re: 50' span PRANDTL 4

Postby klingbergwing2 » Tue Mar 27, 2018 11:30 pm

Here is my two cents based on my engineering, piloting, and designing background.

Most flex wings have very little adverse yaw (but you can force it to happen dynamically). Flex wings and the Prandtl 4 use approximately the same span-wise lift distribution. The span-wise lift distribution is designed such that the tips are very lightly or even negatively loaded. Al Bowers presents this data in one of his papers that I would post here if I could figure out how to upload it!

The result of this type of lift distribution is that when the glider rolls, proverse yaw is generated.

Also, I can note that if a hang glider does show adverse yaw in a turn, it can be hard to detect because it might be just a few degrees and combined with the "odd" sight line from the pilot (no good reference points), it can be very hard to actually see the adverse yaw. If you were sitting in the middle of the wing sighting over the nose at the horizon, then it would be noticeable.

But, in my personal opinion, if there are no vertical surfaces (winglets, fuselage, etc) adverse yaw does not matter one wit. In a pure flying wing the performance does not vary significantly if the nose is not pointed along the velocity vector. In conventional aircraft adverse yaw is a big performance problem, but not so much on pure flying wings. For flying wings the "problem" is mostly one of pilot preference due to common training methods.

For a pure flying wing I say the following about adverse yaw, "Ignore it and it will go away soon enough" (little engineering joke from a pilot)
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