Sep 12, 2013, 04:08 PM
Registered User
Germany, BW, Stuttgart
Joined Mar 2012
1,049 Posts
Quote:
 Originally Posted by aeronaut999 I think I was not rolling the aircraft in a manner that changed the angle-of-attack
I think that would be really hard to gauge. As you roll, chances are good the angle of attack is changing, the sideslip angle is changing, the lift is changing and the side force is changing. Not only that, but the lift vector is now tilted to the side as gravity continues to pull straight down. To keep the glider in position as you roll, you have to be modulating not only the up and down forces, but the lateral forces as well. The human body is notably poor at estimating the force it exerts... . I think you would need some direct indication of angle of attack to really know how much it is or is not changing due to the roll.

Quote:
 Originally Posted by aeronaut999 II think that rolling around a "constant pitch attitude", i.e. a barbecue skewer, would change the angle of attack of each wing in the SAME direction, not in opposite directions. Even in a crosswind situation.
As you stated in post #22, the glider has negative effective dihedral at low angles of attack, and positive effective dihedral at high angles of attack. So even if the roll changes the angle of attack of each wing the same amount, it can still cause a change in sign of the effective dihedral.
 Sep 12, 2013, 06:05 PM Registered User The Willamette Valley, Oregon Joined Dec 2008 1,299 Posts Re the last part of your last post above Shoe-- yes I had that realization just now even before logging back on and reading your post. It seems fair to say that if that's all that is going on, it would never be possible to balance the glider in roll just by banking, during this crosswind experiment, in the specific case where the glider is allowed to be at the trim angle-of-attack. Because at the trim angle-of-attack, the effective dihedral is positive, so the glider wants to roll in the downwind direction. Raising the downwind wing while making no pitch control force, should cause the nose to pitch up until the wing is again at the trim angle of attack. The downwind roll torque should remain. I'm sure that there were some demonstrations where I balanced the glider in roll by raising the downwind wing, while letting the glider fly at trim, pitch-wise. An added note: in these crosswind experiments, I relieved myself of the need to apply a yaw torque on the glider by positioning the glider so a vertical post in the ground prevented the rear end from yawing into alignment with the wind. Due to the effect you've been describing, in all the tests where I let the glider be at trim in pitch, or exerted a fixed pitch torque, the video from the rear camera ought to show the nose lowering when I raised the upwind wing, and show the nose rising when I raised the downwind wing. I'll see if I can observe that. There are many variables at play here and I agree it's difficult to be sure the pilot is exerting a constant pitch force. Therefore I think the tests where the glider was allowed to be at trim in pitch (no pitch force exerted by the pilot) give the strongest suggestion that the wind gradient was playing a role here, rather than the results being solely attributable to the effect you've described. Also of course the fact that when I had the nose pointed straight into the wind, it still was the case that raising one wing created a roll torque toward the low wingtip, making the high wingtip keep rising and the low wingtip keep descending. Here it's clear that anhedral / dihedral was playing no role, so changing the angle-of-attack shouldn't change the balance of roll torques. Thanks for giving me some interesting things to think about! PS thinking through a bit more-- While holding a constant pitch attitude: When the airflow has no crosswind (slip) component, banking the wing decreases the aoa, as long as the pitch attitude is at least a little bit nose-high from horizontal. When the flow has a strong cross component and the pitch attitude is near level, banking upwind decreases the aoa and banking downwind increases the aoa. When the flow has a weak cross component and the pitch attitude is high, banking upwind certainly decreases the aoa, but banking downwind might also decrease the aoa. Certainly it would if the bank angle is high enough. Is this dependent on bank angle, or solely on pitch attitude and slip angle? I'm not sure. It seems there is a grey area, in terms of slip angle and pitch attitude, where the result of banking downwind is difficult to predict. But in my tests the crosswind angle or slip angle was quite high. The pitch attitude was high in the tests where I balanced the glider by banking upwind, but low (near horizontal) in some of the tests where I balanced the glider by banking downwind. The more I think through the tests, the more it becomes clear to me that the changes in bank angle didn't produce strong changes in angle-of-attack, at least nothing on the same scale as my intentional pitch inputs. For example, when the nose was low because I was maintaining a nose-down pitch input, the glider pulled up on the hang strap with less force than when the nose was high because I was letting the glider fly at trim, regardless of whether or not I raised the downwind wing to balance the glider in roll. I don't see how the balancing could have been primarily due to a change in angle-of-attack induced by the bank. But it's definitely something that I need to be aware of in relation to this sort of experiment. I'll be keeping my eye out for any evidence of this sort of effect as I review the videos from the experiments. Steve Last edited by aeronaut999; Sep 13, 2013 at 04:22 PM.