On Mon, 2003-01-06 at 15:26, Andy Ross wrote: > David Megginson wrote: > > I found it easier simply to picture different 2D sections of the > > wing stalling at different times, but I can see how your explanation > > might lead to a programmatic solution faster. > > Right, but what is it about different secions of the wing stalling at > different times that causes the aircraft to snap roll? That's the > part that was non-obvious to me. You get the snap roll effect even > when there is no dihedral or sweep, in situations where the wing > "should" be stalling symettrically. > > It happens because when the aircraft is rolling (even a little bit), > one wing is going down and the other up. This causes a difference in > AoA at the tips. If the wing tips are stalled, then the wing going > down is pushed farther into the stall, while the one going up sees a > lift recovery. So the net effect is that the aircraft tries to roll > *into* the existing roll direction. That's an unstable situation > (like a ball sitting at the top of a hill), and it will diverge if the > roll rate has any non-zero value. > > The more common explanation for washout (that the ailerons are > "masked" by the tip stall) is incorrect.
Hmm, I think that depends mostly on how the flow separation progresses forward on outboad part of the wing. If it's very sudden, then you are probably right. If not, then preservation of roll control could well be the reason. > The snap roll doesn't happen > because of aileron authority, it happens because the aircraft is > literally unstable in roll. Aileron authority can obviously help to > recover, but the tendency to roll off into a divergent snap roll is > the real effect, and it is a different behavior from the normal roll > stability of the aircraft in typical conditions. > > > Isn't the snap roll usually uncoordinated? I've never done aerobatics > > myself. If it is, then I wonder what the role of the uncoordination > > is. > > If there is any wing sweep or dihedral, then a non-zero yaw angle also > changes the relative AoA of the wings to produce the same effect. If > you apply full rudder on such an aircraft, then you don't need to pull > up the nose as far to get the "down" wing tip stalled. > > There might be a yaw rate effect too. When the aircraft is yawing, > the wingtip going "back" also sees a higher AoA and will drop if it is > past the stall. Same deal. The snap roll needs a stalled "down" > wingtip to get the divergence, in any case. > > Andy -- Tony Peden [EMAIL PROTECTED] We all know Linux is great ... it does infinite loops in 5 seconds. -- attributed to Linus Torvalds _______________________________________________ Flightgear-devel mailing list [EMAIL PROTECTED] http://mail.flightgear.org/mailman/listinfo/flightgear-devel
