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.  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

-- 
Andrew J. Ross                NextBus Information Systems
Senior Software Engineer      Emeryville, CA
[EMAIL PROTECTED]              http://www.nextbus.com
"Men go crazy in conflagrations.  They only get better one by one."
 - Sting (misquoted)


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