Andreas Maurer wrote in message
Flexible wings do NOT change their AoA while they are bending -
otherwise flutter would start immediately.


Bye
Andreas

Hi Andreas,

Wing flex must not result in twisting and wing flex does not change
the angle of incidence of any part of the wing. I don’t think
this means that wing flex does not change angle of attack.

Assume a glider is static on the ground and has the tail raised so
that the mean chord is horizontal. Now flex the wings upwards and
release them. The wings move downward through the air. The relative
air motion is at 90 deg to the mean cord so the angle of attack at the
tips is approximately 90 degrees while the wings unflex.

Now assume a flight condition that resulted from a high g pull up that
approached stall speed. I’ll assume the speed is 40kts, that
the wing tips flexed up 6 feet, and that as the pilot pushes forward
to avoid stall the wings return to normal deflection in 1 second. The
wing tip angle of attack change due to the downward motion can be
calculated from the forward speed of 40kts = 67.5 ft per second, and
the downward speed of 6 ft per second. Inverse tan of 6 / 67.5 is 5
so the tip angle of attack was increased by 5 degrees as the wings
unflexed. The effect reduces to zero at the root where there is no
deflection.

If the numbers are valid then it remains to be decided if wing flex
induced angle of attack changes of this magnitude would have an effect
on stall and stall recovery characteristics. I expect that they
would. Others disagree with me.

Note to other posters - I didn't say this *caused* the accident. I
said I believed it was a contributing factor.


Andy