On 14 Jul 2004 15:22:13 -0700, (Andy Durbin)
wrote:

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.

Hi Andy,

In my last posting I was thinking about what you described and decided
that you had been talking about wing twist due to wing bending.


I don't think that the scenario you describe can lead to a wing stall:
The cause that returns the wing to normal deflection is of course that
the pilot reduces AoA by pushing the stick forward. The instance the
pilot reduces g-load this way he also reduces his stall speed - I
doubt that it's possible to stall if the pilot was able to pull a
high-g pull-up only one second before without having a highspeed
stall. Of course the relative AoA-rise indeed occurs when the wing
tips are moving downwards, but the overall AoA is reduced a lot more
with the elevator (otherwise he woudn't lower the AoA enough to cause
the rapid unbending of the wing).

But it's an interesting theory anyway.


One more thought: At 40 kts a high g pullup in an ASW-20 is not
possible anymore - 40 kts is close to its stall speed.
Stall speed of a 20 is about 38 kts, so at 40 kts it wil be able to
generate only 1.02 g without stalling, therefore at 40 kts you simply
do not get any extraordinary wing bending.

For a pullup with 2 g you need at least 53 kts in an empty 20, and for
one of 3 g you need 66 kts. And from 66 kts even a 20 will gain
perhaps 100 ft. How high was that pilot when he started to spin? I'm
pretty sure it was a "standard" spin, caused by too little airspeed
and possibly wrong flap setting.


If the pull-up has been executed at higher speed and the pilot is
pushing the nose down to level flight after a straight climb (with 1g
and loss of speed), the only danger is an inverted stall of the wing
if he tries to push too many negative G's at low speed, but this is
independent of wing bending.



The effect that you describe is imho what causes the smooth ride in a
20: If the wing tips are accelerated upwards, they reduce their AoA,
dampening their movement, and vice versa.





Bye
Andreas