JJ Sinclair wrote:

Kirk,
I agree with you that competent, current pilots, don't inadvertently stall any
ship. But your statement that you like a crisp stall, got me wondering. How do
you like a crisp stall, in the pattern, after some hard maneuvering to avoid a
mid-air? How do you like a crisp stall, when on the rocks, and get hit with a
large tail-gust?

The stall I got in my LS-7, after adding zig-zag to the horizontal stab, was
the worst stall I have ever seen in any sailplane. I mean she went near
vertical on me. Don't care to duplicate that in the above situations.

PS, I suspect the zig-zag delayed the *normal* seperation on the stab, but when
it did seperate, I lost all the down force from the horizontal stab and that
gave me the near vertical pitching moment. The wing may not have stalled at
all. Now someone please feel free to tell me just how full of Ka-ka, I am about
aerodynamics.
:)
JJ Sinclair

I am not an expert in aerodynamics, but I don't agree with your above interpretation.
It implies that just below the stall your elevator was producing a down force,
near the maximum possible or rather the (negative) lift coefficient was near its maximum
possible value and this maximum value was reached when you lost control, before
the wing reached its maximum (positive) lift coefficient. It seems to me that it
is an error to believe that more down force on the tail plane is needed for obtaining
an higher nose up attitude. As airfoils commonly used in sailplanes are unstable,
more nose up attitudes tend to self amplify when you consider only the forces
on the wing and the tail plane has to counter this by a lower down force (or a higher
up force, depending on position of the CG). This is not in contradiction with the
fact that you need aft stick in order to keep a higher nose up attitude.
This change of attitude also change the angle of attack on the tail plane by an
amount exceeding the variation of force needed and the back stick has to compensate for this.
For these reasons I think that when you come closer to the stalling angle of attack
for the wing, the tailplane on the contrary is far from its stalling angle of attack.
When the wing reaches its stalling angle, a further increase in angle of attack
will lower the lift coefficient, thus increasing again the angle of attack due
to the glider beginning to fall and so on. This also increases the angle of attack on the
tailplane which produces an upward force, despite the full aft stick, and this
produces the pitching moment that makes the nose drop. It would be logic to think
that in this case, the increased efficiency of the the tail plane due to the added
turbulator increases the pitching moment and brings the glider more quickly
closer to a vertical dive.