Glider Tail Stall

On Feb 20, 9:00*am, jcarlyle wrote:
Recent threads highlighted tail stalls in powered aircraft
experiencing icing. The thing that concerned me was the recovery being
exactly opposite to the wing stall that we all practice and
demonstrate, and thus have ingrained as almost automatic.

Why would the recovery be different? The tail is an inverted wing
producing a down force. You stall it by pulling back on the stick
increasing its AOA until it stalls. Releasing the back pressure
initiates a recovery - same as a wing stall.

It also
sounded like the thing that distinguished a tail stall from a wing
stall was buffet in the controls rather than in the airframe. This
distinction is pretty subtle to me, and in the heat of the moment I
wonder if I would apply the proper recovery.

It doesn't really matter. With many trainers, the buffet students are
taught to recognize as wing stall is, in fact, tail stall with a
little bit of turbulence from wing root flow separation thrown in.
Allowing the tail to stall limits up elevator authority so the wing
can never get into a full stall. Cessna 152's and SGS 2-33's are
examples.

There's a simple test for this. With the stick full back and the
glider exhibiting pre-stall buffet, apply aileron and if the glider
responds normally in roll, the wing wasn't stalled. If the wing was
stalled, the glider would probably try to spin with the application of
aileron.

Does anyone know, for a modern 40:1 glider, how violent a tail stall pitch up would be?

If the tail stalls, and the CG is within limits, the glider will pitch
nose down, not nose up, and this will help effect the recovery. If
anything, modern gliders are even more benign than older designs.

Also, if a glider has a totatally benign wing stall,
eg, non-violent wing stall break, would this imply that a tail stall would also be non-violent?

Tail stall just runs out of up elevator authority. With one
exception, tail stall is benign.

This is the exception. When the nose up moment is being produced by
something other than the elevator, the stick will be forward as the
pilot tries to limit the pitch up. In this case, the tail is
producing an up force and if the it stalls, the nose will rise further
risking a wing stall.

Two things can produce this situation. One is an aft CG and the other
is a poorly located CG hook used on a winch launch. Slab type all
moving 'stabilators' are more susceptible to this than fixed stab/
hinged elevator type tails. The fix is to be very aware of your CG
and to winch these gliders carefully.