In article et,
says...

If it [stick shaking] happened while thermalling, this suggests it isn't the elevator
stalling. Here's why:

While circling, the elevator's angle of attack (AOA) is greater than
the wing's AOA, because of the differing airflow directions.

This greater AOA tends to increase the upward force on the elevator
(or reduce it's downward force), which is why it is more difficult to
stall a glider in a turn.

Or, if we think of the elevator as an "upside down" wing that is
producing lift downward (pushing the tail down), it's AOA is
_reduced_.

With a lower AOA, it's not going to stall in a turn if it can't do it
in straight ahead flight.

Question: with water, was the CG kept in the same place as without
water, or did it move forward?


Eric, I need to jump in here on JJ's side. I have experienced exactly what
he is describing and interpreted it the same way.

The horizontal tail (high or low mounted) is operating in the wings near
field flow or, in this case the wings downwash. Even if the incidence of
the wing and tail are the same, the tail will be at a larger negative angle
of attack, relative to its local flow, than the wing is at a positive angle
of attack. In the case of a "T" tail, the horizontal will not be in the
turbulent wake of the wing since that turbulence is embedded in the wings
downwash which tends to depart downwards and back from the wing.

Agreed, though I would say the elevator operates at a lower AOA than
the wing, due to the incidence and the local flow. I like to measure
the AOA the same way at both surfaces.

Since the low aspect ratio tail in not an efficient "wing", it must operate
at a larger negative AOA to produce sufficient downforce to balance a
forward CG.

Not agreed.

First, the tail can not control it's angle of attack independently of
wing, at least for the flapped elevators we are talking about on the
LS7. An all-moving elevator does change it's AOA, of course.

Second, the elevator is a flapped surface, and can vary its lift
coefficient, which is how it adjusts it's force, rather than AOA.

Third, the actual force is also dependent on the surface area, and the
torque it produces also depends on the tail boom length. The
efficiency of the surface is relevant only to the choice of tail boom
length, surface size, and airfoil. These are set by the designer.

In a thermaling turn, the negative AOA of the tail must be increased still
further to balance the centrifugal force acting on the CG while maintaining
a low AS.
It is not unreasonable to think that, at some point, the tail
will reach its negative stalling AOA while the wing is still below its
stalling AOA, resulting in the nose dropping and the AS increasing.
(Obviously, as the CG is moved aft, the need for downforce diminishes.)

It is unreasonable to think this. We all know that it is easier to
stall a glider in straight flight than in circling flight. Most
gliders can't be stalled after reaching a certain bank angle, commonly
30-40 degrees. The reason: reduced elevator authority because the
relative airflow is different at the wing and the tail.

In straight flight, the air hits the elevator at the same AOA that it
does at the wing. In circling flight, the air hits the elevator at a
greater AOA than it does at the wing. This greater AOA reduces the
down force available. This greater AOA (what you would call a lesser
negative AOA) means the elevator is further from stalling than in
straight flight.

JJ's "nervous" elevator is more likely to be the airflow separating and
re-attaching to the lower surface of the tail than an effect of the
turbulent wake of the wing.

This is still possible, but not because it _delays_ the stall of the
surface by keeping the flow attached better, but because makes it the
flow separate sooner.

If, as suggested, adding turbulator tape to the
underside of the horizontal tail allows it to develop greater downforce
before stalling, the wing can be brought to a greater AOA and perhaps a wing
stall.

Yes, IF...

snip

I must conclude that, for normal CG locations, the horizontal tail flies at
a negative AOA relative to its local flow

Agreed.

and that this negative AOA
increases as the airspeed diminishes.

Maybe, maybe not. In any case, the elevator must be deflected upwards
to increase the lift coefficient of the surface, as this AOA change
isn't sufficient.

Further, that the horizontal tail
negative AOA can, and often does, reach its stalling AOA at the minimum
sustainable airspeed while the wing flies just below its stalling AOA. This
condition produces very benign "stall" characteristics.

If this were true, it'd be amazing. For example, if it were the
elevator controlling the stall behavior, why is a glider so much
harder (more back stick required) to stall with a forward CG than a
rearward CG? In a forward CG, you use more back stick to get the nose
up, which is when the elevator should be most likely to stall. This
suggests the pitch angle (relative to the fuselage) of the forward CG
glider would be less at "stall" than with a rearward CG, yet is the
same.

The stall behavior of our gliders is controlled by the wing: airfoil
shape (often varying from root to tip), wing twist, winglets, and
planform. The benign behavior of the newer gliders (say, last 20
years) is a tribute to the airfoil designers ability to get improved
performance AND better stall behavior, coupled with the glider's
designers careful balancing of the other factors.

--
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directly

Eric Greenwell
Richland, WA (USA)