Elevator Turbulator tape question

"Eric Greenwell" wrote in message
.. .
In article ,
says...
Robert wrote
How do you know that your feeling and hearing comes from the stab
and not the wing ?

In the LS-8, I can feel it in the stick a good 5 knots above stall. I
don't
believe it is separated air coming from the wing, because the T-tail is
just
too high to allow that. I have felt the same thing about 3 knots above
stall in
a 301 libelle which has a low tail and it was *dirty air* coming from
the
wing.

BTW, I only felt the nervous stick with a load of water and I wasn't
trying to
stall the ship, It happened when I was thermalling.
JJ Sinclair

If it 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?

--
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directly

Eric Greenwell
Richland, WA (USA)

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.

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.

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.)

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. 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.

The counter argument that suggests that the horizontal tail is flying at a
positive angle of attack when the glider is flown near minimum airspeed must
assume that the pitching moment of the wing produces an nose-up pitching
moment that exceeds the nose-down moment of the CG acting ahead of the wings
center of lift - OR that the CG is placed aft of the center of lift. Both
of these conditions would produce serious static pitch instability which
would not pass JAR 22 certification standards.

I must conclude that, for normal CG locations, the horizontal tail flies at
a negative AOA relative to its local flow and that this negative AOA
increases as the airspeed diminishes. 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.

Bill Daniels