"Eric Greenwell" wrote in message
.. .
In article ,

Since the wing is able to develop twice the gliders weight, the
elevator should also be able to develop twice the force it normally
does, shouldn't it? And this explanation would suggest the elevator is
unable to generate more than 2 g's, even in level flight, but we know
it can do that.

I believe the reason the elevator becomes less effective in circling
flight is due to the change in relative airflow between the wing and
the tail. Because the glider is turning partly in the pitch plane
(mostly in the pitch plane at 60 degrees bank), the airflow at the
tail meets the tailplane at a higher angle of attack than it does at
the wing. This higher angle of attack means more "up elevator" is
required to produce the same download. At the point the elevator
reaches it's stop, it is then producing less download than it can in
level flight, and is unable to force the wing to the stall AOA.

A quick glance at "Fundamentals of Sailplane Design" didn't find an
reference to it, but Frank Zaic described the effect 50 years ago for
model airplanes, using the term "circular airflow".
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Eric Greenwell
Richland, WA (USA)

Eric, that's an interesting observation. One should never forget the
effects of local flow around an aircraft. No doubt the effectiveness of the
elevator is effected by the downwash from the wing - even if it's a "T"
tail.

I would tend to think, however, that the wing producing 2x lift would have
an increased downwash which should increase the negative AOA of the
stabilizer thus opposing the nose-down effect of the CG being ahead of the
center of lift. This would tend to increase up-elevator authority. But
then these are difficult things to visualize.

What is clear is that the down (relative to the lateral axis) vector from
the CG is twice as large in a 60 degree bank as in level flight and that the
lift from the center of lift is also twice as large producing a large
nose-down moment that must be opposed by the stab/elevator. If the
up-elevator authority limit is reached before the wing stalls, the case
described by Derek is true.

Bill Daniels