Bill Daniels wrote:
...
However, if the wings center of lift ever gets forward of the CG, you do not
have positive static stability, regardless of any other factor. In this
case any reduction in airspeed will require that the stick be moved forward
to counter increasing tail heaviness - clearly an unacceptable situation.
...

Not necessarily. With a reduction in airspeed there is an increase in AOA
of the wing, so necesarily an increase in AOA of the tail plane. If its
surface and lever arm are sufficient, this will override the increased
pitching (up) moment due to the forward move of the center of lift. This
is the condition for stability and it can clearly be met by a sufficient
high product of tail surface by lever arm. If this occurs, as the pitching
down moment due to the tail override the pitching up moment due to the wing,
in order to obtain this reduction in airspeed, you have to lower this tail
moment by moving the stick back. This is what happens on all stable
aircrafts.


...
Any glider I have ever flown (with the exception of some deliberate aft CG
flight tests) will have the stick positioned further aft at low speeds than
at high speeds indicating that the tail moves toward greater negative AOA as
the glider slows. (I've actually mounted protractors on the stick to prove
this to myself.) For any given trim setting, back pressure on the stick is
needed to reduce airspeed and forward pressure is needed to increase
airspeed - which is essentially the definition of static stability.


I completely agree that the stick moves aft when speed becomes lower and
vice-versa, but this doesn't imply that the tail has a greater negative AOA.
The change in AOA is due to both the stick action and the global change of
attitude of the aircraft relativeley to the airstream due to the need of
increasing the wing AOA. With the airfoils usually used in sailplanes, the
second effect overrides the first one. Again I agree these moves of the stick
associated with the changes in speed and AOA are essentially the definition
of static stability, but this doen't imply that, assuming that the position
of the CG is such that you start with a tailplane at negative AOA, slowing
the glider needs a more negative AOA. If this were true, this would mean that
in order to keep the wing at a higher AOA, you need a higher pitching up moment,
i.e. the wing alone tends to revert to its previous AOA, i.e. is stable by
itself. This is not the case with the cambered airfoils usually used in sailplanes.