"Robert Ehrlich" wrote in message
...
Bill Daniels wrote:
...
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.
...


Here is the point where I think there is a confusion. What do you call
"center
of lift" ? The pitch stability needs only that the neutral point is behind
the
CG, the neutral point is the location of the increment of lift provided by
all surfaces (wing and tail plane) when there is an increment in AOA. If
you
call "center of lift" the point where you can reduce the lifting forces on
the
wing only to a single vector (this is implied by what you say concerning
the pitching moment created by weight and lift) this is a different point
which is
moving forward when the AOA increases and may be ahead of the CG at high
AOA, while the neutral point is always behind it. The confusion is both
betweeen an incremental force and its actual value and by the force
provided
by the wing only and by wing + tail plane.

You can approach the issue with the center of lift of the wing only and
address the tail separately or deal with the aircraft as a whole and talk
about neutral point. Both avenues will arrive at the same conclusions if
done properly. Perhaps it's my very obsolete training, but I still prefer
to deal with each surface separately.

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.

I agree that designers want to reduce trim drag to the minimum by trying to
make the tail fly at a zero angle of attack. To do this, the CG must be
moved aft and static stability sacrificed so there are limits to this
approach. I would disagree that this should occur at L/D max. Gliders
spend little time at L/D max and the best overall contribution to
performance would be to have the tail at zero AOA at high speed. In fact,
most gliders will exhibit lower pitch stability at high speed and some will
even show a tendency to "tuck" (nose down) if the stick is released,
indicating that the neutral point in the static stability curve occurs at
high speed and suggesting that the tail is near a zero AOA.

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.

Bill Daniels