At first glance I would want some reassurance about
the the spinning characteristics and recoverability
of a weight shift pitch control glider with a conventional
wing section! Also I wouldn't fancy flying it inverted.
I think you are proposing a highly unstable and unworkable
configuration i.e. a deathtrap.

The longitudinal control would work differently at
different pitch angles. In a vertical climb or dive
there would be no 'elevator' control. In a steep dive
spin recovery the conventional wing section would bunt
uncontrollably past the vertical. The weight in the
tail would then reverse its effect and increase the
negative g loads. I think it would be so violent that
the glider wings would depart immediately but even
if they didn't you would be in a non recoverable position
just waiting to exceed VNE sufficiently for the wings
to come off a second or two later.

In fact even trying to fly in a fairly level attitude
would be extremely difficult as any forward pitching
would diminish the effect of the weight 'elevator/tailplane'
so that you would then have to send it back past the
postion of stable level flight just to stop the nose
continuing to drop. Then there's the difference in
control forces that would be needed to shift the weight
up and down the tailboom at different pitch attitudes.

Then there's high speed cruising which would increase
the pitching effect of the wing so that the weight
would have to go further and further back as the speed
increased (even neglecting pitch change effects as
above) so the faster you go the futher back the stick.


Conventional glider wing sections are unstable in pitch
and the advantage of an aerodynamic pitch trim and
control device is that it increases its effect with
airspeed and works in all attitudes. If you want to
get rid of the tail you need to use a flying wing chord
section or to use thrust vectoring (maybe that could
be a use for the jet on the other thread but it wouldn't
be a glider)

John Galloway

At 22:24 11 January 2004, Mark James Boyd wrote:
There are some designs which have no horizontal stab:
flying wings
for example. There are also canard setups (the speed
canard, for example).

For a sailplane, I was thinking about how one might
design away
the typical T-tail stabilizer and elevator.

First of all, how much dynamic stability does the horiz.
stab
contribute? If it were eliminated by design, would
it
be absolutely necessary to compensate by using a swept
wing (either forward or backward)? When deflected,
how much torque does an elevator provide?

I'm considering these factors, because eliminating
the
elevator and stab would reduce drag. From there, one
could potentially design a ducted surface, or use moveable
weights in the tail to change C.G and therefore pitch.

In the first case (ducting), there are commonly used
NACA ducts (they look like little triangles on
power planes) that are commonly used as air vents on
power planes. They have the advantage of producing
minimal
drag when the vent is closed. On a glider, they could
be
used in the tail to direct airflow and produce pitching
moments. There is a tail-rotor free turbine helicopter
which uses ducted bleed-air, I believe, to control
yaw this way.

The other option, which is more elegant, is to use
a moveable
weight in the tail for pitch. Move the weight forward
to
pitch down, backward to pitch up. One difficulty is
if the weight must be quite heavy, or the stick movement
needed to move it is too heavy. I suppose this in
some
part is a function of the length of the tailboom.
Another
complication is that a regular elevator is more effective
at
high airspeed, and less effective at low airspeed (more
deflection is required for the same torque). This
isn't
necessarily true with a weight-shift pitch control.

Hmmm...anyone have data about forces provided by the
elevator is flight? Drag caused by the elevator/ vert.
stabilizer
in level flight? How about torque produced by weight
shift
near the arm of the elevator?

I suppose the best way to experiment with this is in
a
model glider first, then in a full scale glider with
BOTH
pitch systems (elev/stab, AND weight shift). Then
finally
with the elev/stab removed.