At 22:36 14 November 2006, Noel.Wade wrote:
Ok, let me put on my 'total newbie' outfit here...

Thinking in terms of real-world situations: In slow
flight I'm sitting
in my glider, holding the stick back and keeping the
angle of attack
high. I'm pulling a large Cl out of my main wing.
The stick-back
condition corresponds to an upward-deflection of the
trailing edge of
the elevator. So am I not generating a negative Cl
with the horizontal
tail in this condition?

Not necessarily. Remember that the angle of attack
of the tailplane increases at the same time that the
angle of attack of the wing increases (in fact the
downwash from the wing affects slightly the alpha on
the tailplane but this can be disregarded for the present
discussion).

While the CL in steady flight for the wing ranges from
about +1.50 to +0.30, the CL range for the tailplane
is only about +0.20 to -0.15 (I am assuming an unflapped
glider, for flapped gliders it is even less). The CL
variation at the tailplane is thus only a fourth to
a fifth of the wing's CL variation.

In this light it is no longer surprising that the elevator
in steady flight is counterintuitively deflected to
cancel most of the lift (negative or positive) that
the tailplane would produce in response to the changes
in angle of attack.

This situation regarding elevator deflection vs lift,
and the slight inefficiency it entails, is the reason
why all-flying tailplanes were popular with designers
in the seventies, until they gave up due to the difficulty
in ensuring nice handling and stability.

Also, my wing airfoil still shows a Cm of
about -0.09 at this high angle of attack. Its small,
but definitely
negative - so I still have a nose-down pitching moment
from the wing -
therefore don't I *need* that 'negative lift' (i.e.
downward force) on
the tail? (I guess this all assumes the CG is ahead
of the wing's
center of pressure/center of lift - but isn't that
usually the case?)

No, for stability all that is required is that the
CG is ahead of the COMBINED centers of pressure of
wing + tail. For modern sailplanes, even the foremost
CG position is still behind the wing's center of pressure
(except maybe for some dedicated aerobatic types, I
don't know).

Now why do designers wish to have the tail producing
some upward lift at slow speed? Mainly because the
spanwise lift on the wing dips a little in the vicinity
of the fuselage. A bit of lift from the tailplane helps
to smooth out this irregularity and leaves a more efficient
wake behind the sailplane.

Spamcans like Cessnas do have tails producing downward
force all the time. Gliders cannot allow themselves
such wastefulness!

Regarding the shape of the elevator itself, Udo already
wrote everything there is to say.

Good questions Noel.