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? 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?)

Thanks, take care,

--Noel



Francisco De Almeida wrote:
Modern gliders are designed so that the tailplane produces a moderate =
amount of lift in slow flight.
In steady flight, the tailplane lift coefficient may range from, say =
+0.2 for thermalling to -0.15 at VNE.
This leaves enough margin for transients, and definitely favours an =
airfoil with positive camber.