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? 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.


Tilters do it with attitude. In a standard class glider you will need to hold up
elevator to achieve slow flight. AOA is controlled exclusively by elevator
pitching the entire aircraft.
On a flapped glider the angle of attack on the mainplane is controlled by
trailing edge flap deflection, which effectively changes the AOA of the
tailplane. There is one speed for each flap position where the elevator is
neutral. Often down under 45kt.
The relative centre of lift / CG position is a good question.