Italia
I must have written my posts very poorly when you are explaining to me what I have just said.
I know well what the ac is and that is precisely why I say that it is the position of the cg with respect to it that determines de degree of stability of the airplane. By the way, I have read, reread and even lovingly caressed that blessed book for many years...
There are many factor affecting stability, but once the airplane has been designed, it is the cg location what will make it more or less stable. Not the angle of incidence of its tailplane.
The conditions for equilibrium are one thing and the conditions for stability are a different thing. The cp is where we consider that Lift is acting. The moment about the cg created by Lift is determined from the cp. Unlike the ac, cp position varies with CL, being zero at zero lift and about 25% at maximun CL.
BOAC You are right, It was a lapse, Drag acts above the CG, of course. Thanks for that, it took me a while to spot it.
Italia
Of course, if we talk about negative AoA, it decreases when the wing's positive AoA increases, but the effect is always in the same sense as the wing. Less negative lift has the same effect as more positive lift, right?
The taiplane is also referred to as the stabilizer for obvious reasons, but it is not due to the sense in which it develops Lift. It stabilizes because when the airplane increases its AoA, the nose up moment of the tailplane about the cg will decrease (or the nose down moment will increase). The effect is the same: opposing to the pitch up. All due to its being behind the CG. A canard plane will always be unstabilising, no matter if its lift is positive or negative.
More nose down or less nose up, the stabilizing effect is the same. If stability depended on the angle of incidence of the tailplane, it would vary every single time we moved an all moving slab or a trimmable horizontal stabilizer. The truth is that with that we would just change is the estate of equilibrium.
BOAC, sorry, I am a sweet water sailor. Inverted flight belongs to emergency, for me...
Last edited by Microburst2002; 13th May 2012 at 13:07.