Expert: Paul Soderman - 1/10/2012

Question
Sir,
I read that the tail plane generally has a negative setting angle and that it produces downforce. But if it does, then how does it produce the restoring moment needed to achieve longitudinal static stability?
Also, Canards, produce positive lift and are at positive setting. If cg is far behind the wing and the canard, then wouldnt canard destabilize the aircraft than provide longitudinal stabiliy?

Answer
Sangeeth - A cambered airfoil will have  a nose-up pitching moment relative to the aerodynamic center (AC) due to the chordwise loading. The aerodynamic center is the point about which the pitching moment is independent of lift. An aircraft center of gravity (CG) aft of the AC will also generate a nose-up pitching moment. So a simple straight wing with aft CG will require a tail that lifts and generates a nose-down pitching moment to stabilize the aircraft. However, if the wing is swept and twisted it likely has a nose-down pitching moment because much of the lift is aft of the AC. The tail would have to generate negative lift  and a nose-up pitching moment. The same would be true if the CG was ahead of the AC. The thrust vector also provides a moment that can be positive or negative depending on engine location. So, the required tail loading for stability might be up or down depending on the aircraft design and loading.

The same is true for a  canard.  A swept wing aircraft would require a lifting canard for stability as long as the CG was not too far aft. We would like a lifting canard to share the load with the wing. But it should stall before the wing does so the aircraft doesn’t flip on its back during stall.
Paul