Originally Posted by
Dick Whittingham
This is a bit complex, and I would like an expert contribution if possible.
A cylinder shape set at an angle of attack generates a nose up moment, and this increases with increasing apha, making it an unstable couple. The plan area of the engines, stub mountings and so on acts like an extra horizontal tail, which is a stabilizing effect.
I understand that the JAA answer is that on balance the total efect comes out stable
Dick
Actually, even the nacelle itself is stabilizing, because it is mounted aft, and regardless of any couple that may or may not be generated about its own axis, there is a lift force generated which is definitely behind the cg and definitely stabilizing.
You can get the same kind of "unstable couple" on an aerofoil shape with reference to itself, but I can't conceive of any aerofoil being net destabilizing if mounted aft of the cg.