HarryC

My Feedback: (1)

Flaps do not increase wing area, unless they are specially designed to slide out from the trailing edge like most airliners. On models, full size light aircraft, most fighters, etc., flaps make no change at all to area.

Flaps work by increasing the wing's camber thereby increasing the lift co-efficient.

Camber is responsible for the nose down pitching that wings create. That is why symmetrical sections with no camber have no negative pitching, unless flaps are lowered of course. The greater the camber, the greater the negative pitch co-efficient. Applying the flaps makes every wing increase its nose down pitching. That is how tailless planes maintain pitch control. But for a plane with a tail there is the wing's downwash to consider. A high wing, low tail like a Cessna blasts the tail with a powerful downwash when it lowers flaps. If the increased downforce on the tail, with its leverage arm, is able to exert more force than the wing's increase in nose down pitch, the plane will pitch upwards. Where the tailplane is high enough to be little affected by downwash, then the increase in the wing's negative pitching wins the fight and the plane pitches nose down.

So a good rule of thumb is that high wing low tail models will pitch nose up when flap is applied, if the tail is level or above the wing then it will probably pitch nose down.

If the model pitches nose down when flaps are applied, it is pointless using elevator to flap mixing to help in violent 3d manouvres, since the flap is reducing the pitch rate and you get a faster pitch rate from elevator alone!

Harry