Leading edge flaps are less efficient than trailing edge flaps. Efficiency here means the ratio of change of lift to change in flap angle.

With every flap deflection, both camber and angle of attack of a surface change. While, for example, a nose-down deflection **de**creases angle of attack while **in**creasing camber such that both effects work against each other, the changes in angle of attack and camber of a trailing edge flap work in the same direction.

Also, a leading edge flap needs to be connected to the wing at its rear end which is a poor choice for a hinge line: The added forces due to deflection will drive the flap to larger deflections. This means it is unstable, always attempting to run into the stops at maximum deflection. To keep the flap under control will require heavy actuators. At the trailing edge the hinge line is ahead of the added lift forces and this will drive the flap to a neutral position regarding tail surfaces while the interconnection of ailerons will keep both sides equally loaded. Trailing edge flaps are inherently stable.

Leading edge devices are only helpful to increase the maximum angle of attack: While the whole airplane will adjust angle of attack to compensate, the flap will reduce the suction peak at the leading edge so flow separation is delayed.