The way the center of pressure moves as lift coefficient changes makes it difficult to use the center of pressure in the mathematical analysis of longitudinal static stability of an aircraft . For this reason, it is much simpler to use the aerodynamic center when carrying out a mathematical analysis.
32.
The " aerodynamic center " is defined to be the point on the chord line of the airfoil at which the " pitching moment coefficient " does not vary with angle of attack, or at least does not vary significantly over the operating range of angle of attack of the airfoil.
33.
In wing designs with the spar sufficiently forward of the airfoil that the aerodynamic center is aft of the elastic axis of the wing, aeroelastic deformation causes the wing to move in a manner close to its ideal efficiency ( in which pitching angles lag plunging displacements by approximately 90 degrees . ) Flapping wings increase drag and are not as efficient as propeller-powered aircraft . Some designs achieve increased efficiency by applying more power on the down stroke than on the upstroke, as do most birds.
