Net lift produced by the airfoil is the difference between lift on the upper surface and the force on the lower surface. Since the two vectors are not located at the same point along the chord line, a twisting force is exerted about the center of pressure. Center of pressure also moves along the chord line when angle of attack changes, because the two vectors are separated. This characteristic of nonsymmetrical airfoils results in undesirable control forces that must be compensated for if the airfoil is used in rotary wing applications. When the angle of attack is increased to develop positive lift, the vectors remain essentially opposite each other and the twisting force is not exerted.
Center of pressure remains relatively constant even when angle of attack is changed.
This is a desirable characteristic for a rotor blade, because it changes angle of attack constantly during each revolution.