Force and moment coefficients

The non-dimensional quantity FI(pV2S) (c. f. Eqn 1.43) (where F is an aerodynamic force and S is an area) is similar to the type often developed and used in aerody­namics. It is not, however, used in precisely this form. In place of pV2 it is conven­tional for incompressible flow to use pV2, the dynamic pressure of the free-stream flow. The actual physical area of the body, such as the planform area of the wing, or the maximum cross-sectional area of a fuselage is usually used for S. Thus aero­dynamic force coefficient is usually defined as follows:

The two most important force coefficients are the lift and drag coefficients, defined by:

lift coefficient Cl = lift/pV2S (1.44a)

drag coefficient Co = dragjpV2S (1.44b)

When the body in question is a wing the area S is almost invariably the planform area as defined in Section 1.3.1. For the drag of a body such as a fuselage, sphere or cylinder the area S is usually the projected frontal area, the maximum cross-sectional area or the (volume)2/3. The area used for definition of the lift and drag coefficients of
such a body is thus seen to be variable from case to case, and therefore needs to be stated for each case.

The impression is sometimes formed that lift and drag coefficients cannot exceed unity. This is not true; with modern developments some wings can produce lift coefficients based on their plan-area of 10 or more.

Aerodynamic moments also can be expressed in the form of non-dimensional coefficients. Since a moment is the product of a force and a length it follows that a non-dimensional form for a moment is QjpV2Sl, where Q is any aerodynamic moment and l is a reference length. Here again it is conventional to replace pV2 by ini/2 in the case of the pitching moment of a wing the area is the plan-area S and the