Drag coefficient

As with lift, it is convenient to refer to a drag coefficient CD defined, in a sim­ilar way to lift coefficient, by

Drag = Dynamic pressure x wing area x CD or D = pV2 x S x CD

where S is the wing plan area.

For an aircraft, a major contribution to the overall drag comes from the wing, and is largely dependent on the plan area. We therefore wish to find ways of minimising the drag for a given wing plan area, and it is sensible to relate CD to the plan area, as in the expression above. Note, however, that for cars, CD is based on the frontal area. Drag coefficient values for cars cannot, therefore, be compared directly with values for aircraft. The drag coefficient of missiles is also normally based on the body frontal area.

The wing drag coefficient depends on the angle of attack, the Reynolds num­ber (air density x speed x mean wing chord/viscosity coefficient), and on the Mach number (speed/speed of sound). For many shapes, the dependence of CD on Reynolds number is weak over a wide range, and for simple estimations, the dependence on Reynolds number is often ignored. For speeds up to about half the speed of sound, the variation with Mach number is normally negligible, and so, for early low speed aircraft, it was customary to treat CD as being dependent only on the angle of attack and geometric shape of the aircraft. However, as we described in the last chapter, ignoring the effects of Reynolds number can lead to serious errors. For high speed aircraft, the effect of Mach number becomes extremely important.