Choice of section

The choice of section shape depends partly on the range of CL values for which efficient low-drag cruising is required. A wide low-drag bucket will be required, if the aircraft is designed to fly efficiently for a large range of speed, weight and altitudes. For steady level flight, CL is equal to weight/(dynamic pressure x wing area), so it is the range of values of weight/dynamic pressure that matters. Note that the weight changes considerably during flight as the fuel is consumed.

The choice of section is also dependent on the maximum value of CL needed. This in turn depends on the weight, the wing area and the stalling speed that can be tolerated without flaps deployed.

The choice of section may be a lengthy iterative process, and at the end, the aerodynamic designer may well be told to go away and think again by the structural designer, who needs a thicker section, with plenty of depth at the rear to accommodate the flap mechanism. The characteristics of typical aerofoil sections are given in the Appendix, p. 361.