However, if high speed flight is the primary consideration, the airfoil must be chosen to have. the highest practical critical Mach number.
Critical Mach number has been defined as the flight Mach number which produces first evidence of local sonic flow. Thus, the air- foil shape and lift coefficient which determine the pressure and velocity distribution-will
have a profound effect on critical Mach number. Conventional, low speed airfoil shapes have relatively poor compressibility characteristics because of the high local velocities near the leading edge. These high local velocities are inevitable if both the maximum thickness and camber are well forward on the chord. An improvement of the compressibility characteristics can be obtained by moving the points of maximum camber and thickness aft on the chord. This would distribute the pressure and velocity more evenly along the chord and produce a lower peak velocity for the same lift coefficient. Fortunately, the airfoil shape
to provide extensive laminar flow and low profile drag in low speed, subsonic flight will provide a pressure distribution which is favorable for high speed flight. Figure 3.12 illustrates the pressure distributions and variation of critical Mach number with lift coefficient for a conventional low speed airfoil and a high speed section.
In order to obtain a high critical Mach number from an airfoil at some low lift coefficient the section must have:
(u) Low thickness ratio. The point of maximum thickness should be aft to smooth the pressure distribution.
(6) Low camber. The mean camber line should be shaped to help minimize the local velocity peaks.
In addition, the higher the required lift coefficient the lower the critical Mach number and more camber is required of the airfoil. If supersonic flight is a possibility the thick- ness ratio and leading edge radius must be small to decrease wave drag.
