text: 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 coe&ient-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 character- 
  istics can be obtained by moving the points of 
  maximum camber and thickness aft on the 
  chord. This would distribute the pressure and 
  the chord and 
  velocity more evenly along 
  produce a lower peak velocity 
  for the same 
  lift coefficient. Fortunately, the airfoil shape 
  flow and low 
  to provide extensive lamiaar 
  profile drag in low speed, subsonic flight will 
  provide a pressure distribution which is favor- 
  Figure 3.12 
  able 
  and 
  illustrates 
  variation of critical Mach number with 
  lift 
  coefficient for a conventional low speed airfoil 
  and a high speed section. 