text: Consider a conventional airfoil shape as 
  is at a 
  If 
  shown in figure 3.9. 
  flight Mach number of 0.50 and a slight posi- 
  tive angle of attack, 
  local 
  velocity on the surface will be greater than 
  less than 
  the flight speed but most likely 
  sonic speed. Assume that an 
  increase in 
  flight Mach number to 0.72 would produce 
  lfrst cvidmc of local son@ flow. This condition 
  of flight would be the highest flight speed 
  possible without supersonic flow and would 
  Thus, 
  be termed the “critical Mach number.” 
  critical Mach number is the bouodary between 
  subsonic and transonic flight and is an im- 
  portant ~point of reference for all compressi- 1 
  bility effects encountered in transonic flight. 
  By delinition, critical Mach number is the 
  “free stream Mach number which produces 
  Therefore, 
  6rst evidence of local sonic flow.” 
  shock waves, buffet, airflow separation, etc., 
  take place above critical Mach number. 
  
  this airfoil 
  
  (3) In supersonic flight, the zero lift drag 
  of an airfoil of some finite thickness will 
  include a “wave drag.” 
  The thickness of 
  the airfoil will have an extremely powerful 
  effect on this wave drag since the wave drag 
  varies as the square of the thickness ratio- 
  if the thickness is reduced 50 percent, the 
  wave drag is reduced 73 percent. The lead- 
  ing edges of supersonic shapes must be sharp 
  or the wave formed at the leading edge will 
  be a strong detached shock wave. 