text: directions. Evidence of this “pressure warn- 
  typical subsonic flow 
  ing’ ’ 
  pattern of figure 3.1 where there is upwash 
  and flow direction change well ahead of the 
  leading edge. 
  the object is travelling at 
  some ,speed above the speed of sound the air- 
  flow ahead of the object will not be influenced 
  by the pressure field on the object since pres- 
  -sure disturbances cannot. be propagated ahead 
  of the object. Thus, as the flight speed nears 
  the speed of sound a compression wave will 
  form at the leading edge and all changes in 
  velocity and pressure will 
  take place quite 
  sharply and suddenly. The airflow, ahead of 
  the object is not influenced until 
  the air par- 
  ticles are suddenly forced out .of the way by 
  the concentrated pressure wave set up by the 
  object. Evidence of this phenomenon is seen 
  in 
  typical supersonic flow pattern of 
  the 
  figure 3.1. 
  
  is seeii in  the 
  
  A factor of great importance in the study of 
  high speed airflow 
  is the speed of sound. 
  The speed of sound is the rate at which small 
  pressure disturbances will 
  be propagated 
  through 
  the air and this propagation speed 
  is solely a function of air temperature. The 
  the variation 
  accompanying table illustrates 
  of 
  the standard 
  in 
  the speed of sound 
  atmosphere. 