text: Parts (c) and (d) of figure 3.8 show the 
  wave pattern and resulting pressure distribu- 
  tion for a double wedge airfoil at zero lift. 
  The airstream moving over the surface passes 
  through an oblique shock, an expansion wave, 
  and another oblique shock. The resulting 
  pressure distribution on the surfaces produces 
  no net lift, but the increased pressure on the 
  the de- 
  forward half of the chord along with 
  creased pressure on the aft half of the chord 
  produces a “wave” 
  drag. This wave drag is 
  caused by the components of pressure forces 
  which are parallel to the free scream direction. 
  The wave drag is in addition to the drag due 
  to friction, separatien, lift, etc., and can be 
  a very considerable part of the total drag at 
  high supersonic speeds. 
  
  In order to appreciate the effect of these 
  various wave forms on the aerodynamic char- 
  acteristics in supersonic flow, inspect figure 3.8. 
  Parts (a) and (b) show the wave pattern and 
  resulting pressure distribution 
  for a thin flat 
  plate at a positive angle of attack. The air- 
  stream moving over the upper surface passes 
  through an expansion wave at the leading edge 
  and then an oblique shock wave at the trailing 
  edge. Thus, a uniform suction pressure exists 
  over the upper surface. The airstream moving 
  underneath the flat plate passes through an 
  oblique shock wave at the leading edge then an 
  expansion wave at the trailing edge. This pro- 
  duces a uniform positive pressure on the under- 
  side of the section. This distribution of pres- 
  sure on the surface will produce a net lift and 
  incur a subsequent drag due co lift from the in- 
  clination of the resultant lift 
  from a perpen- 
  dicular co the free stream.  Parts (e) and (f) of figure 3.8 illustrate the 
  wave pattern and resulting pressure distribu- 
  tion for the double wedge airfoil at a small 
  positive angle of attack. The net pressure 