text: (1) An increase in the section drag coeffi- 
  
  cient for a given section lift coe5cient. 
  
  (2) A decrease in section lift coefficient 
  
  for a given section angle of attack. 
  
  (3) A change in section pitching moment 
  
  coe5cient. 
  
  A reference point is usually taken by a plot 
  of drag coe5cient versus Mach number for 
  a constant lift coefficient. Such a graph is 
  figure 3.10. The Mach number 
  shown in 
  which produces a sharp change in the drag 
  coe5cient 
  is termed the “force divergence” 
  Mach number and, for most airfoils, usually 
  exceeds the critical Mach number at least 5 
  to 10 percent. This condition is also referred 
  to as the “drag divergence” or “drag rise.” 