text: sections have zero lift at zero angle of attack, 
  the sections with positive camber have nega- 
  tive angles for zero lift. 
  
  attack would produce an approximate 0.5 
  change in lift coefficient. Evidently, lift,~curve 
  slope is not a factor important in the selection 
  of an airfoil. 
  
  The importance of maximum lift coefficient 
  is obvious. 
  If the maximum lift coefficient is 
  high, the stall speed will be low. However, 
  the high 
  thickness and camber necessary for 
  high section maximum lift coefficients may 
  produce low critical Mach numbers and large 
  twisting moments at high speed. In other 
  words, a high maximum lift coefficient is just 
  one of the many features desired of an airfoil 
  section. 
  
  lift property affected by the 
  airfoil shape is the section maximum lift co- 
  The effect of airfoil shape on 
  efficient, ci-. 
  ci- can be appreciated by comparison of the 
  lift curves for the five airfoils of figure 1.12. 
  The NACA airfoils 63X06,63-009, and 63i-012 
  ate symmetrical sections of a basic thickness 
  distribution but maximum thicknesses of 6, 
  9, and 12 percent respectively. The effect of 
  thickness on ~1% is obvious from an inspec- 
  tion of these curves : 
  
  An important 