text: In order to obtain maximum glide ratio, the 
  airplane must be operated at the angle of at- 
  tack and lift coefficient which provide maxi- 
  mum lift-drag ratio. The illustration of figure 
  6.6 depicts a variation of lift-drag ratio, L/D, 
  with lift coeficient, C,, for a typical airplane 
  in the clean and landing configurations. Note 
  for each configuration will occur 
  that (LID),,, 
  at a specihc value of lift coefficient and, hence, 
  a specific angle of attack. Thus, the maximum 
  glide performance of a given airplane configu- 
  ration will be unaffected by gross weight and 
  altitude when 
  is operated at 
  the airplane 
  (L/D),az. Of course, an exception occurs at 
  very high altitudes where compressibility ef- 
  fects may alter the aerodynamic characteristics. 
  The highest value of (L/D) will occur with the 
  airplane in the clean configuration. As the 
  airplane is changed to the landing configura- 
  tion, the added parasite drag reduces (L/D)_nz 
  and the C, which produces (L/D),, will be in- 
  creased. Thus, the best glide speed for the 
  landingconhguration generallywill be lessthan 
  the best glidespeed .for theclean configuration. 
  The power-off glide performance may be 
  appreciated also by the graph of rate of descent 
  versus velocity shown in figure 6.6. When a 
  straight line is drawn from the origin tangent 
  to the curve, a point is located which produces 
  to rate 
  the maximum proportion of velocity 
  of descent. Obviously, this condition provides 
  maximum glide ratio. Since the rate of descent 
  is proportional 
  the power required, the 
  points of tangency define the aerodynamic 
  condition of (L/D)m.z. 