NAVWEPS W-ROT-80  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS  OF FLYING  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  that (LID),,,  for each configuration will occur  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 the airplane is operated at  (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.  speed will not cause any significant reduction  of glide ratio. This is fortunate and allows the  specifying of convenient glide speeds which  will be appropriate for a range of gross weights  at which power-off gliding may be encoun-  tered, e.g., small quantities of fuel remaining.  An attempt to stretch a glide by flying at  speeds  above or below the best glide speed  will  prove futile.  As shown by the illustration of  figure 6.6, any C, above or below the optimum  will  produce a lift-drag ratio less than the  maximum.  If the airplane angle of attack is  increased  above the value for (L/D),,,  a tran-  sient reduction in rate of descent  will take place  but this process  must be reserved  for the land-  ing phase. Eventually, the steady-state  condi-  tions would be achieved and the increased  angle of attack would incur a lower airspeed  and a reduction in (L/D) and glide ratio.  The effect  of gross  weight on glide performance  may be difficult to appreciate. Since (L/D)-  of a given airplane configuration will occur at a  specific value of C,, the gross weight of the air-  plane will not affect the glide ratio if the air-  plane is operated at the optimum C,. Thus,  two airplanes of identical aerodynamic con-  figuration but different gross weight could  glide the same distance from the same altitude.  Of course, this fact would be true only if both  airplanes are flown at the specific  C, to produce  (L/D),,.  The principal difference would be  that the heavier airplane must fly at a higher  airspeed to support the greater weight at the  optimum C,. In addition, the heavier airplane  flying at the greater speed  along the same flight  path would develop a greater rate of descent.  The relationship which exists between gross  weight and velocity for a particular C, is as  follows:  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  the maximum proportion of velocity to rate  of descent. Obviously, this condition provides  maximum glide ratio. Since the rate of descent  is proportional to the power required, the  points of tangency define the aerodynamic  condition of (L/D)m.z.  FACTORS AFFECTING  GLIDE  PER-  FORMANCE.  In order to obtain the mini-  mum glide angle through the air, the airplane  must be operated at (L/D)mor. The subsonic  (LIDL  of a given airplane configuration will  occur at a specific value of lift coefficient and  angle of attack.  However, as can be noted  from the curves of figure 6.6, small deviations  from the optimum C, will not cause a drastic  reduction of (L/D) and glide ratio.  In fact, a  5 percent deviation in speed  from the best glide  where  VI= best glide speed corresponding to  some original gross weight. WI  V,=best  glide speed corresponding to  some new gross weight, IV2  -4  VT- w,  VI  w,  (constant C,> 