NAVWEPS Oe8OT-80  BASIC AERODYNAMICS  The configuration of an airplane has a great  effect on the lift-drag ratio.  Typical values  of (L/D),..  are listed for various types of  airplanes. While the high performance sail-  plane may have. extremely high  lift-drag  ratios, such an aircraft has no real economic  or tactical purpose. The supersonic fighter  may have seemingly low lift-drag ratios in  subsonic flight but the airplane configurations  required for supersonic  flight (and high [L/D] ‘ *  at high Mach numbers) precipitate this situa-  tion.  higher  angles of attack the drag coefficient is  much greater and small changes in angle of  attack cause significant changes in drag.  As  stall occurs, a large increase in drag takes  place.  A factor more important in airplane per-  formance considerations is the lift-drag ratio,  L/D.  With the lift and drag data available for  the airplane, the proportions of CL and CD can  be calculated for each specific angle of attack.  The resulting plot of lift-drag ratio with angle  of attack shows that L/D increases to some  maximum then decreases  at the higher lift  coefficients and angles of attack. Note that  the maximum lift-drag ratio, (L/D),,,,  occurs  at one specific angle of attack and lift coefIi-  cient.  If the airplane is operated in steady  flight at (L/D),,,,  the total drag is at a mini:  mum.  Any angle of attack lower or higher  than that for (L/D),,,  reduces the lift-drag  ratio and consequently increases -the total  drag for a given airpiane iift.  Many important items of airplane perform-  ance are obtained in flight at (L/D),...  Typi-  cal performance conditions which occur at  (L/D),.,  are:  maximum endurance of jet powered air-  planes  maximum range of propeller driven air-  planes  maximum climb angle for jet powered air-  planes  maximum power-off glide range, jet or  Prop  The airplane depicted by the curves of Figure  1.13 has a maximum lift-drag ratio of 12.5 at  an angle of attack of 6”.  Suppose  this airplane  is operated in steady flight at a gross weight  of 12,500 lbs.  If flown at the airspeed and  angle of attack corresponding to (L/D),..,  the drag would be 1,000 lbs. Any higher or  lower airspeed would produce a drag greater  than 1,000 lbs. Of course, this same airplane  could be operated at higher or lower gross  weights and the same maximum lift-drag ratio  of 12.5 could be obtained at the same angle of  attack of 6”.  However, a change’ in gross  weight would require a change in airspeed to  support the new weight at the same lift co-  efficient and angle of attack.  The most immediately interesting of these  items is the power-off glide range of an air-  plane. By examining the forces acting on an  airplane during a glide, it can be shown that  the glide ratio is numerically equal to the  lift-drag ratio.  For example, if the airplane  in a glide has an (L/D) of 15, each mile of alti-  tude is traded for 15 miles of horizontal dis-  tance. Such a fact implies that the airplane  should be flown at (L/D)-  to obtain the  greatest glide distance.  An unbelievable feature of gliding perform-  ance is the effect of airplane gross weight.  Since the maximum lift-drag ratio of a given  airplane is an intrinsic property of the aero-  dynamic configuration, gross weight will not  affect the gliding performance. If a typical  jet trainer has an (L/@-  of 15, the aircraft 1  can obtain a maximum of 15 miles horizontal  distance for each mile of altitude.  This would  be true of this particular airplane at any gross  Type  airplane:  (L/D) emz  High performance  sailplane.  25-40  Typical  patrol  or transport..  12-20  High Performance  bomber.  2~25  Propeller  powered  trainer..  1~15  J  et trainer..  14-16  Transonic fighter or attack..  lo-13  Supersonic fighter or attack.  4-9 (subsonic)  32  Revised Januay  1965 