NAVWEPS O&ROT-80  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  in the case of powerplant failure or malfunc-  tion.  When a powerplant failure or malfunc-  tion occurs, it is usually of interest to obtain a  gliding flight path which results in the mini-  mum glide angle. The minimum glide angle  will produce the greatest proportion of glide  distance to altitude loss and will  result in  maximum glide range or minimum expendi-  ture of altitude for a specific glide distance.  In the study of climb performance, the forces  acting on the airplane in a steady climb (or  glide) produce the following relationship:  there is no unbalance of pitching, yawing, or  rolling moment. If the airplane traverses a  sharp wind shear equivalent to a horizontal  gust, the resulting change in airspeed will  disturb such an equilibrium. For example, if  the airplane encounrers a sharp horizontal  gust which reduces  the airspeed 20percent,the  new airspeed (80 percent of the original value)  produces lift and drag at the same angle of  attack which are 64 percent of the original  value. The change in these aerodynamic forces  would cause the airplane to accelerate  in the  direction of resultant unbalance of force.  That is, the airplane would accelerate down  and forward until  a new equilibrium  is  achieved. In addition,  there would  be a  change in pitching moment which  would  produce a response of the airplane in pitch.  GLIDE ANGLE AND LIFT-DRAG RATIO.  The response  of the airplane to a horizontal  gust will differ according to the gust gradient  and airplane characccristics. Gmcrally, if the  airplane encounters a sharp wind shear which  reduces  the airspeed, the airplane tends to sink  and incur a loss of altitude  ‘ before equilibrium  conditions are achieved. Similarly,  if the  airplane encounters a sharp wind shear which  increases the airspeed, the airplane tends to  float and incur a gain of altitude before equilib-  rium conditions are achieved.  In the case of power-off glide performance,  the  thrust, T, is zero and the relationship reduces  to:  where  Y=: angle of climb, degrees  T-thrust,  lbs.  D-drag,  lbs.  W=: lbs.  By this relationship it is evident that the mini-  mum angle of glide-or  minimum negative  climb angle-is  obtained at the aerodynamic  conditions which incur the minimum total  drag.  Since the airplane lift is essentially equal  to the weight, the minimum angle of glide  will be obtained when the airplane is operated  at maximum lift-drag ratio, (L/D)ma,. When  the angle of glide is relatively small, the ratio  of glide distance to glide altitude is numeri-  cally equal to the airplane lift-drag ratio.  Significant vertical and horizontal gusts may  be due to the terrain or atmospheric conditions.  The proximity ’ of an unstable front or thunder-  storm activity ’ in  the vicinity of the airfield is  likely to create significant wind shear and gust  activity at low altitude.  During gusty condi-  tions every effort must be made for precise con-  trol of airspeed  and flight path and any changes  due to gusts must be corrected by proper con-  trol action.  Under extreme gusts conditions,  it may be advisable to utilize approach, land-  ing, and takeoff speeds slightly greater than  normal to provide margin for adequate control.  Figure 6.6 illustrates the forces acting on the  airplane in a power-off glide.  The equilibrium  of the steady glide is obtained when the sum-  mation of forces in the vertical and horizontal  directions is equal to zeta.  The gliding performance  of an airplane is of  special interest for the single-engine airplane  glide ratio=  glide distance, ft.  glide altitude, ft.  glide ratio = (L/D)  D  sin y= --  W  POWER-OFF GLIDE PERFORMANCE  369 