NAVWEPS 00-ROT-80  AIRPLANE PERFORMANCE  sufficient to cause deceleration of the airplane  it can be used in deference  to the brakes in the  early stages of the landing roll, i.e., brakes  and tires suffer from continuous, hard use but  airplane aerodynamic drag is free and does not 1  wear out with use. The use of aerodynamic  drag is applicable only for deceleration to 60  ot 70 percent of the touchdown speed. At  speeds  less than 60 to 70 percent of the touch-  down speed, aerodynamic drag is so slight as  to be of little use and braking must be utilized  to produce continued deceleration of the  airplane.  dry hard surface  runways will require particular  techniques to obtain minimum landing dis-  tance. Generally, the technique involves low-  ering the nose wheel to the runway and retract-  ing the flaps to increase the normal force on  the braking surfaces. While the airplane drag  is reduced, the greater normal force can pro-  vide greater braking friction force to com-  pensate  for the reduced drag and the net retard-  ing force is increased.  The technique necessary  for minimum land-  ing distance can be altered~  to some extent in  certain situations.  For example, low aspect  ratio airplanes with high longitudinal control  power can create very high drag at the high  speeds immediate to landing touchdown.  If  the landing gear configuration or flap or  incidence setting precludes a large reduction  of CL, the normal force on the braking surfaces  and braking friction force capability are rela-  tively small.  Thus, in the initial high speed  part of the landing roll, maximum deceleration  would be obtained by creating the greatest  possible aerodynamic drag.  By the time the  aircraft has slowed to 70 or 80 percent of the  touchdown speed, aerodynamic drag decays  but braking action will  then be effective.  Some form of this technique may be necessary  to achieve minimum distance for some con-  figurations when the coefficient of braking  friction is low (wet, icy runway) and the  braking friction force capability is reduced  relative to airplane aerodynamic drag.  Powerplant thrust is not illustrated on  figure 2.34 for there are so many possible  variations.  Since the objective during the  landing toll is to decelerate, the powerplant  thrust should be the smallest possible positive  value or largest possible negative value. In  the case of the turbojet aircraft, the idle  thrust of the engine is nearly constant with  speed throughout the landing roll.  The idle  thrust is of significant magnitude on cold days 1  because  of the low compressor  inlet air temper-  ature and low  density altitude.  Unfortu-  nately, such atmospheric conditions usually  have the corollary of poor braking action be-  cause of ice or water on the runway.  The  thrust from a windmilling  propeller with the  engine at idle can produce large negative thrust  early in the landing roll but the negative force  decreases with  speed. The .large negative  thrust at high speed is valuable in adding to  drag and braking friction to increase the net  retarding force.  A distinction should be made between the  techniques for minimum landing distance and  an ordinary landing roll with  considerable  excess runway .available. Minimum landing  distance will  be obtained from the landing  speed by creating a continuous peak decelera-  tion of the airplane. This condition usually  requites extensive use of the brakes for maxi-  mum deceleration. On the other hand, an  ordinary landing roll with considerable excess  runway may allow extensive use of aero-  dynamic drag to minimize wear and tear on  the tires and brakes. If aerodynamic drag is  Various devices can be utilized to provide  greater deceleration-of the airplane or to mini-  mize the wear and teat on tires and brakes.  ‘ The drag parachute can provide a large retatd-  ing force at high 4 and greatly increase the de-  celeration during the initial phase of landing  toll.  It should be noted that the contribution  of the drag chute is important only during the  high speed portion of the landing roll.  For  maximum effectiveness,  the drag chute must be  deployed immediately after the airplane is in  contact with the runway.  Reverse thrust of  195  Revised January 1965 