NAVWEPS 00-EOT-80  AIRPLANE PERFORMANCE  where  Si = landing distance resulting from certain  values of landing velocity, Vi, and  acceleration, 6zi  S2=landing distance resulting from some  different values of landing velocity,  V2, or acceleration, a2  propellers is obtained by rotating the blade  angle well below the low pitch stop and  applying engine power. The action is to ex-  tract a large amount of momentum from the  airstream and thereby create negative thrust.  The magnitude of the reverse thrust from pro-  pellets is very large, especially in the case of  the turboprop where a very large shaft power  can be fed into the propeller. In the case of  reverse propeller thrust, maximum effective-  ness is achieved by use immediately after the  airplane is in contact with the runway.  The  reverse thrust capability is greatest at the  high speed and, obviously, any delay in pro-  ducing deceleration allows runway to pass by  at a rapid rate. Reverse thrust of turbojet  engines will  usually employ some form of  vanes, buckets, or clamshells in the exhaust to  turn or direct the exhaust gases forward.  Whenever the exit velocity is less than the in-  let velocity (or negative), a negative momen-  tum change occurs and negative thrust is  produced. The reverse jet thrust is valuable  and effective but it should not be compared  with the reverse thrust capability of a com-  parable propeller powerplant which has the  high intrinsic thrust at low velocities. As  with the propeller reverse thrust, jet reverse  thrust must be applied immediately after  ground contact for maximum effectiveness  in  reducing landing distance.  With this relationship, the effect of the many  variables on landing distance can be apptoxi-  mated.  The effect  of gross wclght on landing distance  is one of the principal items determining the  landing distance of an airplane  One effect  of an increased  gross weight is that the airplane  will  require a greater speed to support the  airplane at the landing  angle of attack  and lift coefficient. The relationship of land-  ing speed and gross weight would be as  follows:  where  Vi=landing  velocity corresponding to  some original weight, W,  Vs = landing  velocity corresponding to  some different weight, W,  FACTORS AFFECTING LANDING  PER-  FORMANCE.  In addition to the important  factors of proper technique, many other vari-  ables affect the landing performance of an air-  plane. Any item which alters the landing  velocity or deceleration during landing toll  will  affect the landing distance. As with  takeoff performance, the relationships of uni-  formly accelerated motion will  be assumed  applicable for studying the principal effects  on  landing distance. The case of uniformly ac-  celerated motion defines landing distance as  varying directly as the square of the landing  velocity and inversely as the acceleration dur-  ing landing toll.  Thus, a given airplane in the landing con-  figuration at a given gross weight will have a  specific landing speed (MS  ot CAS) which is  invariant with altitude, temperature, wind,  etc., because  a certain value of 4 is necessary  to provide lifr equal to weight at the landing  C,.  As an example of the effect of a change in  gross weight, a 21 percent increase in landing  weight will require a 10 percent increase in  landing speed to support the greater weight.  When minimum landing distances are con-  sidered, braking friction forces predominate  during the landing toll and, for the majority  of airplane configurations, braking friction is  the main source of deceleration. In this case,  an increase in gross weight provides a greater 