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. 
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 
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. 
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. 
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