NAVWEPS 00-EOT-80 
AIRPLANE PERFORMANCE 
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. 
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. 
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 
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, 
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