NAVWEPS 00-8OT-80 
AIRPLANE PERFORMANCE 
cause the power required curve to flatten out 
and move to higher velocities and powers 
required. 
The curves of thrust and power required and 
their variation with weight, altitude, and con- 
figuration are the basis of all phases of airplane 
performance. These curves define the require- 
lnent~ of the airplane and must be considered 
with the power and thrust available from the 
powerplants to provide detailed study of the 
various items of airplane performance. 
AVAILABLE THRUST AND POWER 
PRINCIPLES OF PROPULSION 
All powerplants have in common certain 
general principles. Regardless of the type of 
propulsion device, the development of thrust is 
related by Newton’s laws of motion. 
or 
where 
F=ma 
F-d(mV) 
df 
$=force or thrust, lbs. 
m=mass, slugs 
a=acceleration, ft. per sec.% 
d=derivative with respect to time, e.g., 
dr rate of change with time 
mV=momentum, lb.-sec., product of mass 
and velocity 
The force of thrust results from the accelera- 
tion provided the mass of working fluid. The 
magnitude of thrust is accounted for by the 
rate of change of momentum produced by the 
powerplant. A rocket powerplant creates 
thrust by creating a very large change in veloc- 
ity of a relatively small mass of propellants. 
A propeller produces thrust by creating a com- 
paratively small change in velocity of a rela- 
tively large mass of air. 
The development of thrust by a turbojet or 
ramjet powerplant is illustrated by figure 2.5. 
Air approaches at a velocity, Vi, depending on 
the flight speed and the powerplant operates 
on a certain mass flow of air, Q, which passes 
through the engine. Within the powerplant 
the air is compressed, energy is added by the 
burning of fuel, and the mass flow is expelled 
from the nozzle finally reaching a velocity, 
V;. The momentum change accomplished bv 
this action produces the thrust, 
where 
Ttz=Q (V,V,) 
Ta= thrust, lbs. 
Q= mass flow, slugs per sec. 
Vi= inlet (or flight) velocity, ft. per sec. 
V,= jet velocity, ft. per sec. 
The typical ramjct or turbojet powerplane de- 
rives its thrust by working with a mass flow 
relatively smaller than that of a propeller but 
a relatively greater change of velocity. From 
the previous equation it should be appreciated 
that the jet thrust varies directly with the mass 
flow Q, and velocity change, Va-Vi. This 
fact is useful in accounting for many of the 
performance characteristics of the jet power- 
plant. 
In the process of creating thrust by mo- 
mentum change of the airstream, a relative 
velocity, Vz-V1, is imparted to the airstream. 
Thus, some of the available energy is essen- 
tially wasted by this addition of kinetic energy 
to the airstream. The change of kinetic energy 
per time can account for the power wasted in 
the airstream. 
Pw=KE/t