NAVWEPS 00-801-80 
AIRPLANE PERFORMANCE 
Generally, the overall fuel-air ratio of the 
turbojet is quite low because of the limiting 
turbine inlet temperature. The overall air- 
fuel ratio is usually some value between 80 to 
40 during ordinary operating conditions be- 
cause of the large amount of secondary air or 
cooling flow. 
High temperature, high energy combustion 
gas is delivered to the turbine section where 
power is extracted to operate the compressor 
section. Partial or near-complete expansion 
can take place through the turbine section with 
the accompanying pressure and tempcratute 
drop. The exhaust nozzle completes the ex- 
pansion by producing the final jet velocity and 
momentum change necessary in the develop- 
ment of thrust. 
TURBOJET OPERATING CHARACTER- 
ISTICS. The turbojet engine has many oper- 
ating characteristics which are of great im- 
portance to the various items of jet airp!ane 
performance. Certain of these operating char- 
acteristics will provide a strong influence on 
the range, endurance, etc., of the jet-powered 
airplane. Other operating characteristics will 
require operating techniques which differ 
greatly from more conventional powerplants. 
The turbojet engine is essentially a thrust- 
producing powerplant and the propulsive 
power produced is a result of the flight speed. 
The variation of available thrust with speed is 
relatively small and the engine output is very 
nearly constant with flight speed. The mo- 
mentum change given the engine airflow de- 
velops thrust by the following relationship: 
where 
Ta= thrust available, lbs. 
Q=mass flow, slugs per sec. 
vi=inlet or flight velocity, ft. per sec. 
Va= jet velocity, ft. per see. 
Since an increase in flight speed will increase 
the magnitude of Vi, a constant thrust will be 
obtained only if there is an increase in mass 
flow, Q, or jet velocity, Vs, When at low 
velocity, an increase in velocity will reduce 
the velocity change through the engine with- 
out a corresponding increase in mass flow and 
the available thrust will decrease. At higher 
velocity, the beneficial ram helps to overcome 
this effect and the available thrust no longer 
decreases, but increases with speed. 
The propulsive power available from the 
turbojet engine is the roduct of available 
thrust and velocity. t T e propulsive horsc- 
power available from the turbojet engine’is 
related by the following expression: 
pyav -- 
325 
where 
Pa=propulsive power available, h.p. 
T.-*Le..;- ;--;11.1*~ LC‘--LL,IlLSL ‘t”.uiaOK, ibs. 
V= flight velocity, knots 
The factor of 321 evolves from the use of the 
nautical unit of velocity and implies that 
each pound of thrust developed at 325 knots 
is the equivalent of one horsepower of propul- 
sive power. Since the thrust of the turbojet 
engine is essentially constant with speed, tht 
power available increases almost linearly with 
speed. In this sense, a turbojet with 5000 Ibs. 
of thrust available could produce a propulsive 
power of 3,ooO h.p. at 325 knots or 10,000 
h.p. at 650 knots. The tremendous propulsive 
power at high velocities is one of the principal 
features of the turbojet engine. When the 
engine RPM and operating altitude arc fixed, 
the variation with speed of turbolet thrust and 
power available is typified by the first graph 
of figure 2.10. 
The variation of thrust output with engine 
speed is a factor of great importance in the 
operation of the turbojet engine. By reason- 
ing that static pressure changes depend on the 
square of the flow velocity, the changer of 
pressure throughout the turbojet engine would 
116