NAWEPS 0040140 
AlRPLANE PERFORMANCE 
Of course, the development of thrus,t with 
some finite mass flow will require some finite 
velocity change and there will be the inevita- 
ble waste of power in the airstream. In order 
to achieve high efficiency of propulsion, the 
thrust should be developed with a minimum 
of wasted power. 
The propulsion efficiency of the jet power- 
plant can be evaluated by comparing the 
propulsive output power with the input power. 
Since the input power is the sum of the output 
power and wasted power, an expression for 
propulsion efficiency can be derived. 
Pa 
vp=Pa+Pw 
zv, 
')p= v*+v1 
where 
trp = propulsion efficiency 
9=“eta” 
Pa = propulsive power available 
= TCZV~ 
Pw= power wasted 
The resulting expression for propulsion effi- 
ciency, v,,, shows a dependency on the flight 
velocity, V,, and the jet velocity, VZ. When 
the flight velocity is zero, the propulsion 
efficiency is zero since all power generated is 
wasted in the slipstream and the propulsive 
power is zero. The propulsion efliciency would 
be I.00 (or 100 percent) only when the flight 
velocity, Vi, equals the jet velocity, Vz. 
Actually, it would not be possible to produce 
thrust under such conditions with a finite mass 
flow. While 100 percent efficiency of propul- 
sion can not be attained practically, some 
insight is furnished to the means of creating 
high values of propulsion efficiency. To ob 
tain high propulsion efficiency it is necessary 
to produce the required thrust with the highest 
possible mass flow and lowest possible velocity 
change. 
The graph of figure 2.5 shows the variation 
of propulsion efficiency, qP, with the ratio of 
flight speed to jet velocity, VJV,. To achieve 
a propulsion efficiency of 0.85 requires that the 
flight velocity be approximately 75 percent of 
the slipstream speed relative to the airplane. 
Such a propulsive efficiency could be typical 
of a propeller powered airplane which derives 
its thrust by the propeller handling a large 
mass flow of air. The typical turbojet power- 
plant cannot achieve such high propulsive 
ethciency because the thrust is derived with a 
relatively smaller mass flow and larger vcloc- 
ity change. For example, if the jet velocity is 
1,200 ft. per sec. at a flight velocity of 600 ft. 
per sec., the propulsion efficiency is 0.67. The 
ducted fan, bypass jet, and turboprop are vari- 
aCon -which impiove tliC propulsive efIiciency 
of a type of powerplant which has very high 
power capability. 
When the conditions of range, endurance, or 
economy of operation are predominant, high 
propulsion efhciency is necessary. Thus, the 
propeller powered airplane with its inherent 
high propulsive efliciency will always find ap 
plication. The requirements of very high 
speed and high altitude demand very high 
propulsive power from relatively small powcr- 
plants. When there are practical limits to the 
increase of mass flow, high output is obtained 
by large velocity changes and low propulsive 
efficiency is an inevitable consequence. 
TURBOJET ENGINES 
The turbojet engine has foundwidespread USC 
in aircraft propulsion because of the relatively 
high power output per powerplant weight and 
size. Very few aircraft powerplants can com- 
pare with the high output, flexibility, simplic- 
ity, and small size of the aircraft gas turbine. 
The coupling of the propeller and recipro- 
cating engine is one of the most efficient means 
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