NAVWEPS 00-ROT-80 
AIRPLANE PER,FORMANCE 
REQUIRED THRUST AND POWER 
DEFINITIONS 
All of the principal items of flight perform- 
ance involve steady state flight conditions and 
equilibrium of the airplane. For the airplane 
to remain in steady level flight, equilibrium 
must be obtained by a lift equal to the air- 
plane weight and a powerplant thrust equal to 
the airplane drag. Thus, the airplane drag 
defines the thrust required to maintain steady 
level flight. 
The total drag of the airplane is the sum of 
the parasite and induced drags: Parasite drag 
is the sum of pressure and friction drag which 
is due to the basic configuration and, as de- 
fined, is independent of lift. Induced drag is 
the undesirable but unavoidable consequence 
of the development of lift. In the process of 
creating lift by the deflection of an airstream, 
the actuai iift is inclined and a coimponcn: of 
lift is incurred parallel to the flight path direc- 
tion. This component of lift combines with 
any change in pressure and friction drag due 
to change in lift to form the induced drag. 
While the parasite drag predominates at high 
speed, induced drag predominates at low speed. 
Figure 2.1 illustrates the variation with speed 
of the induced, parasite, and total drag for a 
specific airplane configuration in steady level 
flight. 
The power required for flight depends on the 
thrust required and the flight velocity. By 
definition, the propulsive horsepower required 
is related to thrust required and flight velocity 
by the following equation: 
pr= Trv 
3% 
where 
Pr=power required, h.p. 
Tr= thrust required (total drag), Ibs. 
V= true airspeed, knots 
By inspection of this relationship, it is appar- 
ent that each’pound of drag incurred at 325 
knots requires one horsepower of propulsive 
power. However, each pound of drag at 650 
knots requires two horsepower while each 
pound of drag at 162.5 knots requires one-half 
horsepower. The term “power” implies work 
rate and, as such, will be a function of the speed 
at which a particular force is developed. 
Distinction between thrust required and 
pawcr required is necessary for several reasons. 
For the items of performance such as range and 
endurance, it is necessary to relate powerplant 
fuel flow with the propulsive requirement for 
steady IeveI flight. Some powerplants incur 
fuel flow rate according to output thrust while 
other powerplants incur fuel flow rate depend- 
ing on output power. For example, the turbo- 
jet engine is principally. a thrust producing 
machine and fuel flow is most directly related 
to thrust output. The reciprocating engine is 
principally a power producing machine and 
fuei flow is most directiy reiated to power 
output. For these reasons the variation of 
thrust required wil1 be of greatest interest in 
the performance of the turbojet powered air- 
plane while the variation of power required 
will be of greatest interest in the performance 
of the propeller powered airplane. Also, dis- 
tinction between power and thrust required is 
necessary in the study of climb performance. 
During a steady climb, the rate of climb will 
depend on excess power while the angle of 
climb is a function of excess thrust. 
The total power required for flight can be 
considered as the sum of induced and parasite 
effects similar to the total drag of the airplane. 
The induced power required is a function of the 
induced drag and velocity. 
p,,,!g 
where 
Pri= induced power required, h.p. 
D<=induced drag, lbs. 
V= true airspeed, knots 
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