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For finite wings, e is the wing planform e:fficiency factor. For elliptical wing
planform, a - 1. In other words, for a given aspect ratio A, the induced drag
is a minimum for an elliptical wing. However, for the complete airplane, the
definition of e is modified to include the variation of skin-friction and pressure
drag coefficients with angle of attack andis called the Oswald's efficiency factor.
Generally, two types of powerplants are commonly used on modern airplanes,
1) the piston engine-propeller combination or the piston-prop and 2) the tur-
bojeL Piston-props are widely used on light general aviation airplanes, whereas
jet engines are used for commercial transport and rrulitar}r aircraft. In view of
this, the performance analyses presented mt'this text will be based on these two
types of powerplants. We will be referring to an airplane with a piston-prop pow-
erplant as a propeller airplane and that with a turbojet powerplant as ajet air-
plane.
v"The propulsive characteristics of the propeller airplane are normally given
in terms of the power developed by the reciprocating (piston) engine ancl the
propulsive efficiency of the engine3ropeller combination. The power available
for propulsion is th:eYproduct of the power developed by the reciprocating (piston)
engine and the propulsive efficiency ofthe engine-propeller combination. Because
of this, it is common practice to analyze the performance problems of a propeller
airplane in terms of the power available and the power required. The power re-
quired is the power necessary to overcome the aerodynamic drag of the airplane.
In this text, we will use kW as the unit of power and denote the power developed
by the engine in kW using the notation P[kW).
The propulsive characteristics of the jet airplane are normally specified in terms
ofthe t~rust produced by thejet engine, which is referred to as the thrust available.
Hence, the performance characteristics of the jet airplane are usually discussed
in terms of the thrust available and the thrust required. The thrust required is the
thrust necessary to balance the aerodynamic drag of the airp.lane.
The specific fuel consumption of a propeller airplane is the weight of fuel
consumed per unit power per unit time.ln this text, we will use the units of N/kWh
to specify the specific fuel consumption of propeller airplanes. For jet aircraft,
specific fuel consumption is the amount of fuel consumed per unit thrust per unit
time and will have the units of N/Nh.
For a given altitude, the power developed by a piston (reciprocating) engine
is virtually constant with flight velocity. The only variation of power arises due
to the variation of the ram pressure in the intake manifold with fiight velocity.
However, the power developed by a reciprocating engine decreases with an increase
in altitude because ofa fallin air density. With turbosupercharging, this decrease in
engine power can be minimized up to a certain altitude. The propulsive efficienc}r,
in general, varies with flight velocity. However, if the aircraft is equipped with a
variable-pitch, constant-speed propeller, the propulsive efficiency can be assumed
constant over the design operaoing range. For a given altitude, the thrust developed
Pa
AIRCRAFT PERFORMANCE
v
Ta
N
Ta
v
v
. b) Jet aircraft
~g. 2.1 Schematic variation ofthrust and power av:ulable with velocity.
70 PERFORMANCE, STABILITY, DYNAMICS, AND CONTROL
flight, range and endurance, takeoff, landing, and turning flights. We will also de-
termine the fiight variables that optimize the point performance of the airplane in
such fiights. We will not be dealing with path performance problems that are es-
sentially problems in variational calculus. Interested readers may refer elsewherei
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