Page 134 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 121

Airplane Propulsion 121

This is the power that is lost to kinetic energy in the propeller’s
slipstream or the jet’s exhaust.
Again, the power delivered by the engine is the engine power, and
the power that goes into propelling the airplane is the propulsive
power. With a piston engine, for a fixed engine power, the propulsive
power depends only slightly on the airplane’s speed. The first figure in
Figure 5.3 illustrates how the thrust and propulsive power vary with
speed for a typical piston engine/propeller combination at a fixed
power setting. Since thrust is power divided by speed, a fairly
constant propulsive power means that the thrust provided by the
propeller decreases with speed. The thrust from a propeller degrades
with speed but the propulsive power holds up quite well. Of course,
the details are dependent on propeller design, among other things.
As can be seen in the second figure, the jet engine behaves quite
differently with respect to speed than a piston engine. The thrust
available to a jet engine is roughly constant with speed and
The lifetime cost of a Boeing
the propulsive power is proportional to the speed of the
747-400 per seat-mile is the
airplane. This will be discussed in more detail later, though
same for an automobile.
one can anticipate that the differences will impact the
performance of an airplane.

Efficiency
The objective of an aircraft propulsion system is to produce the
required power as efficiently as possible. There are basically two areas
where propulsion systems lose efficiency. The first is in the conversion

Piston Engine Jet Engine
Propulsive
power Thrust

Propulsive
Thrust power

Speed Speed
Fig. 5.3. Propulsive power and thrust as functions of speed for a propeller and
a jet engine.