Page 133 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 120

120 CHAPTER FIVE

to accelerating air or exhaust. Newton’s second law states that the
thrust is equal to the amount of mass, per time, pushed by the engine
(propeller or jet) times the increase in velocity the air experiences. If
you have ever stood behind an airplane with its propeller turning, you
can certainly recognize that a great deal of air is being blown back.

Power

Aircraft propulsion consists of two distinct parts. There is the engine
that converts a source of energy, such as fuel, to work. Then there is
the part of the system that converts the work of the engine into work
on the surrounding environment to produce propulsion. The most
obvious example is a propeller. A piston engine and propeller combi-
nation is an example of a complete aircraft propulsion system. A tur-
bojet is another example, but the parts of that system are a little
harder to distinguish from each other.
Usually engineers, flight instructors, and educators relate flight and
propulsion in terms of forces. In this book, we take the perspective of
power, which is adjusted by the throttle and can be measured by the
pilot. Looking at propulsion from power carries some intuition with it. If
one increases the throttle or fuel flow, the power increases. Power is the
rate of using energy, or doing work, which is the key to understanding
propulsion. Power also lends itself to another fundamental concept:
efficiency.
Looking at the propulsion system from the standpoint of power, it
is convenient to introduce a few terms. You know that power is
required for flight: for supporting the weight of the airplane,
for climbing, for turning or accelerating. This is the required
The rocket-powered Bell X-1 was
power for flight. The power that is actually produced by the
the first airplane to go
engine and delivered to the propeller or is available for
supersonic.
propulsion by the jet we will call the engine power.
The power actually used to produce thrust is the propulsive power,
which is just equal to the thrust produced times the speed of the
airplane. The propulsive power is always smaller than the engine
power because of inefficiencies. The difference between the engine
power and the propulsive power we will call called the wasted power.