Page 131 - Understanding Flight

CH05_Anderson 7/25/01 8:58 AM Page 118

118 CHAPTER FIVE

It’s Newton Again

You learned in Chapter 2 that for a wing to create lift it must divert air
down. This principle is exactly how an aircraft propulsion system
works, except that to create thrust it must push air back. Just as your
household fan pushes air back, so does a propeller and a jet engine. For-
tunately, your house fan does not have enough thrust to propel itself.
Also like wings, aircraft propulsion systems are applications of
Newton’s laws. Remember Newton’s third law states that “for every
action, there is an equal and opposite reaction.” In an aircraft
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propulsion system, the action is acceleration of air or exhaust
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and the reaction is the force, or thrust, produced. Again we
and in 1911 was the first
use the alternate form of Newton’s second law which states
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that the thrust is proportional to the amount of gas accelerated
Channel.
per time times the speed of that air.

Thrust

Though it is not an aircraft propulsion system as such, studying the
rocket engine is an excellent way to understand propulsion. An exam-
ple of how a rocket engine works is shown in Figure 5.1. Fuel and an
oxidizer are pumped into a combustion chamber, producing a large
amount of gas at a high pressure. The gas accelerates to the throat of
the motor, where it reaches a velocity of Mach 1. After the throat the

Oxidizer

Throat
Combustion
chamber Nozzle High-velocity gas

Fuel
Fig. 5.1. Schematic of a rocket engine.