Page 12 - The Jet Engine

Basic mechanics

similar way to the engine/propeller combination. Both
propel the aircraft by thrusting a large weight of air
backwards (fig. 1-3), one in the form of a large air
slipstream at comparatively low speed and the other
in the form of a jet of gas at very high speed.
7. This same principle of reaction occurs in all forms
of movement and has been usefully applied in many
ways. The earliest known example of jet reaction is
that of Hero's engine (fig. 1-4) produced as a toy in
120 B.C. This toy showed how the momentum of
steam issuing from a number of jets could impart an
equal and opposite reaction to the jets themselves,
Fig. 1-2 A Whittle-type turbo-jet engine. thus causing the engine to revolve.
8. The familiar whirling garden sprinkler (fig. 1-5) is
a more practical example of this principle, for the
4. The jet engine (fig. 1-2), although appearing so mechanism rotates by virtue of the reaction to the
different from the piston engine-propeller water jets. The high pressure jets of modern fire-
combination, applies the same basic principles to fighting equipment are an example of 'jet reaction',
effect propulsion. As shown in fig. 1-3, both propel for often, due to the reaction of the water jet, the hose
their aircraft solely by thrusting a large weight of air cannot be held or controlled by one fireman. Perhaps
backwards. the simplest illustration of this principle is afforded by
the carnival balloon which, when the air or gas is
5. Although today jet propulsion is popularly linked released, rushes rapidly away in the direction
with the gas turbine engine, there are other types of opposite to the jet.
jet propelled engines, such as the ram jet, the pulse
jet, the rocket, the turbo/ram jet, and the turbo- 9. Jet reaction is definitely an internal phenomenon
rocket. and does not, as is frequently assumed, result from
the pressure of the jet on the atmosphere. In fact, the
PRINCIPLES OF JET PROPULSION
6. Jet propulsion is a practical application of Sir
Isaac Newton's third law of motion which states that,
'for every force acting on a body there is an opposite
and equal reaction'. For aircraft propulsion, the 'body'
is atmospheric air that is caused to accelerate as it
passes through the engine. The force required to
give this acceleration has an equal effect in the
opposite direction acting on the apparatus producing
the acceleration. A jet engine produces thrust in a

jet propulsion engine, whether rocket, athodyd, or
turbo-jet, is a piece of apparatus designed to
accelerate a stream of air or gas and to expel it at
Fig. 1-3 Propeller and jet propulsion. high velocity. There are, of course, a number of ways

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