Basic mechanics

 

Fig. 1-9-1

Mechanical arrangement of gas turbine engines.

PRINCIPLES OF JET PROPULSION

Fig. 1-4 Hero’s engine - probably the earliest
form of jet reaction.

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 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, thus causing the engine to revolve.

Fig. 1-5 A garden sprinkler rotated by the
reaction of the water jets.

8. The familiar whirling garden sprinkler (fig. 1-5) isa more practical example of this principle, for themechanism rotates by virtue of the reaction to thewater jets. The high pressure jets of modern

Basic mechanics for JET ENGIN

INTRODUCTION
1. The development of the gas turbine engine as an aircraft power plant has been so rapid that it is difficult to appreciate that prior to the 1950s very few people had heard of this method of aircraft propulsion. The possibility of using a reaction jet had interested aircraft designers for a long time, but initially the low speeds of early aircraft and the unsuitably of a piston engine for producing the large high velocity airflow necessary for the ‘jet’ presented many obstacles.

2. A French engineer, René Lorin, patented a jet propulsion engine (fig. 1-1) in 1913, but this was an athodyd (para. 11) and was at that period impossible to manufacture or use, since suitable heat resisting materials had not then been developed and, in the second place, jet propulsion would have been extremely inefficient at the low speeds of the aircraft of those days. However, today the modern ram jet is very similar to Lorin’s conception.

3. In 1930 Frank Whittle was granted his first patent for using a gas turbine to produce a propulsive jet, but it was eleven years before his engine completed its first flight. The Whittle engine formed the basis of the modern gas turbine engine, and from it was developed the Rolls-Royce Welland, Derwent, Nene and Dart engines. The Derwent and Nene turbo-jet engines had world-wide military applications; the Dart turbo-propeller engine became world famous as the power plant for the Vickers Viscount aircraft. Although other aircraft may be fitted; with later engines termed twin-spool, triple-spool, by-pass, ducted fan, unducted fan and propfan, these are inevitable developments of Whittle’s early engine.