BASIC PRINCIPLES Most aircraft engines work on the principle of taking a mass of air and accelerating it rearwards. Newton's third law states that "for every action, there is an equal and opposite reaction". By pushing air rearwards, the reaction will cause the aircraft to move forwards. There are slightly different ways of achieving this. A propeller engine aircraft works by accelerating a LARGE mass of air at a LOW velocity. A pure jet engine works by accelerating a SMALL mass of air at a HIGH velocity. A bypass jet engine is a bit of a cross between both systems where a jet engine drives a fan as opposed to a propeller. So how does this jet engine manage to accelerate the air rearwards? Well air enters the front of the engine and goes through a compressor which raises the air's pressure considerably. Fuel is then added to this high pressure air and when this mixture is ignited, it expands rapidly and leaves the rear of the jet engine at a much higher velocity than which it entered. This all sounds great but this "compressor" must take a fair bit of power to drive it. Where does it get this power from? The answer is that after the air/fuel mixture has been ignited, it passes through one or more "turbines" before it leaves the engine. These turbines are connected to the compressor by means of a shaft so when the turbine rotates, it also rotates the compressor. Unfortunately, the turbine extracts a lot of energy from the airflow while driving the compressor but this is a necessary evil.
		Reproduced with kind permission of Rolls-Royce plc
	Next page Back to "How they work" index Back to "Home"