The basic terms of jet propulsion can be explained using Newton’s second and third laws of motion. The third law declares that ‘for every action there is an equal an opposite reaction’, and the second law states the relationship between force(f), mass(m) and acceleration(a) shown in the formula ’F = m x a’. To put this theory into context for jet propulsion, the air steam is forced to accelerate whilst it’s travelling through the engine which means its kinetic energy is increased; therefore it creates reaction propulsion from ejecting the mass of gas to which the aircraft reacts with an equal and opposite thrust. The velocity of the jet efflux is important in terms of the designed aircraft velocity range as the overall propulsive efficiency percentage is highest when the engine emits an exhaust jet at a speed that is the same as, or nearly the same as, the aircraft velocity. This propulsive efficiency (PE) can be calculated using the following:
PE= Work done in moving the aircraft forward X 100%
Work done in the moving jet stream rearwards
PE= Twice the aircraft speed X 100%
Aircraft speed + jet stream speed
(P2 and part M1)
This is a sketch illustrating the typical working cycle of pressure and velocity changes through an aircraft gas turbine engine.
Compressor – Sucks the air in, Compresses the incoming air to high pressure.
Combustion chamber – Fuel injectors release a steady flow of fuel, burn the fuel and produces high-pressure, high-velocity gas.
Turbine - Extracts the energy from the high-pressure, high-velocity gas flowing from the combustion chamber, turbines drives the compressor.
The working cycle of a gas turbine engine known as the ‘Brayton cycle’ contains three components; a gas compressor, a combustion chamber and expansion turbine. It describes the occurrences that take place as the energy is released from the fuel. This process is achieved by ambient air being sucked...