Aircraft power plants fall into five main types.
- Ramjet engines, for very high speed aircrafts.
- Turbo jet engines, for high speed aircrafts.
- Turbo-fan engines, for Mach 0.3 to Mach 2.
- Turbo-prop engines, for relatively low speeds.
- Piston engines, for simple low speed aircrafts.
Each variantis most suited to a particular aircraft flight speed. The operating efficiency, loosely defined as power absorbed divided by the rate of fuel burn, is maximized when the velocity of the air expelled from the jet, fan or propeller is close to the speed of the aircraft.
In turbo-fan engines, some of the exhaust gases are made up of air that has by-passed the engine core or gas generator, and only passed through a fan. They are therefore called by-pass engines. The higher the by-pass ratio, the larger the engine’s diameter.
Engines can be positioned in many ways. Most transport aircraft have externally mounted engines, leaving the fuselage interior volume clear for payload. Engines can then be rear-mounted, wing-mounted, or a combination of them. Both have advantages and disadvantages. Twin- and four-engined turbo-prop aircrafts will almost inevitable require the engines to be wing-mounted. In combat aircrafts the fuselage is not required to carry an internal payload, so it’s an ideal location for the engines.
Twin- or multi-engined propeller-driven aircrafts must have their engines spaced out along the wing to provide clearance between the propeller tips and the fuselage. The closer the engines are to the fuselage, the more noise is generated inside the fuselage, and the further away they are, the more the aircraft yaws if an engine fails. Wherever the engines are located, they must be supplied with fuel. In twin- or multi-engine installations it is a requirement that the fuel supply can be maintained if any component fails. In case an aircraft should land due to some kind of emergency early on the flight, the aircraft overall weight may be too high for a safe landing. Therefore, the pilot has the possibility to dump fuel. Propellers fitted to many aircrafts often have the capability of varying their pitch. If the pitch is controlled automatically, an engine can be operated at constant speed. If an engine fails, the propeller will windmill. This causes extra drag, and may further damage the engine, so the propeller is feathered: the blade pitch is changed until the blades sit approximately in line with the air stream.