Engines….
Introduction
For an aircraft to remain in level unaccelerated flight, a thrust must be provided that is equal to and opposite in direction to the aircraft drag. This trust, or propulsive force, is provided by suitable type of heat engine. All heat engines have in common the ability to convert heat energy into mechanical energy, by the flow of some fluid mass through the engine. In all cases, the heat energy is released at a point in the cycle where the pressure is high, relative to atmospheric. These engines are customarily into groups or types depending upon: (1)The working fluid used in the engine cycle. (2)The means by which the mechanical energy is transmitted into a propulsive force. (3)The method of compression the engine working fluid.
| Engine Type | Major Means of Compression | Engine Working Fluid | Propulsive Working Fluid |
| Turbojet | Turbine-driven compressor | Fuel/air mixture | Same as engine working fluid |
| Turboprop | Turbine-driven compressor | Fuel/air mixture | Ambient air |
| Ramjet | Ram compression due to high flight speed | Fuel/air mixture | Same as engine working fluid |
| Pulse-jet | Compression due to combustion | Fuel/air mixture | Same as engine working fluid |
| Reciprocating | Reciprocating action of pistons | Fuel/air mixture | Ambient air |
| Rocket | Compression due to combustion | Oxidizer/fuel mixture | Same as engine working fluid |
The propulsive force is obtained by the displacement of working fluid(not necessarily the same fluid used within the engine) in a direction opposite to that in which the airplane is propelled. This is an application of Newtons Third Law of motion. Air is the principal fluid used for propulsion in every type of power plant except the rocket, in which only the byproducts of combustion are accelerated and displaced. The propellers of aircraft powered by reciprocating or turboprop engines accelerate a large mass of air through a small velocity change. The fluid(air) used for the propulsive force is a different quantity than that used within the engine to produce the mechanical energy. Turbojets, ramjets and pulse-jets accelerate a smaller quantity of air through a large velocity change. They use the same working fluid for propulsive force that is used within the engine. A rocket carries its own oxidizer rather than using ambient air for combustion. It discharges the gaseous byproducts of combustion through the exhaust nozzle at an extremely high velocity. Engine fluids further characterized by the means of compressing the working fluid before the addition of heat. The basic method of compression are: (1)The turbine driven compressor(turbine engine). (2)The positive displacement, piston type compressor(reciprocating engine). (3)Ram compression due to forward flight speed(ramjet). (4)Pressure rise due to combustion(pulse jet and rocket).
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