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Jet or turbine engines provide power for commercial and general aviation aircraft around the world. Propeller-driven aircraft have operating altitude limitations due to prop performance, but jet engine performance tends to increase at higher altitudes. Performance of turbine engines is measured by fuel consumption, thrust and drag at various operating altitudes.
A turbine engine will produce a large amount of thrust at low altitudes due to the high density of air. As the aircraft climbs, the air density will drop until the aircraft reaches normal cruising altitudes, often above 30,000 feet (9,100 meters). Although the air density is much lower at these altitudes, the aircraft can travel faster due to reduced drag or air friction.
High jet thrust at lower altitudes is a disadvantage for engine efficiency. A jet aircraft cruising at lower altitudes must reduce power significantly to prevent over speed and airframe damage. The resulting lower thrust with high air density creates poor jet engine performance, and fuel consumption will be higher.
Jet engine performance is optimized when the turbine is operating near 100 percent power. This occurs because only part of the thrust of the engine is due to combustion of the fuel. A large proportion of thrust is the air compressed by the turbine compressor section and passing through the engine or bypassing the combustion process. Most turbine engines are called bypass engines, because only part of the airflow is used for fuel combustion, with the remainder bypassing the combustion section.
As air enters the inlet of the engine, it passes through a series of rotors and blades that compress the air to a higher pressure as it passes through a smaller cross-section. The higher-pressure air is used both for bypass thrust and for combustion air. A discharge nozzle is designed to accelerate the air out the back of the engine as the pressure is converted to velocity, resulting in thrust that pushes the aircraft forward. Combustion gases also drive a series of blades connected to a shaft that runs the inlet compressor section.
Jet engine performance is often measured by specific fuel consumption. This is a defined as the amount of fuel used divided by the net engine thrust. The net thrust is the total thrust of the engine minus the amount of thrust produced by ram effects, or air passing through the engine due to the speed of flight. Specific fuel consumption gives designers standard values of engine performance that can be compared for different altitudes and speeds.
It is also important to understand jet engine performance for situations where one engine fails on a multi-engine aircraft. The remaining engine must produce enough thrust at a specific altitude to permit controlled flight until a landing can be made. In addition, the non-functioning engine creates drag due to air passing through it, an effect called windmilling. Designers must include engine-out performance in engine performance requirements.