Turbofan

The turbofan is a type of airplane engine which has evolved from the axial-flow turbojet engine, essentially by increasing the relative size of the Low Pressure (LP) Compressor to the point where some (or in some cases, most)

Related Topics:
Airplane - Engine - Axial-flow - Turbojet

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of the air exiting the unit actually bypasses the core (or gas generator). This bypass air either expands through a separate propelling nozzle, or is mixed with the hot gases leaving the Low Pressure (LP) Turbine, before expanding through a Mixed Stream Propelling Nozzle.

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If the turboprop is better at moderate flight speeds and the turbojet is better at very high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls Royce). Another term used is ducted fan.

Related Topics:
Turboprop - Rolls Royce - Ducted fan

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The difference between a ducted fan and a propeller is that the duct slows the air before it arrives at the fan. As both propeller and fan blades must operate subsonically to be efficient, ducted fans allow efficient operation at higher vehicle speeds.

Related Topics:
Ducted fan - Propeller

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Depending on specific thrust (i.e. net thrust/intake airflow), ducted fans operate best from about 250 to 1300 mph (400 to 2000 km/h), which is why turbofans are the most common type of engine for aviation use today.

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In a turbofan, the LP Compressor is often called a fan. Civil turbofans usually have a single fan stage, whereas most military turbofans have multi-stage fans.

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Bypass ratio (the ratio of bypassed air mass to combustor air mass) is a parameter often used for classifying

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turbofans, although specific thrust is a better parameter.

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The noise of any type of jet engine is strongly related to the velocity of the exhaust gases. High bypass ratio (i.e. low specific

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thrust) turbofans are relatively quiet compared to turbojets and low bypass ratio (i.e. high specific thrust) turbofans. A low specific thrust engine has a low jet velocity almost by definition, as the following approximate equation for net thrust implies:

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Fn = m * (Vjfe - Va)

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where:

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m intake mass flow

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Vjfe fully expanded jet velocity (in the exhaust plume)

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Va aircraft flight velocity

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Rearranging the above equation, specific thrust is given by:

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Fn/m = (Vjfe - Va)

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So for zero flight velocity, specific thrust is directly proportional to jet velocity.

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Jet aircraft are often considered loud, but a conventional piston engine or a turboprop engine delivering the same power would be much louder. (NASA has a web page with details on jet noise.)

Related Topics:
Turboprop - NASA

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