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Jet engines: why evermore larger fans?

Published on 22/04/2016 01:00 - By

On commercial aircraft in the last decade, the trend has been towards wider jet engines with diameters increased by 6 to 20 inches (15 to 50 cm). Why? To produce engines that consume and pollute less.

As Stéphane Orcel, an engineer at Safran explains to us, everything depends on the size of the fan; the bigger it is, the less the aircraft consumes: "The engine’s efficiency is linked to two effects: one part of the air passes through the primary flow compressors in the combustion chamber and turbine, producing the engine’s energy. The secondary flow, providing the thrust, passes through the fans, and is the most efficient for consumption. So in increasing the fan diameter we supply more air into the secondary flow, and it’s that which produces the engine’s energy efficiency. The larger the diameter, the greater the engine propulsion efficiency, and thus less consumption."

For example on the LEAP, the future engine of the 737 MAX and A320 NEO, the reduction in consumption is 15%.

But bigger, means more weight and drag for the plane. So what are the constraints of larger engines on planes? As Stéphane Orcel explains:  "They have to be placed under the wings, but at the same time allow the aircraft to move on the runway without touching the engine, which is a limitation for current aircraft".

The solution for integrating tomorrow’s larger engines would thus be to increase wing ground clearance by elongating the landing gear. Today, the record is held by the Boeing 777, on which the General Electric engine diameter is as wide as the fuselage of a Boeing 737, measuring over 11 feet (3.40 metres).

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