Cessna’s blue-sky thinking on aircraft power

Washington DC
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Cessna has applied for a US patent for a module that attaches to a turbofan engine to provide electrical power to aircraft systems, eliminating the need for traditional geared power generators.

The Wichita-based company proposes to add a free-spinning turbine wheel to an engine specifically installed to spin up a generator to supply electrical, hydraulic, or other power varieties to an aircraft.

Engineers say the device would be able to run at a constant speed using variable pitch stator vanes just ahead of the turbine. From a maintenance perspective, Cessna says the unit would attach the engine as an independent low-pressure turbine module, but that it would not affect the balance of the existing LPT since the two are not connected via a shaft.

According to the filing, the electrical services turbine could be placed at a various locations throughout the engine, although the application shows the new turbine as the last stage in the engine, just forward of the exit nozzle.

Cessna says the idea is beneficial for many reasons, including that an optimised free-turbine generator will cost and weigh less than a conventional spool-connected generator arrangement, which requires the combination of a generator and an accessory gearbox.

Part of the weight savings comes because there is no need for a power conditioning unit since the free-turbine generator spins at a constant speed and thereby outputs nearly constant power, says Cessna.

The company also says the new design, which would be available as a retrofit or forward-fit option, can provide significantly more electrical power than generators attached to either the high- or low-pressure spools.