In with the old

Future Engines could revive techniques from the past to cut noise levels and improve efficiency, according to Rolls-Royce.

Director of research and technology Ric Parker said the company’s technology strategy for the next five to 20 years includes a proposal to rearrange a gas turbine’s normal layout.

Unlike a conventional jet engine that has its fan at the front to suck in air to the compressors and the turbines, the new design will place it at the rear, near the exhaust.

‘Aft fan’ designs, where the fan is set beside the turbines at the rear of the engine, had fallen out of favour due to their inability to contribute air into the compressors.

Parker said Rolls-Royce was reconsidering the idea because of the importance of thrust, efficiency and environmental impact. Advanced lightweight materials and technology now make it more feasible.

Two aft fans, each turning in different directions, would effectively provide longer duct length, said Parker, which helps to reduce engine noise. ‘This provides acoustic attenuation of the forward-travelling fan noise, which is the dominant source on take-off.’

The principle is also similar to that of contra-rotating propellers which boost thrust on marine vessels and some mid-20th century aircraft such as the Fairey Gannet and ‘flying wing’ designs like the XB-35.

The second propeller ‘downstream’ of the main propeller recovers energy lost in the swirling motion of the air in the slipstream, but adds weight and complexity to the system.

Cranfield University has already studied how contra-rotating aft fans could be fitted to Boeing’s blended wing body concept. The configuration lifts the air intake clear of the wing and so enables top-mounted (rather than underslung) engines to be located closer to the centre line of the aircraft. This improves the aerodynamics and reduces the load on the wing.

‘The top-mounted engine allows the wing surface itself to be an extra noise absorber, protecting those on the ground,’ added Parker.

Rolls-Royce is also studying better ways of starting the jet cycle and producing electrical power for the aircraft. Usually an external electric motor is used to start turning the shaft, compressors and fan.

There is an alternative in the shape of tip-driven fans, said Dr Mike Howse, director of engineering and technology. These would eliminate the starter motor by using the fan itself.

Magnets would be embedded in the tips of the fan blades and an electric field generated around the outer ring surrounding it. This would drive the fan just like a motor.

Such a device could provide significant advantages in aircraft architecture since the fan could be separated from the turbine-driven generator allowing it to be put to other uses.

Modern materials, aerodynamics and seals, said Howse, would help the company solve the associated problems of air escaping over the tips of the fan blades rather than exerting its force on the blade.