Flight plan

A new design of aero-engine fan could make future aircraft more efficient by turning drag into thrust. Siobhan Wagner reports.

Aero-engine fans that are claimed to reduce fuel burn by 10 to 20 per cent by turning aircraft drag into thrust could be possible thanks to a new design from a Cambridge University research team.

The fans, which would be embedded above aircraft wings, would remove the low velocity airflow from the upper surface of the airframe that is responsible for drag. This approach is called ‘boundary layer ingestion’ — the same technique used for torpedo propulsion. If it can be successfully developed it has the potential to reduce fuel consumption significantly.

The way aero-engine fans are designed now makes them unable to ingest boundary layer, which is a non-uniform flow.

‘If you put a non-uniform flow into a fan it just shakes itself to bits and loses its efficiency,’ said Chez Hall, the project’s principal investigator. ‘At the moment, jet engines are away from the aircraft and hang down on pylons. So they capture just clean flow coming in.’

He added that sometimes the engines experience gusts of wind, which is non-uniform flow, but they only have to survive that for a small amount of time.

‘Whereas the fan we want to design has to live with very un-uniform flow for the entire flight,’ said Hall, ‘it’s got to live with that flow and work efficiently.’

The plan is to introduce this new design initially with aircraft that have blended wings. The engines will be able to ingest more air going over the surface than with the tube and wing designs, such as the familiar airline design.

The researchers have a few ideas on what the design of the aero-engine fan should look like. One idea would be to create a duct that will take in the non-uniform flow before it reaches the fan. The duct will be curved in a way that produces a uniform flow approaching the fan.

The researchers believe the duct needs to be circular to match up with the fan, yet they have considered making the inlet semi-circular in order to capture more of the low-energy flow close to the airframe surface.

They are also looking at non-axis symmetric design. The blades and vanes of current fans are axis symmetric and the same all the way around the circumference. The team would vary the designs of the outlet vanes and inlet vanes around the circumference.

‘If we can tune the geometry of the vanes to the non-uniform flow that is coming in, we can correct the flow’s variation and velocity,’ said Hall.

The team will test the design on a low-speed fan rig — first, with uniform flow and then, with a few modifications, with non-uniform flow representative of the boundary layer of a blended wing aircraft.

These sort of non-conventional engine fan designs are only able to be attempted now because of improved computational methods. ‘We can now do calculations of non-uniform flow going into an engine such as this,’ said Hall.

The idea for the engine fan came out of a project that aimed to create a concept aircraft that creates low noise. ‘We realised that if you wanted to design a plane 20 years into the future it has to be very fuel efficient as well as quiet,’ he said.

While the focus is now on designing an engine that is efficient, Hall said that noise is still some concern. ‘The fact that you have got non-uniform flow coming in and interacting with the fan means there is a potential for extra noise,’ he said. ‘That would need to be looked at, but it’s unlikely we’ll end up with a noisy engine.’

Hall admits that in order for these engines to be viable they will need to be used on non-conventional aircraft, such as those with a blended wing body.

‘I can’t see us moving from tube and wing design for a while, but because there are so many advantages with a blended wing and there is a fuel burn advantage with boundary layer ingestion, in 20 years time aircraft manufacturers will be serious enough to introduce it,’ he said