Supersonic flight in a flap

Small flaps mounted in jet-engine inlet ducts may allow supersonic aircraft to fly faster and farther at less cost, say researchers at the University of Illinois.

‘When flying at supersonic speeds, shock waves naturally occur in the engine inlet,’ said Eric Loth, a UI professor of aeronautical and astronautical engineering. ‘The shock waves disrupt the airflow, creating considerable flow separation and significantly reducing engine efficiency.’

To minimise this effect and prevent boundary-layer flow separation, conventional supersonic engines use a bleed system that removes air through holes in the inlet wall and dumps it out the back. While this keeps the boundary layer attached, it also wastes a portion of the ingested airflow.

‘Engine efficiency can be improved by covering the holes with ‘smart’ flaps that bend under certain operating conditions,’ said Loth, project director of a three-year development effort that includes researchers from the UI, NASA, Boeing and the US Air Force.

Flaps downstream of a shock will bend downward, sucking air from the boundary layer into a cavity below, while flaps upstream of a shock will bend upwards, injecting air from the cavity back into the boundary layer, said Loth. ‘Recirculating the air not only prevents flow separation, it also improves the engine’s efficiency, since the air is no longer being thrown away.’

Thousands of flaps would line an inlet. Resembling slips of paper about one centimetre on a side, the flaps are being made from shape-memory materials such as nitinol.

‘Shape-memory alloys are materials that can ‘memorise’ a shape and return to it after repeated thermo-mechanical cycling,’ said Scott White, a UI professor of aeronautical and astronautical engineering. ‘We can design these smart materials to ‘turn on’ and open up under specific conditions of stress and temperature. Their stiffness — and therefore the amount they deflect — can be controlled.’

In a series of recent experiments, White characterised the bending behaviour of miniature flaps under dynamic loading conditions. He and graduate student Sridhar Krishnan monitored the static and dynamic properties of thin nitinol beams as they deflected under various transformation temperatures.

‘Such an analysis is critical to the next stage of our project, where we want to place the flaps under active, closed-loop control,’ said White. ‘A system of smart flaps coupled with a non-linear, adaptive-feedback control system could continually adjust the material properties — and therefore the position of the flaps — for optimum engine performance.’