Reshaping noise reduction on aircraft

A NASA-backed US company is developing aircraft engine noise-reduction chevrons that can change their shape in very high-temperature exhausts.

Noise-reduction chevrons are a well-established technology to mix the fan flow behind an engine, but they can degrade thrust performance at cruise altitude. Aerospace companies such as Boeing have developed shape memory alloy chevrons that change their geometry during take-off, cruise and landing to optimise noise reduction above airports and retain thrust at higher altitudes.

However, today’s shape memory materials are unsuitable for quietening the hot central core of the engine flow, according to Dr Todd Quackenbush, project leader at Continuum Dynamics.

‘The current crop of alloys reach their transition temperature at around 90ËšC which only makes them candidates for manipulating the cooler fan flow around the edge of the engine,’ he said. ‘The challenge is to develop shape memory alloys that work at very high temperatures. Ideally you’d like chevrons on the outside of the engine for fan flow as well as at the centre to mix the high-temperature core flow.’

Continuum plans to investigate suitable materials such as three-component doped alloys including palladium, platinum and niobium, as developed at NASA Glenn Research Centre. The first step will be to identify the mechanical concepts that could generate the required amount of motion and flexion required, leading to a laboratory demonstration during the next two years.

‘The research will build on work we’ve done in the rotorcraft world where we found a way to prestrain the structure so it requires less force to deflect than you would need with brute force,’ said Quackenbush. A prestrained alloy would need less power to hold it in a particular shape, he said.

‘In principle you could dispense with a great many of the high-temperature hydraulic or pneumatic systems in engines and replace them with shape memory actuation.’

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