Paul Collins reveals a new sort of aircraft

The Brunel Award Lecture at the BA Festival of Science in Dublin was a chance to find out more about the Silent Aircraft Initiative, an exciting project aimed at designing a new concept aircraft that is radically quieter than anything flying today.

The Initiative, funded by the Cambridge-MIT Institute (CMI), is a joint research program between the University of Cambridge and the Massachusetts Institute of Technology. The multi-disciplinary project requires a much more extensive collaboration with industry than in the past. In addition to its long-term objective of laying the groundwork for the next-generation-but-one of commercial aircraft, the project is unusual in that it is trying to change the way engineering is taught, as well as inspiring the next generation of engineers.

Hands-on

The lecture involved a lot of hands-on audience participation, with several demonstrations to illustrate the science of acoustics and the technologies being developed. There were many eager volunteers for one of the demonstrations, which centred on a microphone and ten hairdryers. Not, it turned out, for perfecting the elegance of the audience, but for demonstrating the scale of the noise reduction challenge facing the engineers.

‘The way we perceive noise is not at all what you would expect – it takes a big reduction in noise level before we hear an appreciable difference,’ said lecturer Dr Tom Reynolds. The hairdryers were used as miniature noise sources. Cutting from ten to five, one would think, would sound half as loud. In fact, the difference was barely audible. It was not until nine out of the ten sources were switched off that it sounded quieter. ‘Tests on different groups of people have shown that it takes this ten-fold reduction in noise for the sound to be perceived to be just half as loud,’ he said.

How quiet is quiet?

The engineers on the Initiative are aiming for an aircraft that is inaudible above the background noise outside the perimeter of an urban airport. This requires taking the noise down by half not once, but twice and then by a bit further still.

It is not just the engines that need to be treated. About half the noise from a landing aircraft today comes from the body of the aircraft itself as it flies through the air. So the engineers have to reduce the noise from devices like flaps and slats, as well as from a major noise source on approach – the landing gear.

Noise reductions can also be achieved by changing the way the aircraft is flown and the Operations Team is looking at the way aircraft come in to land.

New trial

The Silent Aircraft Initiative has recently announced plans for trials of new arrivals procedures at Nottingham East Midlands Airport. The trial is the result of eighteen months’ work with the large number of partners. It has to meet all the standard industry requirements and go through all the approvals processes, as it will be flown in controlled airspace using commercial aircraft.

‘What is unique about the trial,’ says Dr Reynolds, who is leading the operations research for the team, ‘is that we will provide advanced modelling tools in the design process that will predict the noise “footprint” of a landing aircraft as well as its emissions, allowing the environmental impact to be minimised. During the trial, we will be monitoring arrivals to check our predictions.’

The trial is a ringing endorsement of CMI’s idea of the benefits of increased industrial engagement. It is based on a continuous descent approach, which keeps the aircraft higher for longer and at lower thrust settings than the approach procedures used now. In the traditional stepped approach there are segments of level flight where the pilot has to increase the throttle settings to stop the aircraft descending. This transition itself can cause annoyance, as well as increasing the level of noise.

Some airports are already flying a basic type of continuous descent approach, but are constrained by the congested airspace around many airports. The advantage of the trial at Nottingham is that there is more flexibility, giving an opportunity for more of the aircraft to use the quieter procedure. The research team are hoping to demonstrate that even in busy periods, the new procedure can reduce pilot and air traffic controller workload and do so without reducing the efficiency of the airport.

A DVD resource for teachers and students based around the Brunel lecture is now in preparation.

Dr Paul Collins is Project Manager on the Silent Aircraft Initiative