Turning Down Take Off Noise

Dave -   So far, we've heard how aircraft can impact on weather.  There is also a major impact much closer to the ground and that's noise.  As air traffic grows, it's becoming an increasingly important problem.  So to find out what manufacturers are doing to solve the problem, Ben Valsler has been to speak to Rolls Royce's Senior Project Engineer for Noise, Joe Walsh, and their Chief Noise Specialist, Andrew Kempton.

Andrew -   Aircraft noise has been a problem since the introduction of jet-powered aircraft in the late 1950s, early 1960s.  It can be a serious problem for those living close to airports and it restricts airport growth, and the growth of aviation because of high noise levels.

Ben -   So what's actually causing the problem?  Is it the engines?  Is it the movement of the aeroplanes themselves?  What are the issues that we need to focus on to try and solve this problem?

Andrew -   Engine noise has been the principal culprit, but over time, the noise levels we've managed to reduce significantly from the engine, so that means that other noise sources on the aircraft are now becoming to be serious concerns.  The noise of the air frame itself without engines, the noise of the aircraft as a glider, is a significant contributor to the noise when the aircraft is coming into landing.

Ben -   So taking the engine itself, what is it about that engine that creates so much noise?

Joe -   Well when you consider that the engine is providing thrust for the aircraft and the way it does that is to suck in air, accelerate and pressurise it, and exhaust it.  That provides the forward thrust, so one of the sources of noise is created by the air exhausting, creating turbulence, creating noise.  But as engine designs have developed, we've gone to larger fan diameters which suck in more air, exhaust the air more slowly, and so, generate less noise which is good, but also, the fan is there creating noise in its own right.  So although the overall noise is reduced, we now need to consider noise from the fan and other, what we call turbo-machinery components, and find ways progressively to reduce those noise sources.

Ben -   Jet engine designs have changed over the last 50 years presumably with the main aim to get more power for less fuel, and be able to lift more weight, and stay in the air for longer.  Has reducing noise been a consideration throughout that development?

Joe -   Yes, entirely.  We've had a significant effort into noise reduction at Rolls Royce for many years, as have the other engine manufacturers, and the aircraft manufacturers.  And although we've been quite successful in achieving significant noise reductions, there's still more to do.  So the effort will continue, but the nature of the challenge is changing.  As we've progressively reduced the traditional noise sources, other noise sources come along and we need to continue to address those, looking at the problem as a total system problem.

Ben -   So, as well as the inevitable noise of having to push a lot of air backwards quite quickly, what other parts of jet engines make noises?  What other designs can we put in place that can cut down some of that noise?

Andrew -   The fan noise which is generated by interactions between the fan and hardware behind it can be reduced by careful design of the fan, where we're using computational methods, very big computers to assess what noise is created and to minimise that, at the same time as insuring the fans are very efficient, but we also have acoustic liners which we line the ducting of the engine which help to absorb the sound.

Ben -   So it's not just about reducing the sounds that are created in the first place, but it's about managing how they then travel away from the engine so that we can reduce how much actually gets out into the environment.

Andrew -   Yes, and there are even some designs which try to use the air frame itself to shield the noise coming from the engine to ensure that that noise doesn't get down to the communities around airports.

Ben -   What difference have we been able to make so far?

Andrew -   Well only a small amount of the energy in an engine actually comes out as noise.  My university professor used to tell me that the noise of an FA Cup final crowd was enough to fry one egg, and the energy that's in the engine is very, very much more than that.  We have been able to reduce the noise by 99% or 99.9% over the last 30 or 40 years, but unfortunately, the human ear does not respond linearly to changes in acoustic energy that's reaching it.  So, although we've reduced the sound power to 0.1% of what it used to be, this is only represented by perhaps a factor of 8 reduction in annoyance for people listening to the aircraft.  But considerable strides have been made and work is continuing to drive down noise further.

Ben -   I imagine that there are other things that we can try and do.  For example, make the engines powerful enough to take off very quickly or to perhaps reduce the length of runway that they need so that actually they're closer to the ground for less time.  Is this something else that we consider as well as the noise that the machine itself makes?

Joe -   Yes because ultimately what we're trying to do is reduce the annoyance received on the ground and there are many factors which cause annoyance, not only the sound intensity but also the character of the sound; What frequencies are present? How long does the noise last for?  So when you're considering how to mitigate those things, factors such as aircraft performance, thrust required to take the aircraft off the ground, are all variables, and they're considered as part of the initial designing concept evaluation to look at the most effective way of meeting all of the aircraft requirements and all of the engine requirements including reducing noise.

Ben -   I assume that everything has to be a bit of a compromise.  We can't purely aim for a very quiet engine that actually produces a huge amount of CO₂, and likewise, we can't aim for a super efficient engine that will deafen people nearby.

Joe -   Yes, design trades are involved.  It's useful sometimes to look at what it would take to design a very quiet engine.  So we were involved with some work called the Silent Aircraft Initiative, and that looked at what it would take to design a functionally quiet aircraft that would be imperceptible above typical background urban noise levels.  That aircraft looked a bit like a flying wing.  It had exceptionally low projected noise levels.  It had significant technical challenges associated with it, but in terms of an extreme design, it did show that measures could be taken to generate very low noise levels.  However, there was a downside.  The downside is that the lowest noise design doesn't coincide with the best performance design.  So inevitably, there is some balance required and what we're looking to do is to achieve low acceptable noise levels whilst designing an aircraft which meets many other requirements - operational, safety, economic, emissions, chemical emissions as well, and we're part of that design trade, and we're developing noise technologies that allow the best possible progress on noise reduction whilst allowing progress to also be made on other attributes.

Dave -   That was Joe Walsh and before him, Andrew Kempton, both from Rolls Royce speaking with Ben Valsler.