Aircraft Exhusts...

[chrisk]

I was wondering... as there seems to be so many complaints about aircraft noise, why isn't more effort made to make aircraft engines quieter?

Exhaust systems on aircraft seem basic to say the least, I understand that complex systems may add some weight and possibly reduce the power output slightly, is this the reason? ...maybe the rotation of the prop is responsible for the majority of the sound??
If planes were quieter maybe we'd live in harmony with our land loving friends?

I'd be interested in your comments?

[FlyingForFun]

You mean something like this (http://www.wlac.co.uk/wlacnoise.htm)?

FFF
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[chrisk]

Yes that's what I mean, cheers.

seems to make a lot of sense, bit pricey though but aren't all things planey.

[bluskis]

Most aircraft in Germany I believe are silenced, but I also believe there are problems with approval for silencing aircraft in the UK.

[stiknruda]

A couple of points....

Firstly, the engines were not designed to be sound moderated and any change to exhaust design is generally going to be an expensive affair requiring CAA (UK aircraft) approval. The PFA have a silencing competition running but I must admit that I have not being paying too much attention to it, albeit that this should be a less expensive method of gaining approval to reduce the sound footprint of a permit aircraft.

Penalties include cost, extra weight and drag. An additional six pounds of weight equates to 1 horsepower lost in my smaller aeroplane.

Secondly, it is an unfortunate fact that NIMBYs do exist and that they have been known to complain about gliders and hot-air balloons! A couple of weeks back, I am reliably informed by Ludwig, someone even complained about low flying by the Red Arrows!


Stik - quiet Pitts

[Genghis the Engineer]

The page FFF linked doesn't seem to indicate much of a problem. The prices compare well to any other Lycontinental part.

I have heard some very loud whinges along the lines of "those ***s at the CAA won't approve this silencer, they insist on evidence that it doesn't degrade my performance or a POH supplement changing the performance figures if it does, proof that it's not going to damage the aircraft, a JAR-form 1 and copies of design drawings - and ALL IN ENGLISH TOO when it came from Germany".

Or words to that effect. I'm far from the CAA's greatest fan but I suspect in the issue of approval of silencers they've been somewhat maligned unnecessarily.

But I do agree they're a good thing for GA in general if we can have them.

G

[chrisk]

I guess that we'll never totally stop NIMBYs complaining, but every bit helps surely? maybe we should start complaining about people on the ground spoiling an otherwise beautiful view?

I just seems strange that aircraft have developed thus far with what seems like little consideration to noise, (although I'm sure a lot of consideration has gone into it), there must be a reason why less drag/weight is deemed more important than the extra noise created?

[FlyingForFun]

I think there are a couple of reasons why silencers are not as popular as we might hope they'd be.

First of all, they're actually not all that effective. The ones on the site that I linked to claim to reduce engine noise by 15% - but that's just the engine noise. A lot of the noise our aircraft make is actually propellor noise, not engine noise. So the total noise reduction is in fact quite small.

Then there's the reduced performance. In a car, performance equates to not much more than showing off. But in an aircraft, performance can be a vital safety issue, or the difference between making it out of a short strip or crashing into the hedge at the end. Again using the page I linked to as a reference, the manufacturers claim no loss in performance, and I presume they have proof to back this up, but there is still a perceived loss from the casual pilot-observer's perspective, even if there's no actual loss. And there's the loss of 2lb of useful load - again, a perceived, rather than actaul, loss, because I doubt any of us do W+B accurately to 2lb (do you really weigh yourself with the clothes you'll be wearing on the day of flight, for example?)

But I think the biggest problem is the cost/benefit ratio. The cost is not that high when we compare it to other aviation costs. But there is absolutely zero benefit for the vast majority of us - so the cost would have to be practically zero before we'd consider it.

Compare this to West London Aero Club, who have fitted these silencers to all of their club aircraft for which it is approved. They definitely benefit from it. First of all, they are the sole agent for the silencer, so they have to be seen to promote it, and will hope to make back the cost of the silencers by selling more of them. Also, White Waltham airfield, which is owned by WLAC, has a very serious NIMBY problem, and must be seen to be doing everything possible to reduce noise to protect itself from closure. Of course every pilot can help in this respect, but an individual aircraft having a silencer fitted won't make very much difference on its own, whereas the CFI informing local residents that all club aircraft have had silencers fitted can have quite a large effect (again, probably more perceived than actual) on the residents.

FFF
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[Aerobatic Flyer]

At my club in France we will shortly be fitting a silencer to a Lycoming O320 engined Rallye. The system is manufactured by Chabord, who previously made exhaust systems for F1 cars, and it is claimed that there is no performance degradation.

Whether there is much sound reduction is debatable, though!

I recently attended a fly-in where several aircraft were equipped with silencers. When they overflew at low power settings, the silenced aircraft were noticeably quieter. At full power, however, there was little discernable difference.

This is something that has been seized upon by the National Association of Nimbys in France (a miserable group whose real title is C.A.N.A.L. If you read French, and want to be made angry, click here. (http://jp.pago.free.fr/canal/index.htm) ) They did some tests in Toulouse with a number of light aircraft equipped with different silencers, and found the silencers weren't all that great, especially in the take-off / climb phase. This "comforted" them in their view that draconian limits on private flying are required (no training at weekends or bank holidays, no flying near schools or shopping centres (because of the dangers of us crashing on them...) etc. etc.).

This bunch of killjoys have big European plans, if they are able to get their act together. Obviously it's not good enough to take action against a local airfield that bugs them - they need to make sure that action is taken against ALL airfields, everywhere in Europe so that people who aren't able to complain for themselves are given the peace and quiet they deserve!

What we need is quieter engines, not add-ons to the ones that have been in use for the last few decades. Does anyone know if the new diesel engines are quieter?

[IO540-C4D5D]

It's probably difficult to silence an engine (to the effectiveness we are used to in cars) when it develops its peak power at only say 2500rpm. Of course you can do it but the silencer is correspondingly larger (basic acoustics) and where do you stash away something that big?

But in the end the real answer to lack of silencing is the same one regarding the general lack of innovation in light aircraft: Cessna and Piper have had a virtual monopoly of the U.S. market (i.e. about 90% of the world market) for some 30-40 years, and as any good monopoly would, they sat on their a**e for as long as possible. In fact both are still firmly sitting on their a**e now (looked at a brand new C182TS recently, to see what piece of old junk you get for your £200k+?) while Cirrus are outselling the lot...

[bluskis]

And remember auto engines are water cooled, which contributes to sound deadening, the engine hides under a sound deadened steel enclosure, and have two if not three silencer boxes on a long exhaust pipe, and no propellors.

Can't we divert the attention of CANAL to the nasty little motor cycles their sons and daughters race around towns on, I am sure their decibel level at the recipient's ear well exceeds airplanes, and are immensely more frequent.

[LowNSlow]

I've just fitted a silencer to my Auster and it reduced the exhaust noise from VERY LOUD to quite quiet . Quiet to the point where one could hear the valve gear and the propellor. As the aircraft originally had 4 short open pipes as an 'exhaust system' the percieved noise reduction was huge. The weight gain forward of the C of G (around 15lbs) is actually a bonus for me as the Auster is quite tail heavy in it's natural state! Without the 3rd seat mounted in the back it's virtually impossible to overgross it or get outside the C of G limits (uness you are carrying a cockpit full of lead or newspapaers of course).

[FlyingForFun]

The Rotax in my Europa is far quieter than most aircraft engines. Its water-cooled cylinder heads help dampen the noise. It runs at over 4000rpm in the cruise, with a simple reduction gearbox to turn the prop at more normal speeds, so that allows more effective silencing. Still develops plenty of power, though.

Can't see Cessna or Piper fitting them in the near future, though.

FFF
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[Kolibear]

Designing a exhaust system is not just a question of a taking bit of tube and bolting a silencer on to it.

A typical family hatch car system will incorporate a de-coupler to isolate engine motion from the static section of the exhaust, 5-6 rubber hangers to take up thermal expansion and reduce engine noise in the exhaust gas flow from being transmitted into the body structure. Then at least two boxes - a resonator to tune out undesirable frequencies and a silencer to reduce overall noise. The whole system will be tuned to give an engine sound which is acceptable to the customer - sporty for the GT version, or quietly refined for the Luxury version. It will also be tuned to reduce back pressure and improve performance. Finally, the system has to be manufacturable and be easy to assembly. It must weigh nothing, last for ever and cost peanuts.

And thats before you even start thinking about a catalytic converter and zero-lead fuels.

[Lowtimer]

If car-based diesel installations become mainstream I believe we will see significant noise reductions. Partly because liquid cooling makes a lot of difference, but more because these engines will all be geared, and a geared engine can swing a bigger / and/or more-bladed prop at much lower speeds to deliver the same or better thrust as a small prop on a direct-drive engine. This is the most effective way of reducing prop noise which, as others suggest, is the dominant source. However, I believe exhaust design can make a valuable contribution to noise reduction too.

[ratsarrse]

How about we make a deal with the NIMBY's? We all agree to have silencers fitted if they all agree to not use lawnmowers, refrain from having noisy children, have all their dogs put down, and get rid of their deathtrap 4x4's they use to drive their noisy kids 250 yards down the road to school in. Sounds like a fair trade to me.