Here is my Dumb Question. Exhaust verses Noise Level

[SSCBD]

Example aircraft - say a Tecnam - 912 four stroke. But all of the front end engine aircraft have the same problem including GA.

 

If you run the exhaust pipe down and behind you, say 2 thirds down the fuselage or more would the noise level reduce in the cabin. If not why not?

 

Flying for over 30 years and never really understood why we have to put up with such a high noise level in the cabin with the exhaust ending basically under our feet. Yes I understand a bit of extra weight and attachments and may not look nice - But

 

Would it also not remove the smell of the exhaust more effectively as well?

 

 

[djpacro]

years ago I got involved in some noise certification tests for airplanes in Europe - our focus at the time was propeller noise so engine rpm and propeller diameter/number of blades. Longer exhaust was further down the list - a bit of googling revealed this interesting guide:

 

http://www.akletnany.cz/cs/letiste-w9/hlukova-zatez-w83/?stahuj=47

 

 

[turboplanner]

Example aircraft - say a Tecnam - 912 four stroke. But all of the front end engine aircraft have the same problem including GA.If you run the exhaust pipe down and behind you, say 2 thirds down the fuselage or more would the noise level reduce in the cabin. If not why not?

 

Flying for over 30 years and never really understood why we have to put up with such a high noise level in the cabin with the exhaust ending basically under our feet. Yes I understand a bit of extra weight and attachments and may not look nice - But

 

Would it also not remove the smell of the exhaust more effectively as well?

If you fit a muffler, noise will be exponentially reduced. If you add a tailpipe there is a further substantial reduction of noise plus a reduction of "boom". Acoustic engineers spend a lot of time designing a system for a specific engine. Biggest objection with a steel system is weight. By the time you've added baffles, clamps, mounting brackets and bolts the mass is significant. A solution is to design in titanium which is not as expensive as you might think.

 

 

[Geoff_H]

There is no such thing as a dumb question, only dumb answers! With that I will prove my stupidity. A long exhaust will have more internal and external drag! The internal will reduce HP except where it is done to tune an exhaust to increase cylinder clearing rates. Most noise comes from the propeller so I would expect only slight decrease in noise with a weighty, loss producing system. I actually liked the Vxx engines with each exhaust port just ducted individually outside the cabin. The guy that designed the Cozy IV (taken from a LongEze by Rutan) recommends that you only add to an aircraft anything that when you throw it in the air it stays up. Bit extreme maybe. Saying that my little two strokes are getting tuned exhausts, but for power reasons.

 

 

[fly_tornado]

install a turbo and you fix your noise problem, the need for a bulky exhaust and you get some free* horsepower

 

 

[Bats]

A friend bought an aircraft that had been registered in Switzerland; it came fitted with an extended exhaust which included a small muffler and he was told this was a condition of the registration to address noise. I don't know how the Swiss did their noise testing, but can only say that I could not discern any difference between his aircraft and others of the same model in the circuit, nor from inside the cabin. Eventually the extra chunk of stainless steel was discarded at an annual.

 

Similarly, I recently travelled to Sydney towing a large trailer and of necessity travelled a bit slower than usual. I was twice overtaken by Teslas on the highway and found that there was little to no difference in the sound experienced through my open window, between them and a couple of large saloons which passed at about the same time - it was all tyre and wind noise, not exhaust.

 

From my experience I doubt that there is much effective noise reduction to be found through the use of an extended exhaust etc on an aircraft.

 

 

[facthunter]

They do it more in Europe where the noise requirements are more stringent. Tip speed makes the prop noise. Your biggest problem with an extended exhaust is the engine moves around and the exhaust has to cope with that if you go further aft. If you can mount the lot on the engine it helps. Two strokes and wankels make the worst noise. Some turbo's will damage your hearing depending on the frequencies they emit and the noise energy left in the exhaust. Nev

 

 

[turboplanner]

install a turbo and you fix your noise problem, the need for a bulky exhaust and you get some free* horsepower

You better tell that to the car and truck industry; you could make yourself a millionaire!

 

 

[turboplanner]

..Similarly, I recently travelled to Sydney towing a large trailer and of necessity travelled a bit slower than usual. I was twice overtaken by Teslas on the highway and found that there was little to no difference in the sound experienced through my open window, between them and a couple of large saloons which passed at about the same time - it was all tyre and wind noise, not exhaust.

Those internal combustion engines had mufflers, tailpipes, catalytic converters fitted. With those take off, leaving headers only, or stub pipes it would have been a vastly different story.

Certainly tyres noise, gear noise are equal.

 

 

[Oscar]

years ago I got involved in some noise certification tests for airplanes in Europe - our focus at the time was propeller noise so engine rpm and propeller diameter/number of blades. Longer exhaust was further down the list - a bit of googling revealed this interesting guide:http://www.akletnany.cz/cs/letiste-w9/hlukova-zatez-w83/?stahuj=47

Noise reduction in our aircraft is a very good ambition, to keep us socially acceptable for operating near centres of population. If we want the convenience of access to towns, services etc, we have to play our part.

 

However, as DJP has indicated, exhaust noise may well not be the critical factor. In the case of the Seabird Seeker - a pusher - it was prop. noise that determined its conformance to noise limits: a two-blade prop. close to the wings sets up a 'chop' noise pulse that is way more than the exhaust noise. That limited engine rpm ( well, actually prop. rpm, since a Lycoming and direct drive) and affected climb rpm settings ( and therefore climb performance).

 

The gearbox whine on Rotaxes can be an annoyance: there was a float-equipped trike that used to fly around Pittwater, that was like a leaf-blower gone thermo-nuclear - now banned by Council. It would make your teeth fillings ache.

 

However, there is a further consideration one needs to be aware of, if thinking about changing your exhaust configuration. That is back-pressure from the exhaust system.

 

Engine certification / certifying ( if ASTM self-certified) includes the maximum back-pressure from the exhaust. If you increase this, you will invalidate the manufacturer's warranted performance (TBO etc.) and you are on your own from there. Lycoming - for one - REQUIRES a factory-representative inspection (and sign-off) of installations before it will accept warranty responsibility.

 

I applaud all aircraft owners who seek to reduce the noise from their aircraft - but they need to be aware of all the potential effects. Seek professional ( aero-engineer, mostly) advice before you go changing anything, lest your best intentions end up in tears.

 

 

[johnm]

Some of the C182*s (maybe the older ones) ................... seem to have a banshee whistle to them - which i think is more pronounced than the engine noise - from the ground when they fly past

 

Might be the wing stuts

 

 

[turboplanner]

Some of the C182*s (maybe the older ones) ................... seem to have a banshee whistle to them - which i think is more pronounced than the engine noise - from the ground when they fly pastMight be the wing stuts

In a fly by test, the instrument will read the loudest noise.

Same applies to a group of aircraft; the loudest noise from the loudest aircraft will be what registers.

 

If you are testing for a friendly attempt at minimising complaints, instruments are not much good, because often you find that the complainant is irritated by a particular frequency or sequence e.g. at a motor racing track the complaint may start as a complaint against engine noise, but when you get into the house with measuring equipment against the TV Set and ask a few questions, the complaint usually settles on the PA system and the style of the commentator.

 

Given the OP's question wasn't about fly by noise, but he is interested in in-cockpit noise, it would be about the same as a car with and without a muffler - which is very substantial.

 

 

[Bats]

Those internal combustion engines had mufflers, tailpipes, catalytic converters fitted. With those take off, leaving headers only, or stub pipes it would have been a vastly different story.Certainly tyres noise, gear noise are equal

Sigh, perhaps I should just make wild statements about exponential decreases in noise levels. And I have better than a passing understanding of what goes on behind the scenes in an ICE vehicle, but thank you for your condescension.

 

 

[turboplanner]

Sigh, perhaps I should just make wild statements about exponential decreases in noise levels. And I have better than a passing understanding of what goes on behind the scenes in an ICE vehicle, but thank you for your condescension.

Think about what you posted.

 

 

[skippydiesel]

Example aircraft - say a Tecnam - 912 four stroke. But all of the front end engine aircraft have the same problem including GA.If you run the exhaust pipe down and behind you, say 2 thirds down the fuselage or more would the noise level reduce in the cabin. If not why not?

 

Flying for over 30 years and never really understood why we have to put up with such a high noise level in the cabin with the exhaust ending basically under our feet. Yes I understand a bit of extra weight and attachments and may not look nice - But

 

Would it also not remove the smell of the exhaust more effectively as well?

All very strange -

 

I fly a 912 ULS mounted in an ATEC Zephyr. Composite airframe wooden /fabric wings and control surfaces:

 

I can hold a conversation (without raising my voice) in flight, without the aid of headsets, with my passenger.

 

I don't have exhaust smell/fumes. If I did I would be very concerned about Carbon Monoxide poisoning.

 

Could it be that you are so used to flying in "spam cans" you assume all aircraft are noisy & leak dangerous gas into the cockpit.

 

 

[Yenn]

I fly with an un silenced exhaust on an o-320 lycoming. It my be a bit noisier than a Rotax, but it is not noisy for long. I feel the worst noise makers in the sky are choppers, they make a lot of noise and they seem to take forever to go away.

 

The really bad noise makers are normal motorbikes on the road.

 

 

[ClintonB]

Noise cancelling headset?

 

 

[M61A1]

Example aircraft - say a Tecnam - 912 four stroke. But all of the front end engine aircraft have the same problem including GA.If you run the exhaust pipe down and behind you, say 2 thirds down the fuselage or more would the noise level reduce in the cabin. If not why not?

 

Flying for over 30 years and never really understood why we have to put up with such a high noise level in the cabin with the exhaust ending basically under our feet. Yes I understand a bit of extra weight and attachments and may not look nice - But

 

Would it also not remove the smell of the exhaust more effectively as well?

I would be very interested in trying a decent tuned length exhaust on a 912, the factory one is clearly designed to be compact and is not tuned. Knowing the difference a decent tuned exhaust makes to a four stroke motorcycle, I suspect that there are considerable gains to be had. The pipes would need to be longer, which would get them further back down the fuse. A four into two into one gives very good midrange gains in a tuned system. On that note, a nice tuned length aluminium intake could also give some good gains too.

 

 

[Keith Page]

I fly with an un silenced exhaust on an o-320 lycoming. It my be a bit noisier than a Rotax, but it is not noisy for long. I feel the worst noise makers in the sky are choppers, they make a lot of noise and they seem to take forever to go away.The really bad noise makers are normal motorbikes on the road.

What about the Harleys, they would be the noisiest on the road?

KP.

 

 

[kaz3g]

years ago I got involved in some noise certification tests for airplanes in Europe - our focus at the time was propeller noise so engine rpm and propeller diameter/number of blades. Longer exhaust was further down the list - a bit of googling revealed this interesting guide:http://www.akletnany.cz/cs/letiste-w9/hlukova-zatez-w83/?stahuj=47

"This figure illustrates that a propeller change by itself has little influence on noise heard by people on the ground, and that the biggest influence is due to the addition of silencers systems. Noise from an over- flying aircraft is heard much later as it’s approaching, and is not heard much sooner after it has flown by. "

 

It would be interesting to put the data for a C206 on take off beside this DJP because that situation seems to be a common cause for complaint and caused by propellor tip speed as I understand it.

 

Kaz

 

 

[facthunter]

Tip speed goes supersonic and wastes power and makes lots of noise. Quite a few radials do too, but they are usually geared and have less excuse. They don't have to be. The props on the Electra/Hercules are very quiet, so it can be done. Some light aircraft used 3 blade props to get the diameter down a bit. Helps the nosewheel to be shorter as well. Nev

 

 

[facthunter]

Harleys aren't noisy as supplied, SOME people modify them. Generalising isn't nice to the ones that do the right thing. A noisy motorcycle or plane is a total pain, especially on a long trip. Your hearing is precious and you should protect it as much as possible. Planes need some sound deadening . You wouldn't buy or drive a car that made as much noise. If you ever drive an out and out rally car you will find out how awful such a thing is from that point of view, but you are wearing a helmet.

 

Wouldn't truck compression braking be a high level noise? Nev

 

 

[turboplanner]

Wouldn't truck compression braking be a high level noise? Nev

Yes.

 

 

[Oscar]

Harleys aren't noisy as supplied, SOME people modify them. Generalising isn't nice to the ones that do the right thing. A noisy motorcycle or plane is a total pain, especially on a long trip. Your hearing is precious and you should protect it as much as possible. Planes need some sound deadening . You wouldn't buy or drive a car that made as much noise. If you ever drive an out and out rally car you will find out how awful such a thing is from that point of view, but you are wearing a helmet.Wouldn't truck compression braking be a high level noise? Nev

There's a bikies club ( yes, bikies, not motorcyclists) that does a run past my place every Sunday morning - about 30 mostly Harleys, and most of those with shotgun pipes etc. Yes, they create a deep rumble, that lasts for about two minutes.. But - and as a long-time motorcyclist - the thing that drove me to consider taking a gun out, was the couple of kids on the place about 500 metres away with a moto-cross training track (not an official one, just a track on a 10-hectare block) with no mufflers on their MX'ers, going around and around and around and around for 8 - 10 hours a day in summer, with the typical spasmodic throttle wrenching from fully closed to WFO every 10 seconds. I've lived under the Sydney flight path (near Rozelle) for several years and I'd take that over the bloody MX-ers every day.

 

 

[SSCBD]

"This figure illustrates that a propeller change by itself has little influence on noise heard by people on the ground, and that the biggest influence is due to the addition of silencers systems. Noise from an over- flying aircraft is heard much later as it’s approaching, and is not heard much sooner after it has flown by. "It would be interesting to put the data for a C206 on take off beside this DJP because that situation seems to be a common cause for complaint and caused by propellor tip speed as I understand it.

 

Kaz

The 206 is a well known screamer and get many complaints for noise.