Aircraft Noise-Why?

[dublinpilot]

I’m curious about what percentage of the noise generated from your average spam can is generated by the prop, and what by the engine.

Then following on from that, I’m also curious as to if there is an engineering reason why engine noise is so much louder than your average car engine, even when the car engine is revving hard and generating 100% power.

The reason I ask is this. Recently I started to hear an unusual noise coming from my car. It was slight, but nonetheless there. I brought it to the garage who quickly determined that parts of my gearbox needed replacing (thankfully under warranty). I am sure a lot of problems in cars, are discovered by drivers noticing strange noises.

Now in an airplane, if something is starting to go wrong, and a slight noise was giving a clue to the impending problem, you’d never hear it over the engine/prop noise. I imagine that if the noise levels were lower, it may help some problems to be identified earlier. This would lead to aircraft being safer, and an engineers life being made easier.

If I’m correct in this, and giving that most of the complains about aircraft/airports, are noise related, then the question must arise as to why new airplanes aren’t as quiet as cars? I can understand the problems in fixing existing aircraft, but where’s the issue with new ones? Is it weight, cost or is there an engineering reason, why aircraft engines will always be a lot louder than car engines? Or is it a case that most of the noise is coming from the prop?

[Genghis the Engineer]

Cost primarily.

You'll never make an aeroplane as quiet as you'd like - that amount of energy going into the air - generating lift and thrust - will inevitably produce a fair bit of noise.

But, there are many things that can be done to reduce noise. For example:-

- Exhaust silencers (you've got one on your car!).
- Use of gearboxes to bring propeller tip speed down.
- Propeller shape - square tipped propellers are particularly noisy.
- Intake silencers (use of an intake plenum).
- Positioning engine exhaust upwards (so as to take noise where nobody'll complain) instead of down

Virtually none of these will you see on a light aircraft in Britain, but virtually all of them you'll see on any microlight. Why? - very simple, the microlight chaps have absolutely no choice, they are subject to mandatory noise regulations which don't apply to GA. But since they can cope, on the class of aircraft where weight is most significant, it's certainly possible to introduce all of these mods onto a light aircraft.

A few light aircraft are starting to, compare the noise output of a SkyArrow or a DA40 to that of a C172 or PA28 and you'll find these more modern aircraft are far quieter for similar size and performance.

But the sad fact is that until governments mandate maximum noise output for GA, we'll be stuck with what are quite frankly a very noisy bunch of flying machines. A few people will try and keep their aircraft as quiet as possible, but until it's mandatory (and various authorities are told to expedite noise reduction technologies) most won't bother since that extra cost will (for example) put a flying club at a financial disadvantage compared to those who didn't bother.

G

[FlyingForFun]

All good questions. Afraid I don't know the answers to most of them, but good questions anyway!

As regards your comparison with noticing the strange noise from the gearbox in your car, though, I don't think it's very relevant. Cars have lots of rotating parts which are connected to the engine: clutch, gearbox, differential, propshaft and half-shafts, wheels and brakes in even the simplest car.

On the other hand, most of the aircraft we fly have very few rotating parts, other than the engine and prop. The only ones which spring to mind are magnetos (you'd be more likely to notice a problem during the run-up checks than by hearing a noise), alternator, and vacuum pump. All the other moving parts, apart from wheels and brakes, are control surfaces which can be checked thoroughly without the engine running.

Any thoughts on this from the engineering-types who read the forum?

Having said that, though, I can't disagree with the general idea that aeroplanes should be quieter, from a noise pollution point of view if nothing else.

FFF
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[Genghis the Engineer]

Having said that, though, I can't disagree with the general idea that aeroplanes should be quieter, from a noise pollution point of view if nothing else.
And the less we upset our neighbours, the less our privilege to fly gets constrained.

G

[Kolibear]

NVH (Noise, Vibration and Harshness) gets everywhere, even into Pprune.

Why are car engines quieter? Lots of reasons including having to pass a mandatory drive-by noise test. Car engines tend to be smaller than aero-engines, 1600cc = 98 cu in, whereas 235 cu in (which I hope is a standard Lycoming) = 3850cc. Car engines are liquid cooled so there is an insulating blanket of water to deaden the sound. Exhaust and intakes are designed to reduce noise emissions and engine mounts too are tuned to stop vibration from getting into the body shell. In fact, any part which is connected to the engine will be isolated from the body. Finally, a lot of vehicles have undertrays to keep the noise ( and heat :( ) inside the engine compartment, as well as a hood liner which stops noise from escaping via the hood.

Now it is not impossible to add similar measures to an aircraft, but there are penalties in terms of reduced performance and added weight.

[Flyin'Dutch']

It is relatively easy to make aeroplanes less noisy.

The main 'problems' are weight, performance, money and certification regulations.

Until recently it was mandatory to take kit certified by the LBA in Germany off a machine if you wanted to have the much coveted G on your steed.

Not sure, but would hope that idiocy has stopped with the advent of EASA.

[Genghis the Engineer]

Until recently it was mandatory to take kit certified by the LBA in Germany off a machine if you wanted to have the much coveted G on your steed.
I happen to know the CAA people involved in that. The reason for non-approval, was failure of the Germans to provide copies of their approval reports and operating manual in English. Quite why this was such a problem, since it had already been approved by LBA and English is the international language of aviation was never all that clear.

As I understand it, the CAA are still sat there saying "looks fine, just provide us with a copy of these German reports, and you can have your approval".

Damn the CAA when they've genuinely done something stupid or nasty (and I do, regularly), but not this one - it's not fair on some good people there who were being as helpful as they could.

G

[2Donkeys]

as helpful as they could

German is not a difficult language. Would it really be beyond the CAA to pay the small number of pounds required for a translation in the interests of the greater good?

2D

[Genghis the Engineer]

They'd only have to add it to the approval charge (and I'm not sure if anybody provided it in German either).

This is something by the way that many people don't understand about the way CAA, FAA, PFA or any other authority have to operate. You as the applicant have to provide them with everything that they ask for, or a good reason why it's not needed - they are specifically PROHIBITED from going out looking for information, they are there to check what we provide them with.

The reasons are pretty obvious, otherwise they'd be checking their own work, which is bad practice in any sphere. Also CAA staff time costs £130ish /hr, which is a hell of a lot more than anything I've ever got away with charging - so it makes far more sense for me to sort these things out than CAA. Had I been involved in that particular task, I'd certainly have done so as a matter of course, before I even bothered the Gnomes of Gatwick.

G

[Say again s l o w l y]

It's the usual problems of certification costs and a lack of necessity.
Until we start seeing some limits on G/A noise (like the ones for civil transport a/c) then unfortunately nothing will change radically.

I long for the day that a/c become as quiet as cars, how much simpler will that make public relations? Massively.
Light aircraft are incredibly inefficient, the enormous amount of NVH they produce shows what tractors they really are. Actually that's a slur on modern tractors, modern ones are pretty amazing pieces of kit far more advanced than all but the very latest of spam cans.

If we retrofitted current engines so that they produced less noise, then we would see a big weight disadvantage, but if they were designed properly in the first place, then these problems could be minimised.

There really is no excuse for the amount of noise we produce, I've flown Cessnas etc that have decent exhaust systems fitted and the difference is startling from both inside and outside. I know they are expensive (thanks CAA!) but they are the (short-term) future for GA.

[QNH 1013]

I've recently flown a Cessna 150 fitted with an exhaust silencer and subjectively it doesn't seem any quieter inside than a standard C150, so I assume most of the interior noise is coming from metalwork etc. being vibrated by the engine.
I'm not sure how much quieter it is when it overflies at 1500' though.

What did impress me a few years ago was hearing (or not) how quiet the Rotax engined Europa aircraft were. These, of course, have a part-liquid-cooled engine and a prop gearbox plus exhaust silencer. The noise difference is remarkable when a Europa takes off at an airfield, followed by something powered by a 65hp continental engine, similarly when the two fly overhead.

As has already been pointed out. The public wouldn't notice aircraft if they couldn't hear them, and many things would be a lot easier.

[Flyin'Dutch']

Genghis,

Have a lot of respect for your knowledge but think you are here trying to defend the indefensible.

Noise reducing kits are not brain surgery and the LBA is not a certifying club working for a banana republic.

Not to take a more proactive view on this subject is bad from the CAA whatever their reasoning. It is incumbent upon an organisation like the CAA to do what is in the best interest of the population at large.

Getting folks to take off stuff from an aeroplane which reduces pollution (in this noise) which is perfectly safe and acceptable around the corner and on the same aeroplane over here if it was operated on a different register is not a tenable excuse.

No doubt your friends are doing a great job, however common sense should have prevailed.

Anyway we are now in a brave new world with the inception of EASA, do you know if kit like that can be left on?

FD

[Genghis the Engineer]

I agree that noise reducing kits aren't brain surgery, as a rule all that's required is some general engine handling, a repeat of take-off and climb performance tests, and a noise test - probably no more than 2 hours of flying, an hour or so of data analysis, a survey of the structural and fatigue strength of the device, a parts list, revision to the POH operating data and a supplement to the maintenance manual. A week's work tops.

If you've got A-level standard maths and physics, and an appropriate PPL, I could probably train you to do that in a couple of months - although the training would be a bit specialist, I probably took 8 years to reach that level since my education was rather more general.

But what the hell, no trouble at-all for CAA to make that up, it's not going to worry anybody if they got it wrong and a kilo of metal drops off over a town, or if somebody got a performance write-down wrong and as a result an aircraft goes through a hedge trying to get out of a short field.

Is it????

Seriously, LBA are a competent organisation (they've certainly been very helpful to me on occasion), for which reason it was perfectly fair of CAA to agree a read-across approval, asking only for copies of their paperwork. What I fail to understand is the demonisation of CAA, when people with clear commercial interests failed to provide them with information which should have existed.

What would really interest me - and I don't have an answer to this - is why this data wasn't provided? Did it actually exist, or was there something suspect about the LBA approval? I've certainly had this dealing with parts of the European Mainland - approval is claimed, you ask for the evidence to prove it for a UK job, and no evidence exists.

I'm acting as authorising signatory for (as it happens, non-UK certification) of an aircraft from the Czech republic at the moment - which is already approved in Germany, and have amongst other things had to require large chunks of the pitch control circuit to be redesigned as grossly under strength - all down to doing a bit of independent analysis. The thing had been approved in Germany and Cz on the basis of a few back of envelope sums and no testing, absolutely criminal in my opinion. This sort of thing, which I see with monotonous regularity from all over the world (except generally from the UK, because they are used to dealing with an officious and bloodyminded, but usually technically competent authority - and anyhow hardly anybody builds aircraft in the UK anyhow), leaves you with a suspicious mind - and I'm sure that CAA, who see far more of it than I do, are no less suspicious.

G

Defending the defensible.

[Flyin'Dutch']

Ouch Genghis, don't think you took that very gracefully.

No doubt there are things which could be improved. That however is not just the case for matters under LBA jurisdiction.

To make out that aeroplanes or bits thereof, when flying on other registrations than the 'G' are going to fall on your head by virtue of not having gone through the UK approval system is a bit of shroud waving.

Bit disappointing that you suffer so badly from the 'not invented over here syndrome'

[IO540]

To be fair to little planes, the job IS harder than in cars.

1. 90% of the world GA market is in the USA

2. Way over 90% (probably nearer to 98%) of new *certified* aircraft sales are in the USA.

3. In the USA, personal freedom is highly valued, and the public tolerate planes even though they are noisy

4. In the USA, if you see somebody with money, you think "how can I make even more money", whereas in the UK people tend to think "what can I do to take that rich bastard's money away from him so he can't enjoy himself just like I can't". So noise is tolerated a lot more.

5. To really make inroads, one needs

5a Slowly rotating multi-blade props (less prop noise)

5b Faster revving engines of small capacity (a silencer of a given efficiency has smaller dimensions)

For certified planes, one needs engines which have not yet been proven to work in the higher HP sizes, and one needs gearboxes

6. Given the tiny European sales, nobody wants to pay for the development of the engine or the gearbox

7. The tiny European market doesn't IMV attract very talented people into engine design.

Nowadays, designing a really efficient composite aircraft is mostly a computer exercise. One could do most of it in X-Plane. That's why we see so many really nice new planes, mostly in the permit category, and of course most of them are the same shape!

But designing a reliable engine takes a lot of skill and there is nothing to go on - car engines are a very different animal which spend most of their time at low power, so copying them is no good. I am astonished that Thielert are still having their problems, but this just underlines my point. They are where car engine makers were perhaps 40 years ago. Currently, there is no proven geared power unit of adequate power.

Nothing matches the reliability of a crappy old Lyco with a big heavy metal prop screwed onto the end of the crankshaft. One could silence that but a silencer comparable with a car in efficiency would be huge.

The silencer part is easy :O

[Maude Charlee]

Quite a bit of the noise is caused by the prop, as it generates a significant amount of shear due to the different velocities of the air around it. Maybe the fluid dynamics boffins can explain that better than me.

I also read a while ago, that very small reductions in RPM, I think 100 was the quoted figure, give a disproportionate decrease in noise. Wish I could remember where I read it and the exact figures quoted. Purely from my own experience tooling about doing survey work, it is very noticeable inside the a/c when we reduce from cruise RPM to loiter on site just how much lower the noise levels are for a 300 RPM drop.

[IO540]

That's very true, but one needs RPM to make the prop convert the engine power into thrust. So at a lower RPM one needs more blades. That makes a prop more expensive especially if it is VP.

[BroomstickPilot]

Genghis, I noted on one of your previous posts on this thread your reference to CAA staff time costing £130-ish per hour.

Is this a figure derived merely from spreading total CAA costs evenly across the whole staffing establishment of the CAA, or is it a 'cost of service delivery' applied to front line staff engaged on chargeable work, which the total fee income they earn must cover?

BroomstickPilot

[englishal]

3. In the USA, personal freedom is highly valued, and the public tolerate planes even though they are noisy
Not so sure I agree with that one :O Long Beach City are only to pleased to fine you if you set off the noise monitor on the way into 30 at night. In fact even if you glide in with tickover RPM your windmilling prop will set it off (the answer is to fly the VOR approach and not the ILS at night, this takes you inside the noise monitor ;) )