why are aircraft engines so noisy?

[cooperplace]

having just flown the Jab today, I wonder why its engine can't be made a quiet as a petrol engine in a car. Take a Mazda 3 or Corolla: similar size engine, but almost silent. There's room under the jab engine for a bigger muffler. Would it add that much weight? Some motorcycles have carbon fibre mufflers.

 

A lot of the local opposition to airfields, such as at Aldinga, would vanish if the planes were quiet.

 

Any thoughts, anyone?

 

 

[mAgNeToDrOp]

Most of the noise comes from the Propellor?

 

 

[cooperplace]

the propellor? Ok, maybe it makes a lot of noise, but (i) that engine still seems noisy to me, and (ii) can props be designed for less noise? It would be interesting to see some data on prop noise when they are turned by a silent (eg: electric) motor.

 

 

[facthunter]

If you reduce prop noise it will not be as efficient. ( though if it is done well the prop may be more eficient but more costly} Aero engines are not muffled much either in the intake or the exhaust. A liquid cooled motor has less mechanical noise than an air cooled one.In Europe they have much more stringent standards, but it's extra cost, weight and performance loss. Many planes are very noisy and result in hearing loss for people who work around them. A really quiet cockpit like a glider is quite lovely but you can hear all sorts of "creaking" noises which are a bit off putting. Nev

 

 

[Guernsey]

If you reduce prop noise it will not be as efficient. ( though if it is done well the prop may be more eficient but more costly} Aero engines are not muffled much either in the intake or the exhaust. A liquid cooled motor has less mechanical noise than an air cooled one.In Europe they have much more stringent standards, but it's extra cost, weight and performance loss. Many planes are very noisy and result in hearing loss for people who work around them. A really quiet cockpit like a glider is quite lovely but you can hear all sorts of "creaking" noises which are a bit off putting. Nev

"creaking noises" are why I always talk to myself or sing loudley when working in the garden.

 

Arthritic Alan.

 

 

[eightyknots]

... There's room under the jab engine for a bigger muffler. Would it add that much weight? Some motorcycles have carbon fibre mufflers....

I don't think the weight of the muffler is so much of an issue. The main reason why there's so little muffling in a plane is that designers don't want to lose performance. Even a poorly designed exhaust system on a car can rob up to 10% power from the engine. This is a critical thing on an aircraft.

 

 

[Ferris]

Much of the noise comes from the propellor tips approaching and in some cases breaking the speed of sound. Cessna 210's are restricted at some airports for this reason. The yanks had an aircraft in Vietnam that was very quiet. Supposedly could fly within 200' without being heard. The tradeoff was speed and efficiency.

 

 

[cscotthendry]

having just flown the Jab today, I wonder why its engine can't be made a quiet as a petrol engine in a car. Take a Mazda 3 or Corolla: similar size engine, but almost silent. There's room under the jab engine for a bigger muffler. Would it add that much weight? Some motorcycles have carbon fibre mufflers.A lot of the local opposition to airfields, such as at Aldinga, would vanish if the planes were quiet.

 

Any thoughts, anyone?

Compare how a car engine runs with how an aircraft engine runs and you'll get part of your answer. A car engine spends most of it's working life at 30% throttle or less. An aircraft engine spends most of its working life at 75% throttle or MORE. If you ran car engines like airplane engines, the noise output (from the engine itself) would be similar. Then add to that the prop noise as others have said and there you are.

 

 

[Guest Andys@coffs]

Indeed get an air cooled VW and run them at either 75% or 100% of max rated hp and I doubt anyone will claim pretty quiet!!

 

Trouble is that there are likely many things you can do to reduce noise but none of them, or very few of them, will not weigh something and weight in a light aircraft is something most cant afford

 

Andy

 

 

[kgwilson]

Go for a drive in your Corolla or Mazda but stay in second gear at 110kph on the motorway. Listen to how noisy the engine is even with all that sound proofing with its added weight.

 

Stick your Corolla or Mazda engine with all its peripherals in an aircraft, add a reduction drive & see if it will get off the ground. My guess is that it won't. If it will make a comparison to what happened on the motorway, take away the prop noise & guess what? Probably no difference.

 

 

[turboplanner]

having just flown the Jab today, I wonder why its engine can't be made a quiet as a petrol engine in a car. Take a Mazda 3 or Corolla: similar size engine, but almost silent. There's room under the jab engine for a bigger muffler. Would it add that much weight? Some motorcycles have carbon fibre mufflers.A lot of the local opposition to airfields, such as at Aldinga, would vanish if the planes were quiet.

 

Any thoughts, anyone?

REGULATIONS - that's all.

 

 

[Ultralights]

sad thing is, making aircraft totally silent will not silence the noise complainers. they will then just start to whinge about the pollution they generate... or the contrails, opps, chemical trails..

 

 

[P4D]

I can't get excited about the hum of an electric motor.

 

I can get excited about the roar of a big radial or even a finely tuned two stroke for that matter.

 

I cannot work out why others don't feel the same but there you go horses for courses.

 

 

[boingk]

Aircraft engines are noisy for a few reasons:

 

• High operating rpm
   
   
  
• High continuous power outputs
   
   
  
• Low levels of sound deadening
   
   
  
• Low levels of exhaust muffling
   
   
  

 

 

 

All these contribute to a noisy environment. A car has an effectiv muffling system, layers of sound deadening both inside and outside the engine bay and is running on only a miniscule percentage of what it is capable of operating at. Your typical passenger car will put out 150~200hp but only need 15~20hp to cruise at freeway speeds. Thats a 10% throttle cruise! Imagine doing that in an airplane... it'd be at idle.

 

We cannot afford the luxury of weight and redundance to carry around power we do not use.

 

The propellor also does generate a large amount of noise if tip speed is not kept out of the transonic range over .72 mach. This is the typical loud waaaaaAAAAHHHH! you hear as a warbird or aerobatic plane dives - the blade tips going transonic. For comparison a 72" prop turning 2300rpm will have a tip speed of about .56 mach.

 

Cheers - boingk

 

 

[Powerin]

I know you are talking about cockpit noise, but where I live prop aircraft fly over at several thousand feet. From the ground Jabs are an order of magnitude quieter than your run of the mill Cessnas/Pipers etc. Rotaxes are so quiet you can barely hear them, just a gentle noise from the prop and a bit of a whir from the engine.

 

 

[turboplanner]

Of course the wide open exhaust stubs have nothing to do with it do they.

 

 

[lark]

the c185 prop noise was a horrible sound, think most would have had different props fitted now. Heard this was because the tips exceeded the speed of sound at max rpm!

 

 

[Guest Andys@coffs]

Anything (not turbine) that is used to drag parachutist up....fine pitch prop probably running close to max tip speed = slow and damn loud! Yet engine is the same as something with a more normal prop...

 

Same issue...Glider tugs

 

 

[dazza 38]

Harvards /Texans produce a lot of noise due to the prop tips breaking the speed of sound. A lot more than the run of the mill Cessna 210 sounds when its prop tips break the speed of sound. I reckon

 

 

[boingk]

Harvards /Texans produce a lot of noise due to the prop tips breaking the speed of sound. A lot more than the run of the mill Cessna 210 sounds when its prop tips break the speed of sound. I reckon

I wouldn't think so, dazza; unless I'm mistaken the Texans ran an R-1340 Wasp engine rated to 600hp or so at 2250rpm. That turned a 9ft / 108in prop through a 3:2 reduction to give 1500rpm at max power. This would give a tip speed of about .61 mach. If you overrev the engine to 2500rpm you'd be at around 1670rpm at the prop, giving .71 mach for tip speed - that is in the transonic region and would be making prodigious amounts of noise... but nowhere near supersonic.

 

Cheers - boingk

 

 

[M61A1]

I wouldn't think so, dazza; unless I'm mistaken the Texans ran an R-1340 Wasp engine rated to 600hp or so at 2250rpm. That turned a 9ft / 108in prop through a 3:2 reduction to give 1500rpm at max power. This would give a tip speed of about .61 mach. If you overrev the engine to 2500rpm you'd be at around 1670rpm at the prop, giving .71 mach for tip speed - that is in the transonic region and would be making prodigious amounts of noise... but nowhere near supersonic.Cheers - boingk

You need to also factor in the aircraft airspeed on top of the rotational speed of the prop.

[.Evan.]

I read about this amazing Vietnam aircraft some months back. Looks like it can be done, but many compromises must be made!

 

 

A great history of it's development can be found here:

 

http://www.spyflight.co.uk/yo3a.htm

 

 

[boingk]

You need to also factor in the aircraft airspeed on top of the rotational speed of the prop.

True, but as a function of the speed of sound its still nowhere near it. They specifically design propellor driven aircraft so that these situations are avoided, so even at max power rpm giving a tip speed of .61 mach, you'd have to be flying .40 mach to break the sound barrier with the tip of the prop.

 

Don't know about you, but I sure don't know of any AT-6's that fly at 300mph or more, especially given their regular maximum was 200mph and cruise was 145mpH.

 

Then again, I also don't know much about the ones that they race.... although I think they'd try and avoid it even more due to the inherent inefficiency of running a prop above about .70 mach.

 

- boingk

 

 

[fly_tornado]

wait until you hear a Gold race at Reno. its like a rock concert when they go past the finish pylon.

 

 

[cooperplace]

I wouldn't think so, dazza; unless I'm mistaken the Texans ran an R-1340 Wasp engine rated to 600hp or so at 2250rpm. That turned a 9ft / 108in prop through a 3:2 reduction to give 1500rpm at max power. This would give a tip speed of about .61 mach. If you overrev the engine to 2500rpm you'd be at around 1670rpm at the prop, giving .71 mach for tip speed - that is in the transonic region and would be making prodigious amounts of noise... but nowhere near supersonic.Cheers - boingk

the std jab prop has a sharp cut-off end that looks as tho' it was just cut with a circular saw or something primitive. I wonder if this is the quietest way to do it? I read somewhere that submarine props have lots of research go into their shape, esp. the tips, to make them silent..... I know it's a different situation.