why are aircraft engines so noisy?

[boingk]

Marine propellors also suffer heavily from cavitation, where the low pressure induced by a fast spinning and/or improperly shaped propellor will actually physically erode the prop to the point where it becomes unusable. Many early trans-Atlantic captains succumbed to this due to get-there-itis or wanting to break a new speed record for the crossing.

 

Aviation propellors are a bit different, although somewhat similar in concept. Propellors are generally shaped rather complexly (is that a word?) to give optimum efficiency, and one way of making them both quieter and more efficient is by sculpting them with a scimitar-shaped tip as here:

 

 

The full article is about a Hummel Bird in the US and its extensive modifications undertaken by its builder/owner, David Roe:

 

http://www.eaa.org/experimenter/articles/2010-02_hummel_bird.asp

 

Cheers - boingk

 

 

[facthunter]

That type of blade will put twist and bending loads on it as a result of aerodynamic and centrifugal loads. Lots of Russian designed turboprops ran supersonic and were designed to do so. There is a lot of efficiency sacrificed but at the time they were more economical than pure jet and had better range.. Localised areas of the prop may go supersonic, but obviously they would have to be near the tip. The finish becomes important at well as the section and the thickness of it.

 

Some of the Cessna singles have high tip speeds and make a hell of a racket, and have to limit T/O RPM at some places to keep the locals happy. Nev

 

 

[boingk]

Yeah one of the preeminent Russian passenger liners was commissioned by the Japanese I believe... some crazy cruise speed of about Mach .90 at altitude? Insane stuff.

 

- boingk

 

 

[cooperplace]

Marine propellors also suffer heavily from cavitation, where the low pressure induced by a fast spinning and/or improperly shaped propellor will actually physically erode the prop to the point where it becomes unusable. Many early trans-Atlantic captains succumbed to this due to get-there-itis or wanting to break a new speed record for the crossing.Aviation propellors are a bit different, although somewhat similar in concept. Propellors are generally shaped rather complexly (is that a word?) to give optimum efficiency, and one way of making them both quieter and more efficient is by sculpting them with a scimitar-shaped tip as here:

 

 

The full article is about a Hummel Bird in the US and its extensive modifications undertaken by its builder/owner, David Roe:

 

http://www.eaa.org/experimenter/articles/2010-02_hummel_bird.asp

 

Cheers - boingk

wow, that prop looks beautifully finished around the tips in exactly the way the jab prop isn't

 

 

[cooperplace]

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

thank you, that is absolutely fascinating. Among other things, they went to huge trouble to silence the engine. I think this could be done much better with little increase in weight (carbon fibre) but it would probably add a lot of cost.

 

 

[cherk]

thank you, that is absolutely fascinating. Among other things, they went to huge trouble to silence the engine. I think this could be done much better with little increase in weight (carbon fibre) but it would probably add a lot of cost.

and you can spot the exhaust super-augmenters on the aft fuze , sssh quiet!

 

 

[.Evan.]

thank you, that is absolutely fascinating. Among other things, they went to huge trouble to silence the engine. I think this could be done much better with little increase in weight (carbon fibre) but it would probably add a lot of cost.

The prop looks like large paddles!

 

 

 

[facthunter]

It probably has very low rotational speed. Wood Is Good. Nev

 

 

[Old Koreelah]

and you can spot the exhaust super-augmenters on the aft fuze , sssh quiet!

I'd love to know more about these. Every augmented exhaust I've heard (mine especially) amplified the noise.

Perhaps they functioned more as after- silencers.

 

 

[Old Koreelah]

I'm visiting Parramatta at present and the light aircraft which pass over here are damned loud (mostly prop noise). I recall an episode of "Inspector Rex" where the hero parachutes out of a plane over Austria. The Cessna had a car- type exhaust system and muffler the length of the fuselage. Ugly as sin.

 

 

[facthunter]

The augmenters were used to pull cooling air through the engine. Examples Aero Commander and Caribou. They are all exceedingly noisy. The aim was efficiency not noise control. Nev

 

 

[dazza 38]

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

Sorry mate, but Texans/Harvard propellor tips DO break the speed of sound when the aircraft goes into a hard turn. (Wilkipedia)

 

 

[boingk]

I don't see how a hard turn would make a difference, unless of course you are operating high throttle/rpm settings during that turn.

 

Mind you if you're breaking Mach with any part of your prop you'll actually be going *slower* than if you were holding a lower throttle setting due to the sonic shockwave seperating the airflow over the prop and reducing its efficiency. Of course if you have specialized laminar-flow props then that may not be of such critical importance... although you will start to reach the mechanical operating limit of your prop, in any case.

 

- boingk

 

 

[Old Koreelah]

The augmenters were used to pull cooling air through the engine. Examples Aero Commander and Caribou. They are all exceedingly noisy. The aim was efficiency not noise control. Nev

That's why I built augmentors into my Jodel. I could idle on the ground indefinitely without overheating and the engine always ran cool in flight, but the noise was intolerable.

 

 

[facthunter]

boink, the blade angle at higher angles of attack is increased on the downgoing blade (as when pulling "G" in a turn). The speed governor responds to the average blade angle , so there is probably some bad airflow over the down going blade tip. If the engines are direct drive at 2400-2600 rpm the tip speed is high. They certainly are noisy and the best way to get big noise is go supersonic. It only has to be in a localised area. Nev

 

 

[Sapphire]

boink, the blade angle at higher angles of attack is increased on the downgoing blade (as when pulling "G" in a turn).

Answer

 

So if you increase the weight of your a/c, the downgoing blade will progressively increase in angle of attack and produce yaw.

That's if the prop is turning turning clockwise viewed from pilot seat.

 

[Guernsey]

That's if the prop is turning turning clockwise viewed from pilot seat.

So if the aircraft was coming towards you it would be anti-clockwise and thus be totally silent.

 

Alan at it again.

 

 

[boingk]

I understand that, but we're back to the same issue.

 

The blades are turned at a maximum power setting of 2250rpm - meaning blade speed of 1500rpm due to the reduction gear - generating Mach .61 tipspeeds. As I also said earlier, anything over mach .70 will generate VERY large amounts of noise as you try and fight transonic buffeting. I have no doubt that the tipspeed would exceed this.

 

I've also no doubt that the things are noisy, but then again a lot of warbirds and vintage aircraft are. Modern prop aerofoils have come a long way since the 1940's, and often generate less noise and more efficiency at higher rotational speeds.

 

- boingk

 

EDIT: If we're talking supersonic tipspeeds, check out the XF-84H 'Thunderscreech' experimental fighter. It drives a prop at supersonic speeds and reportedly made pilots, groundcrew and anyone in the general vincinity violently nauseated due to the continuous sonic shockwaves. Legend has it more than one person voided their bowels when exposed to the noise.

 

 

[facthunter]

Aren't some of the Texan engine options direct drive ? The whole calculation is based on that. I think you will get shock waves on a metal prop at tip speeds of around M.85 Most other materials are worse than that. This is only due to finish and accuracy and thin-ness? Also you wouldn't just add forward speed to tip speed. It is a vector ( long side of a triangle). and don't forget that Mach no is dependent on temperature.

 

Regarding yaw with the high AoA situation... Yes there is considerable yaw in those situations approximating what happens when one of two engines fails. You have a speed where the yaw is sufficient to exceed the induced roll (aileron capacity and the available rudder ( yaw) This is why so many p51's roll on go around when full power is applied at low speed. The reaction due to torque is minor compared with the thrust assymetry... A wave -off on a carrier is a similar situation where contra props became common. ie Fairey Gannet... Nev

 

 

[Sapphire]

So if the aircraft was coming towards you it would be anti-clockwise and thus be totally silent.Alan at it again.

 

It is totally silent because the prop chewed you up into minced meat anticlockwise.

 

 

[boingk]

I believe the R-975, R-1340 and R-1820 are all reduction drive engines, specifically designed as such to gain maximum efficiency from the prop.

 

Very few radial engines are direct drive, I think its something to do with the housing offering a very convenient platform for the reduction gear... not to mention the very high power outputs!

 

Cheers - boingk

 

 

[facthunter]

Yes I do understand the reduction gear thing, but some research that I have seen indicates that some direct drive engines are out there. The main reason for the reduction is prop size and tip speed, and the need to allow the engine to do more revs and deliver more power. Nev

 

 

[boingk]

You're probably right... I don't know enough about engine lineage to wade into this one. I do know that there are certain aircraft, though, in which you can easily exceed the speed of sound by using high throttle and rpm settings - Glasairs for example. A Glasair 3 with an IO-540 and 80" prop will cruise very easily at 15,000ft and easily be able to break through the Mach limitation of its prop if you aren't careful. Basically you'll notice increased noise and a lowered speed despite higher rpm setting.

 

What I've always wondered though - all this tipspeed stuff aside - is how slow you can get a prop before it becomes unable to provide meaningful thrust. There must be an optimum 'minimum' speed at which you can operate very efficiently but still maintain useful thrust... does anyone know anything about this side of the equation?

 

- boingk

 

EDIT: Part of the reason I'm asking that is I'm thinking of making my own ultralight aircraft with a reduction-drive engine... or two. Part of the whole 'revitalise 95-10' type movement.

 

 

[Sapphire]

By observation I've see microfilm planes with a prop-speed of maybe 60 rpm. They stay up for hours on a few thousand turns of a rubber band. Never seen anyone extrapolate that to a pilot carying a/c.

 

 

[Gnarly Gnu]

I read somewhere the Lockheed mentioned above had a prop speed around 800 - 1200rpm with a reduction drive using a big rubber belt.