Why Helicopters Make Noise: Germans Show How

[Lonewolf_50]

This is an ingenous bit of research.
DLR Portal - News - DLR researchers first to make causes of helicopter noise visible

A tip of the cap to our friends in Germany for using this tool to illustrate noise / sound propagation.

Will quieter helicopters result? Maybe not next week, but one hopes they can improve the state of the art.

[jimjim1]

State of the art

NOTAR

The MD NOTAR helicopters (e.g. MD 902) are hugely quieter than any other helicopter that I have observed.

I understand that they are slightly more costly to operate and so no one in the UK other than a few air ambulances countenance their use. I suspect that the reason that the air ambulances use them is nothing to do with their low noise footprint but is due to the reduced risk of striking the tail rotor on an obstacle during off-airport landings. There is no tail rotor and so it cannot strike anything:-)

My life would certainly be a lot quieter if the police also selected these aircraft, however I suppose that they perceive that increased noise is a benefit to their mission.

[riff_raff]

Interesting link. Schlieren techniques have long been used to image flows in wind tunnel tests, but this seems to be the first serious attempt with a rotorcraft in hover flight. One limitation with this technique is that it only provides an image of pressure gradient, and the results can be affected by variations in the ambient conditions. In a test cell it is fairly easy to control ambient conditions, but it is very difficult to control ambient conditions outdoors. That's why this test was conducted in a quarry pit.

[ShyTorque]

Interesting stuff, I'd be the first to advocate quieter helicopters.

It has been known for many years that interaction between rotor vortices was the main noise culprit. The early version of the Lynx had a tail rotor design where the blades rotated "forwards" at the top of it's tip path plane. This aircraft made one helluva distinctive racket as it violently chopped up its own main rotor vortices in forward flight. Later versions had the tail rotor rotation reversed so it was going "backwards" at the top.

[John R81]

JimJim- are you suggesting that Surrey Police are missing a trick with their 902? Should they swap it out for a noisier machine?

[RotaryWingB2]

DA

I suspect that the reason that the air ambulances use them is nothing to do with their low noise footprint but is due to the reduced risk of striking the tail rotor on an obstacle during off-airport landings.

I think you'll find it's more to do with politics and what the person doing the procurement had been fed that day. (PR, lies, or a bung.... you decide)

[Robbo Jock]

Bolt a vertical flat plate on the end of each rotor blade - that'll cut down on the vortices and hence noise. Shimples.

Or fit one of those Airwolf 'silent mode' buttons. I never could figure out why they didn't fly around with that pushed in all the time.

[Peter-RB]

RwB2,

Bungs..! surely they went out with the last Labour Government and the Harold Wilson/ George Brown type Politics, come to think of it Toni Bliar could have had a hand in that sort of Brown Envelope type of Business!..Allegedly

Peter RB
Lancashire

[Lonewolf_50]

Originally Posted by Robbo Jock

Bolt a vertical flat plate on the end of each rotor blade - that'll cut down on the vortices and hence noise. Shimples.

As you probably know, each bit of mass one adds to a blade has an impact on vibration signature and the three dimensional loading back at the hub/cuff.

With that considered, quite a few rotor blade designs currently in use apply geometric variations to reduce vortices. (The one I am most famiiar with is on the S-70 family by Sikorsky). The idea of putting something like a "stall fence" out near the blade tip has probalby been tried, but not adapted due to what is does to some other feature of the design. (There are trade offs everywhere, eh? )

Hopefully, the continuing quest for noise abatement can find some areas to improve without putting a damper on performance.

[RVDT]

And some not yet in use……………………….

[MightyGem]

My life would certainly be a lot quieter if the police also selected these aircraft,

True, generally because they seem to be more unserviceable than the 135s.

[Lonewolf_50]

In re blue edge:
1. How much more does it cost per blade?
2. What does that complexity in geometry do to periodic repair / refurbishment / overhaul costs?

On the other hand, looks neat.

[riff_raff]

Rotor aero engineers have a good understanding of what the sources of rotor noise are. The difficult part is coming up with a low noise solution that does not compromise other areas of rotor performance too much. The digital design and analysis tools available now are quite good. And better methods for visualizing flows during testing, like this Schlieren technique, will make it easier to validate the modeling results.

In addition to improved aero design of blade tips, another very effective approach being considered for noise reduction is variable speed main rotor drivetrains.

Here's a fairly recent paper from NASA on reducing noise from rotor tips.