Helicopter acoustic signature (noise)

[thebig C]

My area is overflown by a couple of helicopters every day, on average. The noise levels from the different types varies enormously. The Bells - Jetranger, 222 etc. - make the most noise: you can hear them in the distance and still have time to make a cup of tea before they can be seen, and you can still hear them long after they've disappeared. At the other extreme, the EC-120/EC-130s are incredibly quiet: if you don't look out as soon as you hear them, too late, they've gone.

I haven't seen or heard the new Air Corps helicopters - the Alouettes had such a distinctive sound - but I wonder where they would fit in on the noise scale? From a military point of view, I presume a low acoustic signature - being quiet - is a major advantage at times.

[GoneToTheCanner]

Hi there
You would be surprised how quiet a helicopter can be, if it's coming directly at you, especially if it doesn't have the Alouette's big intake and big exhaust.What people don't realise is that the intake directs a lot of noise forward.If you look at S.African AIIIs, they had a big filter over the intake and a turned up exhaust, which diffuses the exhaust signature(noise and IR)thru the rotor. The Brits had it on their Gazelles too, and a fast-moving, low-flying Gazelle is hard to see, hear and hit. The Bells have large, broad blades(the Huey signature noise!) but now have more than two blades and have rigid rotors like the French helis and are much quieter(see Bell 407).
regards
GttC

[Goldie fish]

The teaction caused by the disturbed airflow between the main and tail rotors also causes a lot of noise. One of the reasons FENESTRON equipped aircraft are so quiet.

[hptmurphy]

Dauphin had the classic signature lol

Used to love Irish Helicopter Bell 212s over flying Haulbowline as a recruit....could hear it coming for miles

The quietest I remember was Irish Helicopters MBB 105...but when it got close to the ground..then it got noisy

[thebig C]

The Fenestron seems to make a big difference. That's one of the reasons I was wondering how the Air Corps' EC-135 (with Fenestron) compares with the AW139 (traditional tail rotor).

[pym]

Indeed the Dauphin had a classic sound. Low bass "chop chop" sound in the distance at first, then the high pitched growl as it went overhead.

By comparison the 139 & 135's sound relatively quiet. Although I guess it depends on circumstances, for example a few weekends back I was rudely awoken by the Garda 135T2 at 5am. No other sound around at the time + low flight = grumpy pym.

GttC - in your opinion would modifying the exhausts on the 139's to diffuse the IR signature be a relatively straightforward task?

[Vmax]

As I understand it,the distinctive whistling sound of the the Dauphin as it went overhead was caused by the fenstom, believe ir or not. This was cured in later eurocopter fenstroms models like the 135 by staggering the tail rotor blades at different intervals, interrupting the harmonics of the unit, so to speak.

[GoneToTheCanner]

Hi there
It should be.If you look at the current Apache or Lynx, they use a diffuser type exhaust which mixes cold air with the hot exhaust stream. This has the effect of cooling the exhaust stream, scattering it and reducing noise.Big airliners use the same idea to reduce their noise output. If you look at the hush kits on the old BAC 1-11 or the 737-200, they had "mixer" kits which mixed cold outer air and hot core air and knocked a few DB off the noise levels...It wouldn't surprise me if there were already drawings for a suppressing exhaust, for the 135/139s,which would bolt straight on.It'd be remiss of the manufacturer not to offer on to it's military clients......in older, fully-articulated rotors such as the Alouettes', the rotor blades "flap" up and down more than a rigid or semi-rigid (Bo105) rotor and disturb more air and create more noise. The Gazelle and Bo105 and Lynx are old hat now, but all came about in the mid-70s and revolutionised helicopter flight.
regards
GttC

[Victor]

Although I guess it depends on circumstances, for example a few weekends back I was rudely awoken by the Garda 135T2 at 5am. No other sound around at the time + low flight = grumpy pym.

I regularly get this at 1am (but it wouldn't wake me) - it echoes rather badly in urban areas.

I suspect a lot depends on how the aircraft reacts the environment - different noises reflect differently based on altitude (distance, angle and angle of reflection), urban / rural, open/wooded