Unusual heli over SW London last night?

[Seloco]

Working late last night in my attic office - started hearing a rather strange high pitched fan noise around 00:30Z (22/05) like a cross between a 737/320 in initial climb and a model aircraft, but self evidently neither as it did not seem to be moving much. After a few minutes looked out of window; all that was visible was nav/strobe lights of what appeared to be a helicopter hovering over the Sunbury/Walton area, from which direction the noise was coming. I have never heard a heli make this sort of noise before (and we get a lot here coming along the Thames corridor) - they are usually considerably lower in pitch, with the blade noise more dominant than the engine. With this one the blade noise (at least from my position a few miles downwind) was almost inaudible, with the high bandsaw whine dominating.

Anyone else hear it or have any ideas (it was around for about 15-20 minutes)? I guess it could have been an atmospheric effect of the relatively high wind removing the low frequencies in some way, but my experience of observing helicopters over the years is that the opposite is normally true, and that it is the higher frequencies that get lost with distance. Since it was so distracting, I am intrigued to find out what it was.

[retiredandloveit]

Seloco,
What type of helicopter does your local Police unit operate. Many now use various Eurocopter models to avoid complaints about noise at night. They tend to 'whisper' rather than throb (personal opinion).

[treadigraph]

Met are currently using AS355s which are (in my opinion!) bl@@dy noisy. However I believe some species of Eurocopter (145?) has recently been delivered - maybe that's what Seloco saw/heard? I hope they are quieter!

[Seloco]

The Met Police have Twin Squirrels. We are well used to them keeping us awake at night, but the sound is nothing like that of last night. Surrey Police have EC135s. These have a fenestron of course, which does affect the noise signature but again, I have never heard one at the frequency of last night's.

[My Dad's Little Boy]

I'm just guessing but this is a possibility and I stand to be corrected. It could have been a MD900 because that makes a noise similar to that you were describing.

[Seloco]

It could have been a MD900 because that makes a noise similar to that you were describing.
I think that it is unlikely MDLB. I have heard Explorers a number of times and they appear quieter than "normal" helos. This one seemed considerably noisier!

[batfink2]

Sounds daft, but it could've been any of the above, but due to atmospherics and acoustics, the engine noise has been altered. I've heard an EC135 on a clear day and it appears pretty damn quiet - but the slightest bit of moisture in the air, and suddenly the noise is transmitted a bit further and a lot clearer - giving it a completely different engine sound.

[jayteeto]

The 145s will not be in Met service for a little while yet.

[Huyin]

Could it have been a Gazelle?? Noisy,high pitch whine from fenestron??

[Seloco]

Thanks for all the ideas to fuel my inquisitiveness. I'm coming to the conclusion that it was probably a number of unusual atmospheric conditions (lots of low cloud and high wind) that combined in some way to remove the low frequency rotor noise almost completely and let the fenestron HF "drone" predominate. Given the frequency similarity, I'm wondering how the rotational speed of a fenestron compares with that of, say, an IAE2500 on take-off power - it sounded spookily alike.

[teeteringhead]

5601 rpm strikes a chord fpr the Gazelle fenestron ...

.... amazing what gets stuck in the mind .... (hope it's right!)

[sidtheesexist]

I believe I was waiting at the hold at LHR on that eve when I and my colleague on the flightdeck spotted what we thought was a chinook! Looking at the previous posts more carefully would suggest that it's rather unlikely that anyone could have mistaken a chinook for owt else so this post is probably a waste of time!!.........Rgds, Sid

[treadigraph]

On Friday 26th there was Police colours (dark blue & yellow) EC135 orbiting at low level in the Croydon area - I wonder if the Met have borrowed it to replace a sick AS355? I was hoping it was one of their new toys, nice and quiet! Is the 145 as quiet?

[ATCO Two]

treadigraph,

The Surrey Police operate an EC135 from Fairoaks - UKP36.

[Seloco]

It was back again last night, around 23:30-24:00Z on 27/05. Doing a series of rather consistent clockwise orbits over a number of points just south of the Thames from East Molesey westwards, and sounding for all the world like a large radio-controlled model toy! Very strange noise that carries a long way downwind......

[ThomasT]

They be looking for mus. terrs. Those guys who switched off all the security cameras in the trains, and had a terr. exercise running at the same time and in the same place, to mask their naughtiness. The Cambridge Evening News reported that a wounded eyewitness on the train saw a gaping hole, blown in from underneath the carriage, and NO ONE standing there at the time of the blast. Naughty musis, confusing your honest gov.

[Seloco]

Er thanks ThomasT; wrong thread/forum perhaps?

[Squadgy]

It was back again last night, around 23:30-24:00Z on 27/05. Doing a series of rather consistent clockwise orbits over a number of points just south of the Thames from East Molesey westwards, and sounding for all the world like a large radio-controlled model toy! Very strange noise that carries a long way downwind......
I reckon that http://www.airteamimages.com/15431.html could be it - think it is based at Blackbushe. It was up here a couple of days ago and the noise you mention ties in. Hope this helps

[treadigraph]

Have the Met now got their EC145s - I saw a AS355 early yesterday morning, but at about 0100 this morning there was a very strange sounding helicopter orbiting somewhere over South Croydon (and out of my sight)? Sounded rather like one of those electric R/C models with a the odd bit of rotor-slap thrown in...

[batfink2]

Are you sure it wasn't an electric RC model thing? :rolleyes: