Chinooks. Not exactly stealthy, are they?
 

Caveat: I know naaaathiing, this is a genuine question.

I was working in my garden office this afternoon when I heard *that* sound (not unusual in my area). I was due a break, so I wandered out and looked west. A *long* time later a Wokka hove into view at reasonably low level. It occurred to me that in a hostile environment I'd had enough notice to order a MANPAD on eBay with a pretty good chance of it showing up in time. Is it just me, or does these things announcing their arrival quite so well make them a little... vulnerable?

Stealth wasn't in their spec, so it wasn't designed for it. *shrugs*
Comanche was designed as Low Observable and was a LOT quieter than any other helicopter I've ever heard (or haven't heard) but that one went away 20 years ago.

Reminds me of the story about the RNZAF in their UH-1Hs chasing deer poachers in NZ's national parks.
The poachers were in 500s - and had ample time to disappear due to the Hueys being so audible from miles away.
That 50 foot of blade slap was something else.
And yet the one time I strapped into one of those side-facing seats on the transmission tunnel with the sliding side door open - the big Huey was surprisingly quiet to ride in.
Noise projected outwards I `spose...
Anyone ridden in the Chinook- are they also relatively quiet inside in flight compared to outside when heard from ground?
Have heard a few Aussie ones flying over here in Sydney and up in Townsville.
What struck me was that they weren't so much really loud, just had a very distinctive sound... unmistakeable.

There is no way on earth you could call a Chinook quiet inside!

minigundiplomat is well placed to answer this question but from my memory, the noise signature of a CH47 over the sea in still wind (ie as controlled as it gets) was assessed to be either 10nm or 15nm for planning purposes (ie at 10.1 or 15.1nm you would not hear anything). wind direction and speed can have a profound effect on the way the noise travels.

Mentioned over on "spotters forum" that on Tuesday morning I heard a distant and familiar whopping noise while walking through some local woodland and surmised Chinook - check on ADSB confirmed it was indeed (apparently there were actually two) some 15 miles away over Park Royal in West London, heading south towards the Thames heli lane. Even allowing for me being on the highest point in Croydon, reasonably divorced from traffic on a nearby main road with (had there been no trees) a clear line of sight and a favourable wind, I was quite astonished that the sound could carry across the bulk of London and all the ambient noise of a weekday morning.

(Always used to amuse me that on the reality Cop TV programmes when the Met Police's "India 99" EC-145 was being used for tracking suspects on foot, the voice over would breathlessly announce that "they have absolutely no idea the chopper is there!" Really? They must be :mad: deaf then as the EC-145s, (and the earlier AS-355s and Bell 222s) are very audible from miles away - having had the things orbiting my area in the wee hours on countless occasions, I'd be amazed if many locals were still asleep.)

Many years ago, when based at a certain not so secret helicopter base, our dog would bark, the room divider window panes would visibly begin to vibrate, a certain faint rumble would be heard, then shortly afterwards a Chinook would call up to rejoin the MATZ.

A Chinook in level flight is quieter the heavier it is.

I read somewhere in Vietnam that the UH-1's would approach on an angle so that it was hard to differentiate from which direction they were coming from, might be true but I do not know.



..

On my one Chinook ride, we had the rear door open thoughout and it wasn't too bad, mind you everybody was wearing headsets which would have muffled the noise a bit.
A few years ago (about 2016 I think) I was out in my back garden 'slugging' (unfortunately we've never been blessed with a resident hedgehog) at 4.25am and on H3, about half a mile north of me and about 2 miles east of Bagshot Mast a pair of wokkas ambled along westbound at about 1000ft and I was amazed how little noise they made in the otherwise dead calm of the dawn; somebody on here said it was a new design of rotor blade.

Bell's 214ST Flight Test.
 

Spring - Summer 1982: Three flight test 214ST aircraft located South of Bell's flight test facility in Arlington Texas, 18401, 18402, and 18403 were finishing up their morning's flight cards and joined up to return to the test center. As we approached the control tower gave us the 'knock-it-off' command as we had just cracked the tower's window! Ott

I doubt that you heard them from 15 miles.
You probably heard the other two chinooks, 4 miles away, with their transponders swiched to stby and not showing on ADSB.

Given the time of day, the sound could have been reflected upwards by a layer of cold air at ground level. The converse is also true in that I can hear traffic from many miles away on a cold and frosty morning. Once the sun gets to work, the sound disappears.

I’ve heard some excuses for bad navigation before but that’s a really good one! :p

Not that it needs to be stealthy, but the S-92 isn't much better for ~1/2 the weight & the A400M can clearly be hear at around 3 million miles away.

I recall being told that when the BV234s were being trialled in the Frozen North someone, scheduled for the next rotation, heard one coming and went outside to watch it land. He gave up and came inside fifteen minutes later frozen to the core - and it landed thirty minutes after that.

Travelling to a Coningsby Airshow in the late 1980s when their ATC contacted us and said they had audible but not visual with our UH-1H on a bright sunny day….we were some 12 miles away!

Some sideways action allows the ‘buzz’ of the Tail Rotor to shine forth!

While they were introduced to Lippitts Hill as ”the quietest in their class” to the local residents, EC145’s were a clattery cacophony on the ground and only slightly noisier than their predecessor AS355Ns. However, once in the air they were quite quiet - especially from below and surprisingly inaudible at observation distances using that nice 15x(?) camera/footstep thingy.

Still not as quiet as the Explorers used by some ambulance helicopters.

A US study into D model rotor noise, amongst other things.

https://apps.dtic.mil/sti/pdfs/ADA191059.pdf


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