Silent rotor technology

originally posted by: Zaphod58
a reply to: GD

I saw a helicopter flying over California recently that could be heard within about 500 feet or so, as it passed over. Beyond that, you couldn't hear a thing.

The US has had noise cancelling on their special forces choppers for years. I seen a Pave hawk flying towards me up the Florida coast at night, not a big deal as the Coast Guard flies them up and down our area all the time, then it turned out to sea and like someone had just flipped a switch it went completely dark and silent, from maybe 1/4 mile away over the ocean.

No lights however you could hear a light wisp of the blades so I could hear that it was still flying. Made my jaw drop, I was waiting to hear it smack the water but it kept flying out to sea.

Either the special forces were training out of the base near us or the Coast Gaurd has had this gem installed so they can sneak up on drug dealers.

a reply to: JIMC5499
Sounds like a modern case of control flutter..All about resonance control.

a reply to: RudeMarine
Yes the little birds in Nam played around with the tech to begin with.

a reply to: Blackfinger

We were doing low frequency vibration studies on our aircraft back in the mid 80's.

a reply to: RudeMarine

Noise thrown by helicopters is surprisingly directional. You get out of the zone that BVI is being thrown, and it can get pretty quiet. While there is some considerable effort dumped into silencing a PaveHawk, I would guess by your story that you happened to find one of quieter angles when it turned out to sea. Airspeed (relative to the blade tip) is also a factor, so local winds can make a helicopter traveling in one direction louder or more quier than another.

originally posted by: Blackfinger
a reply to: RudeMarine
Yes the little birds in Nam played around with the tech to begin with.

Visual stealth as well with the dim lights on the helo at twilight.

a reply to: RadioRobert

Local winds relative to the listener affect how ALL aircraft, trains, boats and every other noise generating machine is heard. For the listener, local winds might be the single largest factor in what is heard.

With moving noise generating machines, the Doppler effect also comes into play, depending upon speed of the machine.

a reply to: Salander

Well, yes, because wind is moving the medium of propagation. What I was talking about, though, was relative velocity of the blade (especially the tip where it is highest) to the air it is meeting. A small change in that relative speed/direction can have interesting effects on the noise "footprint". Everyone who has been around a helicopter who makes a sudden change of direction has heard how drastically the sound (both type of sound and volume) changes depending on the orientation of the helicopter to the observer and to some degree its relative movement through the air.