I just read a USA Today news report quoting supposedly reliable Pentagon sources as saying that American/British special ops troops have been searching for Osama bin Laden inside Afghanistan since Day Two of the WTC attack. Apparently, small capture-or-kill patrols are using “super-quiet military helicopters” to conduct their missions.
What is the word on these helicopters? I vaguely remember reading a Tom Clancy book many years ago in which a character claims that they are virtually silent–or at least greatly muffled. How loud are they? Are they black? Do they run with any lights? How large? Do we presently have any helicopters with “stealth” features? (Also: why in hell are Pentagon sources giving away this information?)
A friend and I were standing outside at night, in deep fog, circa 1978. We heard something pass overhead, something like we’ve never heard since.
Hard to describe, but it sounded like a giant BOOMERANG, it made a slow swishing sound, and it was flying low. It was not loud - you’d never hear it if you were indoors.
We were near a couple af military bases at the time, so I’ve always wondered…
Anyways, how long has this technology supposedly been around?
I have no notion of what their called but I live very close to a airforce base and I have seen them. They are not black they are greyish and they look like a fat apache helicopter. Also their blades are much much shorter than the normal long ones. And they are next to silent. This is just an idle observation by an avid hiker and fisherman in the southwest.
An H-53 is just about the loudest thing imagineable. 3 engines and seven main rotor blades produce a sound like ten train wrecks at once. But in a desolate country with sparse population, you could just about land a meteor undetected.
Having lived in the desert, I can tell you that sound can travel a long way. Especially in the winter then it’s very cold. On the other hand, high winds will make it much harder to hear things downwind of your position.
FWIW, the quietest helicopter I’ve heard is the McDonald-Douglass MD-500 NOTAR. It seems most of the noise helicopters make comes from the little tail rotor revolving very fast and the engine. The main rotor makes noise as well, but not as much as the engine and tail rotor. (Oh, and the engines I hear most are piston.)
My brother - who among other things is a highly decorated helicopter pilot - nearly spat his drink out when I asked him about “quiet” helicopters : apparently the words “stealth” and “helicopter” are pretty much mutually exclusive.*
Before anyone queried this statement I should point out that my brother was actually a fighter and helicopter gunship pilot. He’s flown all the phantoms and other military planes - if he says something isn’t stealth, I’m inclined to believe him.
As Dr. Lao already pointed out “quiet” is a relative term here. Less quite than a standard UH-60 doesn’t mean the thing is anywhere near silent. Still, I suppose every little bit helps.
I have no idea about this but one would think if you wanted a really quiet helicopter (again, relatively speaking) for covert insertions wouldn’t one guess they’d use a smaller helicopter than the quite large MH-60? Granted you lose a lot of cargo hauling capability but if my goal is just to drop someone off with equipment that one man could carry I would think a smaller helicopter, possibly a NOTAR design, would be preferrable.
Here is a picture of a civilian quiet helicopter. It doesn’t look terribly “special”, but I notice that the rotor blades look unusually broad. NoiseNews has this quote from an civil Helo Idustry spokesman back in '97:
So, while normally, helos are damned noisy, I’m willing to bet that there are designs and techniques for quieting them that are not widely known, even within the rotary-wing community. Now, how effective they are, that’s an unknown, as nowhere can I find a cite for actual db(a) numbers.
I always thought the trick was to simply reduce the rotor RPMs until the tips of the rotor blades are no longer exceeding the speed of sound. If I remember rightly, the term is called “fanning.”
Another solution mentioned in Popular Science years ago is to use ducted thrust from the turbines to replace or augment the tail rotor.
These answers are just from my memory. I’m sure I’ll be corrected if I am wrong here, but I don’t believe I am.
It is quite common for Aircraft in general that are on a combat mission to run without any running lights. How large?
The new RAH66 Comanche incorporates some “stealth” characteristics, but it not a true stealth aircraft in the sense of the word… its just “less” visible to radar than say, an Apache helicopter.
I am not a helicopter pilot or aviation engineer but it is my understanding that helicopter rotors never break the speed of sound. Indeed, the speed of sound seems to be a major limiting factor in how fast a helicopter can fly. It’s not just the rotor speed but also the forward motion of the helicopter that has to be taken into account. The blades are going forward on one side of the aircraft and backwards on the other. The blade moving forward has its speed from the rotor plus the speed of the forward motion of the helicopter to consider.
IIRC helicopter blades deteriorate quite quickly in normal operations. If you had a blade breaking the sound barrier on every pass forward I think the blade would get chewed up REALLY quickly. I also can’t imagine the turbulence created as it passes the sound barrier would be very good for the flight characteristics.
As to the ducted thrust for the tail rotor it has already been done. NOTAR stands for NO TAil Rotor. Here’s a few picture of them (towards the bottom of the page).
Whack-a-Mole is correct that the advancing blades do not break the speed of sound. However, the limiting factor in helicopter speed is more commonly a phenomenon called “retreating blade stall.”
The advancing blade has an easier time producing lift because of the increased airspeed of forward motion of the helo. The retreating blade has the forward speed of the helo subtracted, so it needs to increase its angle of attack to generate the same amount of lift as the advancing blade. As with any airfoil, the blade reaches a point where increasing angle of attack further results in a stall, much like in an airplane wing at low speed.
In an H-60 in a hover, the outermost blade tip is going about 560 MPH (166ft circumference rotor arc X ~5 revs per second = 832 feet/sec = 567MPH). The helo would have to be going close to 200MPH for the tips of the advancing blade to break the sound barrier (approx 760-770 MPH at sea level depending on temperature). In practice, it is generally hard to push an H-60 faster than about 175 MPH (155 knots) due to retreating blade stall.
It’s been a while, but I flew SH-60s (Navy variant H-60) for about 5 years, so I should have a vague idea of what I’m talking about.
I work on these aircraft (as an engineer). I hope you understand that I’m not going to talk about anything that’s not already commonly known.
The MH-60K has no special design features to reduce noise. If you could make one of these as “quiet” as your average semi-tractor rig, it would be a vast improvement. The RAH-66 does have design features that are intended to reduce noise, radar and IR signatures. Things like cost, maintainability, payload, etc. have been “given up” in order to make it less detectable, and to a greater extent that other helo designs, but even the company reps rarely use the word “stealth” to describe it. The rotor is designed to operate at different speeds for different levels of noise vs. maneuverability.
I have heard H-53’s on low passes that actually set off car alarms in nearby parking lots.
I’ve never understood the facination with the “black-ness” of “black helicopters”. We’re talking about guys that come to work, put on a pot of coffee, then check the automatic weapons lockers. They have mottos like “Death waits in the Dark” - you think they want to fly around in mauve and fuscha?
I have a patch from a project undertaken by VX-4 at NAS Point Mugu. It’s all black, with the legen in red “Si ego certiorem facem, mihi tu delendus eres.” Which supposedly means “If I told you, I’d have to kill you.” (“If I made it a certainty to you, I would have to delete you.”?)
