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Acoustic Stealth or how i get a aircraft quiet

Michel Van

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there Stealth for Radar
but how about Acoustic quiet stealth aircraft ?

Lockheed had build the YO-3A "Quiet Star" propeller-driven monoplane designed to be as quiet as possible.
that was in year 1970, now 37 year later Wat can they do ?

can US quiet they jet stealth aircraft ?
or famous "Black Helicopters" how hover quiet in conspiracy internet pages.

is it possible to build a quiet helicopter with subsonic rotorblade ?
or are there other technology ?
 

sferrin

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Michel Van said:
there Stealth for Radar
but how about Acoustic quiet stealth aircraft ?


Fly it REALLY high. :)

Michel Van said:
is it possible to build a quiet helicopter with subsonic rotorblade ?
or are there other technology ?

All helicopters have subsonic blades.
 

doggedman

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There is a means to treat the acoustic signature of fixed wing jet aircraft. The physics is quite simple. Conventional low/high bypass jet engines emit low frequency noise which readily propagates through the atmosphere (you can easily hear it). By shifting the emitted noise to high frequency, the atmosphere will attenuate the noise quite rapidly and hence the receiver on the ground will only hear a faint whisper (if anything at all).

That was the good news. The bad news is that in order to accomplish the frequency shift, you need to make a nozzle with many smaller holes vice the one large aperture that you find with typical jet engines. The thrust loss and installation penalty is usually a hefty price to pay which is why the concept has certain practical limits.

Helicopters are quite another thing as the bulk of the emitted noise that's transmitted over long distances comes from the rotating blades. While there are blade tip treatments that have demonstrated effectiveness, at close range on a cold day when acoustic conditions are favorable to an observer on the ground, there isn't much that can be done to address the emitted noise.

That said, one of the more innovative concepts I've seen to address the problem of emitted noise from rotating wings is something called the stop-rotor. The stop rotor aircraft takes off and lands just like a normal helicopter; but during cruise (and while penetrating enemy airspace), the wings are locked in a fixed position and so the aircraft is configured as a normal fixed wing aircraft. Except in an LZ (landing zone), the aircraft could be configured as any stealth aircraft.
 

sferrin

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doggedman said:
That said, one of the more innovative concepts I've seen to address the problem of emitted noise from rotating wings is something called the stop-rotor. The stop rotor aircraft takes off and lands just like a normal helicopter; but during cruise (and while penetrating enemy airspace), the wings are locked in a fixed position and so the aircraft is configured as a normal fixed wing aircraft. Except in an LZ (landing zone), the aircraft could be configured as any stealth aircraft.

Yeah but then that's not really reducing rotor noise because it isn't on. Kinda like saying you could reduce commercial jet noise by not flying them.
 

Just call me Ray

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You can use a sound generator to emit sound waves on a reciprocal frequency to those made by the engine, therefore nullifying them. Probably the most practical way. You can also physically shield from noise the same way you would from radar or IR - the B-2 is a very, very quiet machine.
 

yasotay

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sferrin said:
doggedman said:
That said, one of the more innovative concepts I've seen to address the problem of emitted noise from rotating wings is something called the stop-rotor. The stop rotor aircraft takes off and lands just like a normal helicopter; but during cruise (and while penetrating enemy airspace), the wings are locked in a fixed position and so the aircraft is configured as a normal fixed wing aircraft. Except in an LZ (landing zone), the aircraft could be configured as any stealth aircraft.

Yeah but then that's not really reducing rotor noise because it isn't on. Kinda like saying you could reduce commercial jet noise by not flying them.

And the stopped rotor technology has been put on the back burner for a while. Another way to reduce the noise signature of rotorcraft is to change the plan of the rotor tip (as in tilt the rotor). While the V-22 is by no means silent, I can tell you from first hand experiance that it is exceptionally quiet compared to a conventional helicopter.
 

Jemiba

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"You can use a sound generator to emit sound waves on a reciprocal frequency "
This method works relatively good in a small environment, like a passengers cabin, but
to reduce the noise outside, it won't work. noise is emitted by every single rotor blade,
which is constantly changing its position in space and relative to the fuselage of the
helicopter. You would have to emit the sound caused by the blades with a 180° phase
shift in all directions with the original, very high amplitude, not to mention phase shifts
caused by interactions between rotor and fuselage. Calculating this "contra sound"
probably couldn't be done in real time, not ot mentione emitting it .
 

doggedman

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While I'm not sure I understand what a reciprocal frequency is, I believe the method you might be referring to is active acoustic attenuation which is also referred to as active acoustic cancellation.

An active acoustic attenuation system employs a receiving array, adaptive filter, processor, and an emitter to receive emitted signals (noise), measure signals, and then emit signals 180 degrees out of phase to the incoming signals thus producing destructive interference waves. The result is an interference zone wherein all noise that can be perceived by a local receiver within the zone is nullified.

Relative to the intended application, I see two practical limitations imposed by mother nature.

1. The receiving array (for capturing the signals to be cancelled) cannot be collocated with the emission source as the interference zone would of such small size as to render system effectiveness of no practical use. The active acoustic attenuation system as described would be useful for a public address system as long as the the receiver/emission array is of sufficient distance away from the noise source such that a receiver (listener) can be positioned within the cancellation zone.

2. More importantly, in order to function, the emitted noise must come from a stationary source. The concept, as described, would not be useful for an emission source with any velocity. In a situation where the emitted noise (acoustic waves) are propagated simultaneously to the input device, the canceling acoustic wave generated, even with a small time delay, would be lagging behind the original acoustic wave issued from the emission source. If the time delay for producing the canceling attenuation signals are Δt millisecond, and the velocity is V, then the canceling signal waves travel behind the original emission source by distance of D=VΔt. The canceling signals simply cannot catch up with the original sound waves from the emission source because both are propagating in the air with the same velocity.

With respect to the noise emitted from the V-22, I note the following: 1) the V-22 blade design is composite and the characteristics of the composite material does diminish emitted noise; 2) the design of the nacelles also uses methods to reduce emitted noise from the propulsion system and APU; 3) the V-22 can make a low noise approach which calls for the wings to be in a 40 degree position for most of the decent and begin a 90 degree rotation close to the ground. The proximity to the ground when the thrust is directed vertically limits the size of the "acoustic bounce" and the amount of noise generated. These and other reasons (range primarily) are why the Special Operations Forces fought so hard for, and managed to save the V-22 when former Defense Secretary Dick Cheney fought so hard to have the program killed.

If you are in search of a true low acoustic design that is compatible with RF low observables and offers the capability of vertical lift, might I suggest the fan-in-wing concept. While this concept does offer most of the practical characteristics you seek, I note that prior designs needed to tow a gas station behind them in order to achieve practical range. That said, concepts were conceived in the late 1980's and early 1990's before the advent of advanced composites. So it might be possible today to design a more structurally efficient airframe that could make the concept practical.
 

doggedman

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With respect to the stop rotor concept being on back burner...it's not over until the fat lady sings.

http://www.seeop.com/spinwing.htm
 

fightingirish

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quellish

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Jemiba said:
"You can use a sound generator to emit sound waves on a reciprocal frequency "
This method works relatively good in a small environment, like a passengers cabin, but
to reduce the noise outside, it won't work. noise is emitted by every single rotor blade,
which is constantly changing its position in space and relative to the fuselage of the
helicopter. You would have to emit the sound caused by the blades with a 180° phase
shift in all directions with the original, very high amplitude, not to mention phase shifts
caused by interactions between rotor and fuselage. Calculating this "contra sound"
probably couldn't be done in real time, not ot mentione emitting it .

Oh no, this last bit is not correct at all. Many modern consumer electronics have active noise cancellation, which is exactly what you are talking about. The signal processing requirements were a problem in the 1970s and 1980s, but are not today. In the 1980s and 1990s there was a lot of work done behind closed doors with exotic signal processing techniques to solve some of these problems. Merging sound and optical signal processing, all kinds of very exotic stuff. Now you can buy off the shelf chips for this sort of thing.
Still, yes, trying to use this approach for the sounds generated by, say, a rotorcraft is not practical. For the *sources* of the noise, such as internal vibration, harmonics, etc. yes, this is actually a viable approach. Many of these sound sources are small in strength or amplitude but produce large effects, especially at a distance. Countering the sources inside the airframe using active techniques does work.

The same principals also work for other kinds of active signature reduction.
 

Lauge

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I would also submit that it depends on what kind of "stealth" you want. Do you want to conceal the fact that an aircraft is operating in the area, or do you "just" want to make it harder to pinpoint exactly where ?

As an example, I've been in close proximity to an AS550 Fennec light helicopter, operating at low altitude and taking appropriate cover behind terrain and trees about half a mile from our position. Could we hear it ? You betcha. Could we tell where the sucker was at ? No dice. BTW, it's the same thing with an MBT. Lots of noise, but non-directional. It's spooky to know that there's a 60 ton tracked vehicle with a big gun in the area, but not knowing where..... :p

But if you want "weally, weally quiet", you need some more advanced stealth technology. With a rotorcraft, I would assume you could reduce noise (or at least change it enough that it doesn't sound like a rotorcraft) by reducing rotor speed. With all the technological challenges that carries with it.

Regards & all,

Thomas L. Nielsen
Denmark
 

Mr London 24/7

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Hello all,

The CIA's Vietnam era "Quiet One" OH-6's are interesting as far as the Heli discussion goes:

http://www.airspacemag.com/military-aviation/the_quiet_one.html?page=1
 

quellish

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mr_london_247 said:
Hello all,

The CIA's Vietnam era "Quiet One" OH-6's are interesting as far as the Heli discussion goes:

http://www.airspacemag.com/military-aviation/the_quiet_one.html?page=1

That program was really the genesis of both LO rotorcraft and special operations-specific helicopters. The "Quiet One" later lead to the CIA SEASPRAY special mission MD-500s, which directly lead to the AH-6 and MH-6 used by the Army. Hughes continued to work on quieting technology, some of this surfaced in a 1990 AW&ST article on new quieting technology for the MD-500.
Some of the technologies developed for the A160 UAV also can be used to reduce the noise signature, from what I understand.
 

500 Fan

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Does anybody have a link to the AW&ST magazine article from 1990 mentioned in relation to the "new" quiet technology for the MD500? Thanks.

500 Fan.
 

Orionblamblam

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quellish said:
Many modern consumer electronics have active noise cancellation...

Which works when the receiver - a specific human ear, for example - is at a well defined point in space. The source of the sound produces spherical waves; the cancellation system produced spherical waves. At specific locations, the waves cancel; at others, they amplify. Like so:


This works at the small scale, where the receiver is essentially *inside* the sound cancellation system. But to cancel the noise of a rocket, a jet engine or a helicopter blade for the *outside* universe, the source of the sound cancellation would have to be precisely co-located with the source of the sound to be cancelled. But rocket, jet engines and helicopter blades are *not* point sources, but instead large complex structures that make noise not only in themselves, but attoparsecs and even fathoms away. A lot of the noise for rockets and jet engines is created at the shear plane between the supersonic exhaust and the static air; as the exhaust grinds to a relative halt, several percent of the kinetic energy is transformed into noise. And this can take place a rod or two aft of the exhaust nozzle. You'd have to perform active sound cancellation all along that length. This would not only require a whole lot of computational power, but powerful Star Trekkish tractor beams that can manipulate air at the molecular level over an area of perhaps several dozen picoTexas.
 

quellish

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There have been a couple of recent attempts to use active noise cancellation on helicopters, such as:
http://www.mendeley.com/research/helicopter-active-noise-control-system/
But these have been focused on gearbox/transmission noise, which is a minor component of a helicopter's acoustic signature.

Griffin_314 posted a link on DLR to a document that is a 1975 survey of helicopter quieting techniques. In his post he summarizes it well (the document itself is very garbled at points):
http://dreamlandresort.com/forum/messages/36322.html
 

Abraham Gubler

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Orionblamblam said:
quellish said:
Many modern consumer electronics have active noise cancellation...

Which works when the receiver - a specific human ear, for example - is at a well defined point in space. The source of the sound produces spherical waves; the cancellation system produced spherical waves. At specific locations, the waves cancel; at others, they amplify.

Active noise cancellation is used in moving devices but inside the vehicle to cancel the noise before it can be radiated outwards from the outer shell. Particularly in submarines. For an aircraft such an application could be set up around the engine and transmission to reduce noise. The Lockheed Q-Star used a belt transmission to reduce noise.
 

Orionblamblam

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Abraham Gubler said:
Active noise cancellation is used in moving devices but inside the vehicle to cancel the noise before it can be radiated outwards from the outer shell. Particularly in submarines.

That would be *relatively* easy compared to an aircraft. The noise-makers on a sub are largely *in* a sub, and have to transmit through the shell. A fast enough system could detect the vibration of the shell and cancel it out. it would require a metric buttload of little "speakers" mounted all over the inside of the shell, but it would be doable.

Note that the prop, on the other hand, is an external noisemaker, and these days are surrounded by a shroud, in part to keep the noise down.
 

shockonlip

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Also wanted to point out that Ufimtsev's PTD is also an acoustic theory,
and not just an E-M theory.
 

quellish

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Orionblamblam said:
Abraham Gubler said:
Active noise cancellation is used in moving devices but inside the vehicle to cancel the noise before it can be radiated outwards from the outer shell. Particularly in submarines.

That would be *relatively* easy compared to an aircraft. The noise-makers on a sub are largely *in* a sub, and have to transmit through the shell. A fast enough system could detect the vibration of the shell and cancel it out. it would require a metric buttload of little "speakers" mounted all over the inside of the shell, but it would be doable.

Active vibration control and active vibration cancellation are both active areas of research:
http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A9534456AH&q=helicopter+active+vibration&uid=790707678&setcookie=yes
http://data.mecheng.adelaide.edu.au/avc/publications/public_papers/1999/active99cqh.pdf
Sikorsky has an active rotor vibration control system in several of its models, designed to increase aircrew comfort. The same system could be adapted to signature control. Sikorsky did a project for the Navy, testing integration on the MH-60S:
http://www.defenseindustrydaily.com/68m-to-sikorsky-to-rd-mh60s-active-vibration-control-02143/
https://www.fbo.gov/index?s=opportunity&mode=form&id=f42ed977d5828377eb0c9543ba70783e&tab=core&_cview=0
 

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