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Stealth technology for black helicopters?


Jack White
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I have just finished mowing my lawn and sat down to cool off.

Suddenly I heard "a helicopter" almost directly overhead. I did not hear

"it" approaching. Looking up, I saw FIVE BLACK HELICOPTERS with no

markings visible, flying in single file very fast. The sound of all five was

no greater than a single chopper. As soon as they passed my location,

any noise they created was no longer heard. Many helicopters fly over

my house daily, but I have seen none like these, which were virtually

silent. They clearly were neither military nor civilian ones to which I

am very accustomed.

Has anyone heard of development of "stealth" helicopters?

Jack

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There have been, and continue to be, pushes to make helicopters quieter. Like jet engines, people dislike the noise they create. The culprits are the main rotor, and to a lessor extent, the tail rotor. Sweeping the blade tip helped reduce noise and increase blade efficiency. You can see it on aircraft like the Lynx, Seahawk, Blackhawk, Merlin, etc.

I don't suppose you rang up your local Air Traffic Control centre to ask who they were, and what type they were? Helicopters in a black paint scheme are reasonably popular, so it's hard to tell what type they were and how much noise they make.

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This might be of interest:

The U.S. Army and CIA developed what could be considered a stealthy helicopter during the Vietnam War. There, they were primarily interested in reducing the amount of noise that the helicopter generated, and they named the helicopter The Quiet One. Light, quiet and stealthy helicopters could be used for clandestine missions, quick in-and-out assignments without being noticed. A Special Forces A-Team performing an extraction could grab their target, climb a rope, and be extracted by a stealth helicopter. Other stealthy helicopter has focued on reducing detectability by radar and infra-red sensors, including the suppression of hot engine exhaust gases.

In the 1980s, Hughes and other American aircraft manufacturers investigated concepts for the construction of radar-evading "Stealth" helicopters. The US designed stealthy helicopter-type aircraft. One program was the McDonnell Douglas X-wing. Stealth features of this hybrid craft included using the stalled blades, when in aeroplane mode, as radar reflectors, and using McDonnell Douglas NOTAR (no tail rotor) technology to eliminate tell tale tail rotor radar signature. The X-wing project had an unclassified counterpart, which allowed components of the 'black' X-wing to be obtained under cover.

According to one report, a classified stealth helicopter was being tested at the Groom Lake Air Force base as early as 1990. The code name for the helicopter as "T.E.-K," standing for "Test and Evaluation Project K." The F-117 stealth fighter was reportedly known as "T.E.-A," and the B-2 stealth bomber, known as "T.E.-B." The 2/6/95 issue of Aviation Week & Space Technology reported that the Air force had a silent NOTAR helicopter and a stealth helicopter inside Area 51.

For the conventional helicopter, there are two fundamental systems that contribute to the generation of near-field and far-field noise, the main rotor and the tail rotor. Each rotor emits unique and recognizable sounds due to its highly individualized operating condition. Engine noise is typically of secondary significance.

A helicopter main rotor generates primarily low frequency noise and, in certain operating regimes, high amplitude low-to-mid-frequency noise modulated at the blade passage frequency. The low frequency rotor noise is made up of basic loading noise and broadband turbulence noise, each a function of lift and rotational speed. These sources are present in any lifting rotor.

Additional sources, such as Blade Vortex Interaction (BVI) noise and High Speed Impulsive (HSI) noise, become dominant in specific operating regimes, namely in descents and at high forward airspeeds, respectively. BVI noise can be the most significant contributor, because it occurs during a helicopter’s approach to the landing area.

Operational noise reduction modifications involve the use of known low noise techniques and modifications to flight paths in an attempt to minimize the noise “footprint.”

Current technology for noise reduction employed in new rotorcraft designs, must ensure that the cost, performance, and other impositions on the design are met in concert with reduced noise.

Control of main rotor noise has traditionally been accomplished by the judicious selection of rotor blade configurations and rotational tip speed. Airfoils, blade planforms, and tip shapes are chosen which mitigate the effects of HSI noise and BVI noise. For a given design gross weight, increasing the blade chord and changing the number of rotor blades are means of reaching an acoustically desirable rotational tip speed. The blade number change also alters the frequency distribution of the sound generated.

The most direct method of controlling BVI noise is by reducing or diffusing the tip vortex. Tip shapes such as the sub-wing, Ogee tip, and others have been shown to cause measurable reductions in BVI noise by modifying the vortex structure.

Conventional means of noise reduction, e.g., tip speed reduction, tip shapes and airfoil tailoring, are inferior to several innovative design concepts: modulated blade spacing and x-force control when used to significantly reduce noise with minimal performance degradation and no vibration increase.

Helicopter main rotors have historically been designed with equally spaced blades. This equal spacing from one blade to the next translates to a main rotor acoustic spectrum characterized by a single fundamental blade-passage frequency and its harmonics. As many as 20 or 30 harmonics are commonly present in a main rotor's acoustic spectrum, each of which is a multiple of the fundamental blade-passage frequency. In a typical spectral plot, these frequencies appear as pronounced, ordered “peaks” spread evenly across the acoustic spectrum.

Since the acoustic frequencies associated with the rotating blades are directly related to the blade spacing, intuitively the use of unevenly spaced blades holds the potential of lower sound levels and less perceptibility. The acoustic effect of uneven or modulated blade spacing is to generate several blade-passage frequencies, one for each unique angle between blades. Each blade passage frequency, in turn, generates its own set of harmonics. The total acoustic energy is thereby spread over a broader range of frequencies, rather than being concentrated at one blade-passage frequency and a single set of harmonics.

Main rotor designs that incorporate modulated blade spacing have reduced peak noise levels in most flight operations. X-force control alters the helicopter's force balance whereby the miss distance between main rotor blades and shed vortices can be controlled. This control provides a high potential to mitigate BVI noise radiation. A main rotor design, incorporating the modulated blade spacing concept, offers significantly reduced noise levels and the potential of a break-through in how a helicopter’s sound is perceived and judged.

The advantages of the modulated blade spacing concept are many: it has minimal impact on performance and potentially reduces vibration; it reduces sound levels and improves sound quality when incorporated on tail rotors; it lessens perceptibility; and it potentially has aural detection benefits. It is believed that the lower source frequencies associated with a main rotor can be altered similarly to those of a tail rotor.

One configuration studied had five blades, a radius of 19.5 feet, a thrust weighted chord of 12 inches, and a rotational tip speed of 665 feet per second. This rotor incorporated modulated blade spacing with angles between blades of 72, 68.5, 79, 65, and 76.5 degrees. If incorporated on the baseline helicopter, the rotor results in a 16 percent payload penalty for the full fuel case. The cruise airspeed would be reduced by 6.2 percent and the maximum airspeed by 17.2 percent. The reduction in peak noise levels is predicted to be 4, 8 and 4 dBA during takeoff, flyover and approach, respectively. The noise reductions up-range (15-20 seconds before overhead) are even greater: 16, 16 and 9 dBA during takeoff, flyover and approach, respectively.

In new R&D acquisitions, were the Government is trying to develop a new weapon system, such as a stealth helicopter, and then enter into production but has no previous experience with the program objectives, is a good candidate for a CPFF contract type. This is to share the technical and resulting cost risk with the contractor. This allows the contractor to concentrate on the job at hand (i.e., to try and prove that the idea can be developed into a working model by solving the technical problems they are facing) versus focusing on how much money they have spent to date under the contract.

http://www.globalsecurity.org/military/sys...rcraft/mh-x.htm

The canceled RAH-66 Comanche helicopter would have been a stealth helicopter, not only in noise levels but also in radar signature.

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Thanks. Five helicopters in formation are likely to be governmental.

They made virtually no noise, especially coming and going.

My understanding is that all aircraft must carry ID numbers; these

did not...just solid black. Fort Worth is the home of Bell Helicopter,

so I see many helicopters daily...4 TV station copters, 2 medical

helicopters, one police helicopter, many Bell, military and corporate

copters. These were unlike any I have seen, and much quieter.

All five made less noise than a single conventional chopper.

Jack

Edited by Jack White
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Thanks. Five helicopters in formation are likely to be governmental.

They made virtually no noise, especially coming and going.

My understanding is that all aircraft must carry ID numbers; these

did not...just solid black. Fort Worth is the home of Bell Helicopter,

so I see many helicopters daily...4 TV station copters, 2 medical

helicopters, one police helicopter, many Bell, military and corporate

copters. These were unlike any I have seen, and much quieter.

All five made less noise than a single conventional chopper.

Jack

Is it possible that you missed the id numbers? Or that they were less visible due to the paint scheme? Even governmental aircraft must have id marks, afaik.

My first thought was perhaps they were Eurocopters (but that is probably just because I recently saw a show on Green TV comparing the Bell 206 and the Eurocopter and they made a point of the Eurocopter being quieter.)

http://www.luftrettung-hamburg.de/html/heli_expo_2005.html

http://www.flickr.com/photos/doctor_keats/2309718634/

Could you describe some of the characteristics (if you remember) of the copters to see if we can narrow down the model? Was the tail enclosed or open? How big did they appear (how many people would you estimate they could hold)? Was the nose rounded or pointed? Were they all the same model? How many windows? Did they have skids or wheels? (wheels may have been retracted giving a smooth bottom, some models leave them out) Rounded or angular features? Narrow body or did they have pods or winglets?

Pure speculation here, could they have been a new Bell model? Perhaps one they were testing for noise specifically in groups?

Edited by Matthew Lewis
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Thanks. Five helicopters in formation are likely to be governmental.

They made virtually no noise, especially coming and going.

My understanding is that all aircraft must carry ID numbers; these

did not...just solid black. Fort Worth is the home of Bell Helicopter,

so I see many helicopters daily...4 TV station copters, 2 medical

helicopters, one police helicopter, many Bell, military and corporate

copters. These were unlike any I have seen, and much quieter.

All five made less noise than a single conventional chopper.

Jack

Is it possible that you missed the id numbers? Or that they were less visible due to the paint scheme? Even governmental aircraft must have id marks, afaik.

My first thought was perhaps they were Eurocopters (but that is probably just because I recently saw a show on Green TV comparing the Bell 206 and the Eurocopter and they made a point of the Eurocopter being quieter.)

http://www.luftrettung-hamburg.de/html/heli_expo_2005.html

http://www.flickr.com/photos/doctor_keats/2309718634/

Could you describe some of the characteristics (if you remember) of the copters to see if we can narrow down the model? Was the tail enclosed or open? How big did they appear (how many people would you estimate they could hold)? Was the nose rounded or pointed? Were they all the same model? How many windows? Did they have skids or wheels? (wheels may have been retracted giving a smooth bottom, some models leave them out) Rounded or angular features? Narrow body or did they have pods or winglets?

Pure speculation here, could they have been a new Bell model? Perhaps one they were testing for noise specifically in groups?

The looked "Bell-like". They were medium to small size, but looked not much different than

most choppers I see. I don't remember noticing the style of landing gear.

They were very fast, about 300 feet high, and from appearance to disappearance I

only saw them about 20 seconds. If they had any markings I could not see them at 300 feet.

I normally do not see BLACK helicopters around here. I'd guess they held 2 or 3 persons.

They were flying single file, in formation. The tail was not "open".

I just now looked at the Bell website. They most resembled the JetRanger.

Jack

Edited by Jack White
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Okay - resembled the Jetranger. That gives us a shape to look at. You say the tail was not open. Do you mean like this?

250px-Heli.fenestron.750pix.jpg

Matt makes a good point about the reg numbers; there is a general size they have to be, but in some circumstances they can be quite small. For example, ex-military aircraft in authentic markings. In cases like this, the rego is very small. I'll see what the FAA regulations say.

I would strongly suggest calling the local Air Traffic Control section about the aircraft; they may be able to answer your questions quite easily.

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How about the tail rotor? Enclosed like the first image I posted?

ETA: I wanted to confirm it was not a NOTAR (NO TAil Rotor) aircraft; they achieve significant noise reduction with that system.

Edited by Evan Burton
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How about the tail rotor? Enclosed like the first image I posted?

ETA: I wanted to confirm it was not a NOTAR (NO TAil Rotor) aircraft; they achieve significant noise reduction with that system.

No. Nothing like that. Just ordinary looking helicopters, like most of the Bell machines

which predominate around here.

Jack

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So it was like this?

heli-tail-rotor3.jpg

Sorry, but I'm trying to understand what you meant by "not open"; it made an impression on you so it's something we can use to help try and identify what make of aircraft it was.

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It seems there are times when a registration is not required to be displayed on an aircraft:

(a) When display of aircraft nationality and registration marks in accordance with §§45.21 and 45.23 through 45.33 would be inconsistent with exhibition of that aircraft, a U.S.-registered aircraft may be operated without displaying those marks anywhere on the aircraft if:

(1) It is operated for the purpose of exhibition, including a motion picture or television production, or an airshow;

(2) Except for practice and test fights necessary for exhibition purposes, it is operated only at the location of the exhibition, between the exhibition locations, and between those locations and the base of operations of the aircraft; and

(3) For each flight in the United States:

(i) It is operated with the prior approval of the Flight Standards District Office, in the case of a flight within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for the takeoff airport, or within 4.4 nautical miles of that airport if it is within Class G airspace; or

(ii) It is operated under a flight plan filed under either §91.153 or §91.169 of this chapter describing the marks it displays, in the case of any other flight.

(B) A small U.S.-registered aircraft built at least 30 years ago or a U.S.-registered aircraft for which an experimental certificate has been issued under §21.191(d) or 21.191(g) for operation as an exhibition aircraft or as an amateur-built aircraft and which has the same external configuration as an aircraft built at least 30 years ago may be operated without displaying marks in accordance with §§45.21 and 45.23 through 45.33 if:

(1) It displays in accordance with §45.21© marks at least 2 inches high on each side of the fuselage or vertical tail surface consisting of the Roman capital letter “N” followed by:

(i) The U.S. registration number of the aircraft; or

(ii) The symbol appropriate to the airworthiness certificate of the aircraft (“C”, standard; “R”, restricted; “L”, limited; or “X”, experimental) followed by the U.S. registration number of the aircraft; and

(2) It displays no other mark that begins with the letter “N” anywhere on the aircraft, unless it is the same mark that is displayed under paragraph (B)(1) of this section.

14 CFR 45

Edited by Evan Burton
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It seems there are times when a registration is not required to be displayed on an aircraft:

(a) When display of aircraft nationality and registration marks in accordance with §§45.21 and 45.23 through 45.33 would be inconsistent with exhibition of that aircraft, a U.S.-registered aircraft may be operated without displaying those marks anywhere on the aircraft if:

(1) It is operated for the purpose of exhibition, including a motion picture or television production, or an airshow;

(2) Except for practice and test fights necessary for exhibition purposes, it is operated only at the location of the exhibition, between the exhibition locations, and between those locations and the base of operations of the aircraft; and

(3) For each flight in the United States:

(i) It is operated with the prior approval of the Flight Standards District Office, in the case of a flight within the lateral boundaries of the surface areas of Class B, Class C, Class D, or Class E airspace designated for the takeoff airport, or within 4.4 nautical miles of that airport if it is within Class G airspace; or

(ii) It is operated under a flight plan filed under either §91.153 or §91.169 of this chapter describing the marks it displays, in the case of any other flight.

(B) A small U.S.-registered aircraft built at least 30 years ago or a U.S.-registered aircraft for which an experimental certificate has been issued under §21.191(d) or 21.191(g) for operation as an exhibition aircraft or as an amateur-built aircraft and which has the same external configuration as an aircraft built at least 30 years ago may be operated without displaying marks in accordance with §§45.21 and 45.23 through 45.33 if:

(1) It displays in accordance with §45.21© marks at least 2 inches high on each side of the fuselage or vertical tail surface consisting of the Roman capital letter “N” followed by:

(i) The U.S. registration number of the aircraft; or

(ii) The symbol appropriate to the airworthiness certificate of the aircraft (“C”, standard; “R”, restricted; “L”, limited; or “X”, experimental) followed by the U.S. registration number of the aircraft; and

(2) It displays no other mark that begins with the letter “N” anywhere on the aircraft, unless it is the same mark that is displayed under paragraph (B)(1) of this section.

14 CFR 45

I must admit that I would not be able to read 2-inch high lettering at about 300 feet.

Jack

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Neither I think would many others.

I was able to find some info on noise reduction.

http://www.aviation.com/technology/070727_...r_industry.html

One area in which manufacturers are concentrating their technological development efforts is in reducing the aerodynamic noise made by helicopters' main and tail rotors. Aerodynamic noise is responsible for most helicopter noise that people on the ground hear.

Manufacturers are now designing new helicopters with four or more main rotor blades. This avoids the distinctive chopping noise that the older, twin-bladed Bell UH-1 "Huey" helicopter makes, which can be heard miles away.

This at least suggests that Bell would be working on ways to decrease noise.

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