Onbekende (stealth) helikopter betrokken bij aanval compound Bin Laden?

Gestart door Elzenga, 03/05/2011 | 21:12 uur

jurrien visser (JuVi op Twitter)

How to Make Helicopters Stealthy

The main challenge lies in making them quiet flyers.

.By Jesse Emspak
Wed Aug 17, 2011 09:44 AM ET

THE GIST The crashed helicopter left on Osama bin Laden's compound revealed a peek into the latest in stealth helicopter technology.
There have been several approaches to making helicopters stealthy, though none have become widespread.
The main challenge lies in keeping the helicopters quiet.


Black Hawk Helicopters land in Khowst Province, Afghanistan. One of the key challenges to making a helicopter stealthy is making them quieter. Click to enlarge this image.
Corbis

Pictures of helicopter wreckage from the raid on the compound where Osama bin Laden was killed drove speculation that it was the first look at the latest in "stealth" helicopter design.

Reports suggest that Pakistani authorities even allowed Chinese spies to examine the wreckage for clues on how the craft was designed.

So what does it take to make a helicopter stealthy?

For starters, it takes cutting back the noise. Unlike high altitude aircraft, helicopters aren't so vulnerable to being seen by radar, as they fly low and are often hidden by the terrain. The big issue is the sound.

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"They tend to produce noise in frequencies that are easily detectable by the human ear," says J. Gordon Leishman, a professor of aerospace engineering at the University of Maryland and author of a textbook on helicopter aerodynamics. The problem isn't the engine itself; that is actually relatively quiet. It's the rotors that make the classic "whup-whup" sound.

The simplest way to dampen a helicopter's noise is to increase the angle of the rotor's blades, also called the pitch, and slow them down. But there is a trade-off: raising the blades' pitch eventually causes a stall and decreases the lifting power. So there are limits to how much you can do that, Leishman says.

Another way to reduce the noise is to modify the tail rotor. One could make it larger, allowing it to spin more slowly. But a tail rotor can only get so large before it becomes impractical. Newer helicopter designs use a "fenestron," which is an enclosed tail rotor that has many more blades, and it is quieter.

But this, too, can affect performance, as it is heavier, though it uses less power to achieve a given thrust. This design has become common, though it doesn't look like the tail section in the pictures from the Bin Laden raid.


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Then there's increasing the number of rotor blades. This helps because each additional blade carries less of the weight of the helicopter, and thus produces less noise. But there are limits to how many blades can be tacked onto the rotor's drive shaft, and once again weight becomes a consideration.

One approach for quiet helicopters that has appeared in McDonnell-Douglas designs is the No Tail Rotor, or NOTAR, which uses a jet of low-pressure air to provide thrust against the torque of the main rotor. Such helicopters are much quieter. As the pictures of the helicopter in the Bin Laden raid show the tail section, it's clear that this design wasn't used in the raid.

None of these noise-reduction methods relies on particularly new technologies, so why haven't stealth helicopters been more common? The answer lies in a combination of politics, budgets, and utility.

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The first stealth helicopter was a Hughes OH-6A, a modified version of one called the "Loach" for its acronym -- LOH for light observation helicopter. Called "The Quiet One" It incorporated a number of changes that would be adopted later. Adding a rotor blade (it had five on the main rotor and four on the tail) and using a modified exhaust system were among them.

The problem was that modifying the copter was expensive, and there was only one mission for it -- to help secretly drop wiretapping equipment on a North Vietnamese communications cable. Only two were built and later dismantled.

Later, there was the Comanche program. Leishman notes that the Boeing/Sikorsky RAH-66 Comanche was the first helicopter designed from the "ground up" to be a stealth vehicle. It has odd angles (reminiscent of a stealth bomber) to reduce its radar cross-section, and was coated in radar-absorbing material. It also featured an enclosed tail rotor. The program was started in the late 1980s and the first models flew in 1996.

But the Comanche was designed for reconnaissance, and by 2004, when the program was canceled, the Army decided it could better use the funds on unmanned aerial vehicles (UAVs). By that time, $6.9 billion had been spent, plus another half-billion in contract cancellation fees to Boeing and Sikorsky.

Several reports have speculated that the U.S. helicopter wreckage in Bin Laden's compound is a modified Blackhawk. The helicopter's producer, Sikorsky, hasn't acknowledged that the helicopter is one of theirs, and the U.S. Army's Special Operations Command hasn't confirmed it either (and won't).

The U.S. government recently accused the Pakistanis of allowing Chinese military personnel access to it -- and possibly even pieces to take back and examine.

"If Beijing has had the opportunity to examine components and materials this will be of obvious interest in terms of materials technology for signature reduction," wrote Douglas Barrie, a fellow at the International Institute for Strategic Studies and an expert on the military aerospace industry in an email. "However copying or reverse engineering in this area is far from a simple task."

http://news.discovery.com/tech/stealth-helicopter-technology-110817.html

A.J.

Laten we het eens anders benaderen, zou jij een dusdanig belangrijke actie uitvoeren met een "experimenteel" toestel?

jurrien visser (JuVi op Twitter)

August 14, 2011 4:27 pm

Pakistan lets China see US helicopter

By Anna Fifield in Washington

Pakistan allowed Chinese military engineers to photograph and take samples from the top-secret stealth helicopter that US special forces left behind when they killed Osama bin Laden, the Financial Times has learnt.

The action is the latest incident to underscore the increasingly complicated relationship and lack of trust between Islamabad and Washington following the raid.


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"The US now has information that Pakistan, particularly the ISI, gave access to the Chinese military to the downed helicopter in Abbottabad," said one person in intelligence circles, referring to the Pakistani spy agency. The Chinese engineers were allowed to survey the wreckage and take photographs of it, as well as take samples of the special "stealth" skin that allowed the American team to enter Pakistan undetected by radar, he said.

President Barack Obama's national security council had been discussing this incident and trying to decide how to respond. A senior official said the situation "doesn't make us happy", but that the administration had little recourse.

As Navy Seals raided Bin Laden's compound in the military city of Abbottabad, just outside Islamabad, in May, one of their modified Black Hawk helicopters crashed into the wall of the compound, rendering it inoperable.

The Seals used a hammer to smash the instruments then rigged up explosives to detonate it in an effort to keep classified military technology secret, but the tail section landed outside the compound wall and remained intact. John Kerry, chairman of the Senate foreign relations committee, went to Pakistan two weeks after the raid to secure the tail's return.

At the time, Pakistani officials, who were livid that the US carried out the raid without informing Islamabad first, hinted that the Chinese were interested in looking at the wreckage, and photographs of the tail circulated on the internet. But people close to the White House and the Central Intelligence Agency have told the FT that the Chinese were in fact given access to the helicopter.

"We had explicitly asked the Pakistanis in the immediate aftermath of the raid not to let anyone have access to the damaged remains of the helicopter," said the person close to the CIA.

Senior US officials confronted General Ashfaq Kayani, head of the Pakistan military, about this but he flatly denied it, according to a person with knowledge of the meeting. A senior Pakistani official also denied it to the FT. China declined to comment, as did the White House and CIA.

Beijing has a strong military relationship with Islamabad and is a major supplier of weapons to the Pakistani military.

"The Chinese would have enormous interest in this newfangled technology," said the person involved in confronting the Pakistanis. "They [Seals] did not blow the thing up for no reason," he said.

However, the senior government official said it was "hard to say" how useful the information would have been. "Most of the helicopter was virtually destroyed during the operation," he said.

Additional reporting by Matthew Green and Kathrin Hille

Copyright The Financial Times Limited 2011.

jurrien visser (JuVi op Twitter)

Stealth Helicopter
MH-X Advanced Special Operations Helicopter

On 1 May 2011, the United States announced that it had launched an operation into Pakistan from Afghanistan to apprehend Al-Qaeda leader Osama bin Laden. Bin Laden was killed during the operation, reported to have been conducted by members of the Naval Special Warfare Development Group (DEVGRU; commonly referred to by an older title, Seal Team Six). The Naval Special Warfare operators were reported to have been inserted and/or extracted by elements of the 160th Special Operations Aviation Regiment (Airborne). During the raid, it was reported that one of the force's MH-60 helicopters was damaged and had to be destroyed in place. The remnants of the helicopter were later recovered and moved from the site by the Pakistani military. Subsequent pictures of the wreckage suggested that the helicopter might have in fact been a new type or a highly modified existing type, possibly incorporating stealth technology. The features on the wrecked tail rotor could also have been for noise reduction purposes. The photographs of the wreckage also suggested a silver infra-red suppressant finish, similar to that used on fixed wing stealth aircraft such as the F-22, had been applied.

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

http://www.globalsecurity.org/military/systems/aircraft/mh-x.htm

Elzenga

Citaat van: A.J. op 24/10/2011 | 20:25 uurIk heb niet veel meer gezien anders. Nog even en we gaan geloven dat de marsmannetjes geland zijn.
Naast de staartrotor was ook de staart, roervlakken en een deel van de staartboom intact gebleven. Dat gaf ook meer informatie.
Citaat van: A.J. op 24/10/2011 | 20:25 uur
Een bevestigde veronderstelling? Maar vertel, jij hebt met Sikorsky en zijn testpiloten aan de koffie gezeten en eea is even tussen neus en lippen door aan bod gekomen?
Ik niet...maar verschillende luchtvaartjournalisten en experts klaarblijkelijk wel. Al ging dat ook om piloten die ze kennen. Al is er daarbij geen compleet beeld van de helikopter uitgelekt. Maar goed ook voorlopig. Want ze zijn nog wel vaker nodig vrees ik.

Maar goed...geloof dat het hier over de NH90 gaat. Waar men bepaalde stealth-technieken ook heeft toegepast om de zichtbaarheid waar mogelijk te beperken.

yelloow

Dat hele gedoe over stratego vind ik een beetje vermoeiend eigenlijk. Stratego is helemaal niet zo'n strategisch spelletje, eerder tactisch

Elzenga

Citaat van: 5m@sh_1up op 24/10/2011 | 20:08 uurAllerminst eenvoudig te beantwoorden vragen. In het hoofd van een leek zoals jij misschien wel, maar voor mij geldt dit absoluut niet. Dus... Zeg maar, leek, wat denk jij zo goed te weten?
Merkwaardig dan..dat deze leek alle antwoorden goed bleek te hebben..en niet bij toeval. Zou toch niet mogen kunnen als ik zo dom ben op dit vlak als jij beweert.
Citaat van: 5m@sh_1up op 24/10/2011 | 20:08 uurMaar natuurlijk, kom maar eens met een bron+bewijs, voordat je dit soort bullcrap verder blijft geloven.
Waarom zou ik mijn tijd nu verdoen die bronnen en bewijzen weer terug te gaan zoeken als ik toch al weet dat het geen zin heeft, want worden door jou al bij voorbaat afgewezen. Het ging om Amerikaanse helikopterpiloten..die de toestellen kenden. Bevestigden dat de UH60 de basis is, maar gaven verder geen extra info prijs over hoe ze er exact uit zien. Gek trouwens dat je deze informatie niet hebt gezien...kwam nadrukkelijk voorbij in de hele berichtgeving en discussies over het voorval...of weer zo geloven in eigen gelijk/oordeel dat andere info er niet toe doet? 
Citaat van: 5m@sh_1up op 24/10/2011 | 20:08 uur
Hou jij je maar gewoon bezig met politiek... Misschien heb je daar wél verstand van.
Wat ik stel over strategische zaken en het strategospel IS politiek. Dusssssss...

A.J.

Citaat van: onderofficier op 24/10/2011 | 20:19 uur
Wanneer kom jij eens met bewijs?   
Jij zegt het beter te weten ...........maar van enig bewijs is van jouw zijde ook geen sprake. 

Er is bewijs genoeg dat de Amerikanen met de Blackhawk vliegen. Ik zie echter geen bewijs dat de Amerikanen plotseling eoa stealth-achtige heli in de bewapening hebben.

A.J.

Citaat van: Elzenga op 24/10/2011 | 20:08 uur
Klopt..behalve voor de speciale projecten en missies blijkbaar ;)

En die conclusie trek jij uit een foto van een staartrotor?

A.J.

Citaat van: Elzenga op 24/10/2011 | 20:06 uur
Het was wel iets meer dan alleen een aangepaste staartrotor. En ja de nodige artist impressions waren nogal onrealistisch en overdone. Uitgaande dan van de UH60 als basis.

Ik heb niet veel meer gezien anders. Nog even en we gaan geloven dat de marsmannetjes geland zijn.

Citaat
Wat tot op zekere hoogte ook een veronderstelling is...al is er het nodige bevestigd in die dagen vanuit de groep van (oud)gebruikers en ontwikkelaars van de toestellen.

Een bevestigde veronderstelling? Maar vertel, jij hebt met Sikorsky en zijn testpiloten aan de koffie gezeten en eea is even tussen neus en lippen door aan bod gekomen?

onderofficier

Citaat van: 5m@sh_1up op 24/10/2011 | 20:08 uur
Maar natuurlijk, kom maar eens met een bron+bewijs, voordat je dit soort bullcrap verder blijft geloven.

Wanneer kom jij eens met bewijs?   
Jij zegt het beter te weten ...........maar van enig bewijs is van jouw zijde ook geen sprake. 
Tegenslag is de beste gelegenheid om te tonen dat je karakter hebt; vele tonen (helaas) aan dat ze weinig karakter hebben.

5m@sh_1up

Citaat van: Elzenga op 24/10/2011 | 20:02 uur
Citaat van: 5m@sh_1up op 24/10/2011 | 14:31 uur
Wat ik het eerste dacht, toen ik dat staartdeel zag, was: Man, dat ziet er nep uit... En, hoe kan de heli daar nou gecrashed zijn? En waarom is dan ALLEEN dat stukje staart nog intact?
eenvoudig te beantwoorden vragen...die had jij toch zeker moeten weten :devil:
Allerminst eenvoudig te beantwoorden vragen. In het hoofd van een leek zoals jij misschien wel, maar voor mij geldt dit absoluut niet. Dus... Zeg maar, leek, wat denk jij zo goed te weten?
Citaat van: 5m@sh_1up op 24/10/2011 | 14:31 uur
Maar goed... Je hebt het verder over 'experts'. Welke experts? Bedoel je al die journalisten die elkaar nalullen? Of die mannetjes die al hun 'kennis' op wikipedia-niveau hebben?
Citaatbla bla bla...zaten experts bij die meer verstand hebben van helikopters en er in meer gevlogen hebben dan jij beste 5m@sh_1up.
Maar natuurlijk, kom maar eens met een bron+bewijs, voordat je dit soort bullcrap verder blijft geloven.
Citaat van: 5m@sh_1up op 24/10/2011 | 14:31 uur
En hoe kun jij daar nou forse discussies over gehad hebben? Je hebt er toch geen bal verstand van?
Blijkbaar meer dan voldoende om er zinvolle zaken over te zeggen beste 5m@sh_1up. Gezien de reacties toen. Je hoeft niet perse 5m@sh_1up te zijn om te zien of zaken wel of niet kunnen of kloppen.

Hou jij je maar gewoon bezig met politiek... Misschien heb je daar wél verstand van.

Elzenga

Citaat van: Poleme op 24/10/2011 | 19:02 uurDe US Army trok hieruit de conclusie dat de Comanche niet te veroorloven was en steelsheid niet nodig was, ook niet in de toekomst ;)
Klopt..behalve voor de speciale projecten en missies blijkbaar ;)

Elzenga

Citaat van: A.J. op 24/10/2011 | 14:32 uurWelke restanten? Die aangepaste staartrotor? Dat is het enige waar een foto van is, dat er vervolgens de wildste theorieën opduiken met de meeste fantastische artist impressions en waarbij alle "experts" een duit in het zakje willen doen dan moet het wel waar zijn?

Ik geloof er niet zo in.
Het was wel iets meer dan alleen een aangepaste staartrotor. En ja de nodige artist impressions waren nogal onrealistisch en overdone. Uitgaande dan van de UH60 als basis. Wat tot op zekere hoogte ook een veronderstelling is...al is er het nodige bevestigd in die dagen vanuit de groep van (oud)gebruikers en ontwikkelaars van de toestellen.

Elzenga

Citaat van: 5m@sh_1up op 24/10/2011 | 14:31 uur
Wat ik het eerste dacht, toen ik dat staartdeel zag, was: Man, dat ziet er nep uit... En, hoe kan de heli daar nou gecrashed zijn? En waarom is dan ALLEEN dat stukje staart nog intact?
eenvoudig te beantwoorden vragen...die had jij toch zeker moeten weten :devil:
Citaat van: 5m@sh_1up op 24/10/2011 | 14:31 uur
Maar goed... Je hebt het verder over 'experts'. Welke experts? Bedoel je al die journalisten die elkaar nalullen? Of die mannetjes die al hun 'kennis' op wikipedia-niveau hebben?
bla bla bla...zaten experts bij die meer verstand hebben van helikopters en er in meer gevlogen hebben dan jij beste 5m@sh_1up.
Citaat van: 5m@sh_1up op 24/10/2011 | 14:31 uur
En hoe kun jij daar nou forse discussies over gehad hebben? Je hebt er toch geen bal verstand van?
Blijkbaar meer dan voldoende om er zinvolle zaken over te zeggen beste 5m@sh_1up. Gezien de reacties toen. Je hoeft niet perse 5m@sh_1up te zijn om te zien of zaken wel of niet kunnen of kloppen.