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Helicopters are so loud because their blades create turbulence as they pass behind one another in their own wakes. The phenomenon is called a “blade-vortex interaction,” and it makes for that thwop-thwop-thwop sound signature to helicopters.
By changing the shape of the rotor blades — the new product is called “Blue Edge” — Eurocopter has reduced the blade-vortex interaction enough to allow for a sound reduction of just three to four decibels.
Now that’s quiet.
This revolutionary main rotor blade provides a passive reduction in noise levels, using a double-swept shape that is very different from present-day blades. The aim of this program is to reduce the noise generated by so-called blade-vortex interactions (BVI), which occur when a blade impacts a vortex, created at the tip of the blade of any helicopter.
A five-blade Blue Edge main rotor has been flying since July 2007 on an EC155 testbed, logging 75 flight hours and demonstrating noise reductions of 3 to 4 dB, as well as very good performance of the blade. With this safe and simple means of measureable noise reduction for helicopters now validated, Eurocopter is ready to move Blue Edge into production applications.
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, possibly incorporating stealth technology.
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....
The Boeing/Sikorsky RAH-66 Comanche was an advanced U.S. Army military helicopter intended for the armed reconnaissance role, incorporating stealth technologies. It was also intended to designate targets for the AH-64 Apache. The RAH-66 program was canceled in 2004 before it was fielded
Prototype flight testingThe first Comanche prototype was rolled out of the Sikorsky Aircraft's helicopter production facility on 25 May 1995. The prototype's maiden flight occurred on 4 January 1996. Flight qualification tests and evaluations were conducted for the two DEM/VAL (Demonstration and Validation Phase) prototypes. Through the early 2000s, the U.S. Army planned to purchase over 1,200 Comanches to fill the scout and light attack roles, with deliveries of operational RAH-66s beginning in 2006....
CancellationOn 23 February 2004, the U.S. Army announced their decision to cancel the Comanche helicopter program in view of the need to provide funds to renovate the existing helicopter fleet of aging attack, utility, and reconnaissance aircraft. The Army also planned to use Comanche program funds to speed up unmanned aerial vehicles (UAVs) development. UAVs have been providing reconnaissance during military operations in Afghanistan and Iraq. About US$6.9 billion had been invested in the Comanche program at the time of its termination. An additional US$450–680 million was paid for contract termination fees to main program partners Sikorsky and Boeing.
Technology developed for the Comanche is expected to be integrated into the Apache and other U.S. military helicopter developments  The Army was developing the Bell ARH-70 to replace the OH-58D in place of the RAH-66, but after cost overruns, the ARH-70 was canceled in October 2008 as well.
It incorporated stealth features to avoid detection, such as retractable weapon stations and main gun, and stealth faceting and radar absorbent materials...
The Comanche's very sophisticated detection and navigation systems were intended to allow it to operate at night and in bad weather...
Originally posted by ADVISOR
Then again what do I know, was only in the Cavalry...never seen a helo up close..nah never.