Bin Laden Raid Copters Effective, But Not New

 - May 29, 2011, 1:45 PM

The pair of “low-observable” Sikorsky Black Hawks used in the raid by U.S. forces against terrorist Osama bin Laden’s Pakistani compound May 2 were kitted with a variety of stealth and radar-defeating features that had been in development since the 1970s, and maybe longer. The helicopters came to light after one had to be left behind and intentionally detonated after apparently clipping a compound wall and making a hard landing. While the explosion destroyed most of the helicopter, parts of the empennage, stabilizer, tail rotor and tail-rotor hub survived and were extensively photographed by the international news media, giving rise to initial speculation that the U.S. used a new stealth helicopter. However, in the weeks that followed, experts, including those who spoke with AIN, dismissed the idea that a totally new helicopter had been used. Serial numbers on parts at the scene indicate that the destroyed helicopter was an MH-60 built in 2009. 

The U.S. developed stealth helicopter technology concurrent with the F-117A stealth fighter and the B-2 stealth bomber and much of it came into public view on the two prototype Boeing/Sikorsky RAH-66 Comanche gunships built in the 1990s. While $14 billion was spent on that program before it was canceled in 2004, the Comanche did incorporate the familiar faceted radar-minimizing body cladding of the F-117, along with a fenestron tailrotor, a T-tail vertical stabilizer and quiet main-rotor technology. Kits with some of that technology, along with radar-resistant coatings and a modified vertical fin, were believed applied to the “low observables” by at least two third-party contractors, most likely Boeing and Lockheed Martin. The Army’s fascination with “low observables” began during the Vietnam War and was rekindled by the AH-6 Little Bird program, a Boeing-built MD500, in the 1980s. Before that the CIA had used a specially modified 500 dubbed “The Quiet One” to conduct electronic surveillance in North Vietnam.

Getting the main rotor and tail rotor technology right would have been the first step in adding stealth to the Black Hawk, said James Van Horn, whose company makes composite aftermarket blades for a variety of helicopters. Van Horn said that achieving significant reductions in a helicopter’s acoustic signature is a fairly straightforward affair. “The more air you move, the more noise you make.” Van Horn said to reduce a helicopter’s acoustic signature, you curve sweep and taper the outboard five percent of a rotor blade, thereby eliminating its familiar “crack” sound, which is basically the resonance of the blade tips going supersonic. “The concept is not to present high Mach number air to any perpendicular surface if you can avoid it.”

Van Horn said large reductions in acoustic signatures could be achieved by modifying the rotor tip, using his company’s own composite replacement Bell 206 tail rotor as an example. That reshaped blade attains a 2.4-decibel reduction–or about a 40-percent overall reduction in the helicopter’s noise profile. Programs such as Eurocopter’s Blue Edge and Blue Pulse blade technology can achieve even greater noise reductions. Blue Edge is a passive system that features a redesigned main rotor blade that uses a double-swept shape to reduce the noise generated by blade-vortex interactions (BVI), which occur when blade-tip vortices interact with the rotor blades. A five-blade Blue Edge main rotor on an EC155 has demonstrated noise reduction of three to four decibels. Blue Pulse is an active system that the company says can achieve a significant reduction in noise and vibration. Blue Pulse uses a piezo-active rotor control system designed to reduce noise levels generated by BVI. Blue Pulse uses three flap modules located at the trailing edge of each rotor blade. The piezoelectric actuators move the rotor flaps 15 to 40 times per second to completely neutralize the familiar “slap” sound that is typically associated with helicopters during descent. Blue Pulse has been flying since 2005 and produces a measured noise reduction of up to five decibels on an EC145.

The low-observables were no doubt using some sort of quiet blade technology. Eyewitness accounts said the helicopters were not heard until they were directly over bin Laden’s compound. Photos of the wreckage suggest the crashed Black Hawk had a five- or six-blade tail rotor as opposed to the stock four-blade design. The unusual-looking tail-rotor hub fairing on the destroyed Black Hawk appears to have been designed as a radar cross-section reducer, Van Horn said. But he also noted that hub fairings could be used to reduce the amount of ship vortex striking the tail rotor and the vertical stabilizer.

All of these stealth features likely added considerable weight to the helicopter and, along with higher-than-anticipated ambient air temperatures at the time of the raid, may have contributed to the lost helicopter’s crash, now widely being attributed to airflow disruption. The helicopters are believed to be operated by personnel from the Army’s 160th Special Operations Aviation Regiment at Fort Campbell, Ky. President Obama visited there on May 6.