As experts have gotten a look at photos of the pieces of a U.S. helicopter left behind at the compound where Osama bin Laden was killed, they've come to the conclusion that commandos were using a stealth version of the famous Black Hawk, Army Times and IHS Jane's report.
Jane's says that:
"Following the raid on 1 May only the tail section of the helicopter remained — the main body having been destroyed in situ by US special forces — but aspects of its design do not tally with the generally held belief that Sikorsky MH/UH-60 Black Hawk-type helicopters were used in the mission.
"Specifically, the tail rotor of the crashed helicopter has a five-bladed assembly, whereas the Black Hawk tail rotor has four blades. Also, where the Black Hawk's vertical tail section tapers towards the top, the helicopter lost in the raid has a vertical tailplane than has an even chord from top to bottom capped off with what appears to be a large aerodynamic surface."
And Army Times reports that:
"The helicopters that flew the Navy SEALs on the mission to kill Osama bin Laden were a radar-evading variant of the special operations MH-60 Black Hawk, according to a retired special operations aviator.
"The helicopter's low-observable technology is similar to that of the F-117 Stealth Fighter the retired special operations aviator said. 'It really didn't look like a traditional Black Hawk,' he said. It had 'hard edges, sort of like an ... F-117, you know how they have those distinctive edges and angles — that's what they had on this one.' "
Update at 10:40 a.m. ET. More on the technology:
Aviation Week says that "stealth helicopter technology in itself is not new and was applied extensively to the RAH-66 Comanche. Priorities are usually different versus fixed-wing aircraft. Reducing noise and making it less conspicuous is the first job (more main and tail blades reduce the classic whop-whop signature). Noise can also be reduced by aerodynamic modifications and flight control changes that make it possible to slow the rotor down, particularly in forward flight below maximum speed."
As for avoiding radar, Aviation Week writes that "radar cross-section reduction is also possible — you can't make a helo as radar-stealthy as a fixed-wing airplane, because of all its moving parts, but on the other hand it is generally operating at low altitude in ground clutter, and is not an easy target."