A previously undisclosed, classified stealth helicopter apparently was part of the U.S. task force that killed Osama bin Laden in Pakistan on May 1.

The exact type of helicopter is unknown but it appears to be a highly modified version of an H-60 Blackhawk. Photos disseminated via the European PressPhoto agency and attributed to an anonymous stringer show that the helicopter’s tail features stealth-configured shapes on the boom and the tail rotor hub fairings, swept stabilizers and a “dishpan” cover over a five-or-six-blade tail rotor. It has a silver-loaded infrared suppression finish similar to that seen on V-22s.

See AviationWeek.com/ares for some photographs.

The aircraft was damaged during the mission and abandoned. The mission team destroyed most of the airframe but its tail section landed outside the wall of the target compound and escaped demolition.

Stealth helicopter technology is not new and was applied extensively to the Boeing/Sikorsky RAH-66 Comanche, cancelled in 2004. Compared with fixed-wing stealth, more emphasis is usually placed on noise and infrared signatures.

Noise can be reduced and made less conspicuous by adding blades to the main and tail rotors. It can also be reduced by aerodynamic modifications and flight control changes that make it possible to reduce rotor rpm, particularly in forward flight below maximum speed. Infrared reduction measures are crucial - the Comanche had an elaborate system of exhaust ducts and fresh-air ejectors in its tailboom.

Radar cross-section (RCS) reduction measures include flattened and canted body sides, making landing gear and other features retractable, and adding fairings over the rotor hubs. It usually is not possible to achieve the same - you can’t make a helo as radar-stealthy as a fixed-wing airplane, but helicopters generally operate at low altitude in ground clutter. Reducing RCS also makes jamming more effective, whether from the aircraft itself or from a standoff jammer.