LOS ANGELES — When a U.S. military helicopter was destroyed in the backyard of Osama bin Laden's compound, it left not only a pile of smoldering wreckage but tantalizing evidence of a secret stealth chopper.
The quest for a helicopter that can slip behind enemy lines without being heard has been the Holy Grail of military aviation for decades and until this week nobody had thought such a craft existed.
But aviation experts are now convinced that the Pentagon may have developed such an aircraft. They say the U.S. military went to extraordinary lengths to protect its new technology by destroying a helicopter that had been damaged in the raid.
A section of the craft survived intact, and photos of it leave no doubt in analysts' minds that the U.S. had modified a MH-60 Black Hawk into some kind of super-secret stealth helicopter — the likes of which have never been seen before.
CIA Director Leon Panetta has said that the only helicopters used for the operation were Black Hawks, and he acknowledged that one of them had to be destroyed.
While stealth jets are designed to evade radar, stealth helicopters are built to be quiet. Some experts have concluded that the military and CIA may have succeeded in their decades-old quest to develop a helicopter without the ear-splitting thump-thump-thump that has signaled the presence of rotorcraft from miles away.
Aerospace analysts say the surviving tail section appears nothing like that of the standard $30-million Black Hawk chopper made by Sikorsky Aircraft Corp. in Connecticut. Notably, the tail rotor was partially covered by a plate or hub, possibly part of a noise muffling system.
"What we're seeing here is a very different type of design than what we normally see in rotorcraft," said Loren Thompson, defense policy analyst for the Lexington Institute in Arlington, Va . "It appears that the military went to great lengths to reduce the radar and acoustic signature of the helicopter."
The tail section hints at what other modifications might have been made to the far more important main rotor.
Farhan Gandhi, aerospace engineering professor at Pennsylvania State University and deputy director of the Penn State Vertical Lift Research Center of Excellence, said tremendous advances in helicopter noise reduction have been made in recent years.
"You can never have helicopters make zero noise, but there is a tremendous possibility to make them much quieter than they are now," Gandhi said.
To reduce noise, rotors can be slowed down. Advanced computation has enabled engineers to refine the shape of rotor tips. And research is being conducted into active controls that can make minute changes to the shape of rotors many times per second.
"The technology has been lab tested and flight tested, but it is not on any military aircraft that we know about," Gandhi said.