Making a whisper-quiet helicopter is still beyond the means of modern technology, but military defense analysts say the Sikorsky choppers that Navy SEALs used in the raid on Osama bin Laden’s compound in Pakistan were clearly an advance toward that long-sought goal.

One of the helicopters was

damaged during a hard landing and the commandos destroyed it before leaving with the infamous terrorist’s body. Military aviation experts have been poring over photographs of the debris, particularly the tail section.

Precisely how the Black Hawk helicopters were modified is not known, but the photographs of the wreckage, analysts say, are offering new clues to the military’s cutting-edge technology.

Several analysts agreed the aircraft used technology that appeared to stem from the Comanche, a $39 billion, joint project between Sikorsky and Boeing until it was scrapped in 2004 due to high costs.

“It’s clearly something unusual,” said Dan Goure, a defense analyst with the Virginia-based Lexington Institute.

But with only the tail boom to look at, picturing the entire machine is a bit like trying to rebuild a dinosaur from one bone, Goure said.

“Nobody could afford to do a purpose-built stealth helicopter,” Goure said. “If you’re not going to spend $1 billion on a purpose-built helicopter,” then a modified aircraft is the next best thing.”

Aviation analyst Richard Aboulafia said it’s clear to him that Comanche technology was incorporated in the modified Black Hawks.

“I was hoping that something would be salvaged from that thing,” Aboulafia said.

The curved, bulbous structure of the chopper is crucial to confounding radar, he said.

“You don’t want sharp angles and you don’t want gaps to appear in the structure — that’s where radar emissions can bounce around,” Aboulafia said.

U.S. officials confirmed that the marquee helicopter of Stratford-based Sikorsky Aircraft was the platform for the warbirds that delivered 25 SEALs to the compound. But that’s all they would say.

“We have no additional operational details, or comments on operational details, to make at this time,” U.S. Army Lt. Col. Elizabeth Robbins, a Department of Defense spokeswoman, said Friday.

Sikorsky also declined to comment other than to say the company has not been asked to help investigate why the aircraft sustained a hard landing.

The photos showed a drastically modified tail boom that reduced the helicopter’s radar-cross section and a silence plate to muffle noise. Analysts say there’s evidence of radar-evading technology used on other stealth aircraft such as B-2 Bomber and the F-35 Lightning II.

“The biggest thing was noise suppression,” said Mark Bobbi, an aerospace and defense analyst based in Florida. “If you look at the tail section, they spent a lot of time on noise. There’s a circular shroud that covers the hub of the rotor, the whole tail boom has got a much different configuration than the traditional Black Hawk.”

Bobbi said it appears they worked hard to reduce the “radar signature” of the rotor blades in the tail section.

“They also likely did something to the main rotor. You can’t cover the entire blade, but you can put a composite cover over the main rotor hub… There’s lots of metal parts in there, a major radar return, so I’m sure they did a lot to shield that.”

Aboulafia, of the Teal Group in Virginia, said he sees some evidence that radar-absorbing composite coverings were used on the mission choppers. He and other analysts agreed that the Black Hawk was the obvious choice as a platform to add stealth technology. It is, he said, “the assault chopper of choice for special operators” because it’s reliable, relatively small and speedy.

“What we’re seeing here is a very different type of design than what we normally see in rotor craft,” 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.”

In Monday’s mission, two Black Hawks were to hover over the compound while SEALs rappelled to the ground. One chopper developed problems, due in part to temperatures that were 17 degrees higher than expected, and had to land abruptly.

One retired military helicopter pilot told Reuters that the Black Hawk likely ran into an issue called “settling with power,” when high temperatures, a heavy load and high altitudes force an unplanned landing. “Those conditions just suck the RPM out of the rotor,” he said.

Sikorsky’s Black Hawks, which typically have a range of 360 miles, are considered reliable and have been real workhorses during the last decade of war, one congressional aide told the news service.

The Black Hawk, which first began flying in 1978, has a crew of three or four and can carry 11 soldiers equipped for combat. It has a maximum gross weight of 22,000 pounds and can carry up to 9,000 pounds on an external cargo hook. It has a top speed of 187 mph.