The main challenge lies in making them quiet flyers.
The crashed helicopter left on Osama bin Laden's compound revealed a peek into the latest in stealth helicopter technology.
There have been several approaches to making helicopters stealthy, though none have become widespread.
The main challenge lies in keeping the helicopters quiet.
Pictures of helicopter wreckage from the raid on the compound where Osama bin Laden was killed drove speculation that it was the first look at the latest in "stealth" helicopter design.
Reports suggest that Pakistani authorities even allowed Chinese spies to examine the wreckage for clues on how the craft was designed.
So what does it take to make a helicopter stealthy?
For starters, it takes cutting back the noise. Unlike high altitude aircraft, helicopters aren't so vulnerable to being seen by radar, as they fly low and are often hidden by the terrain. The big issue is the sound.
"They tend to produce noise in frequencies that are easily detectable by the human ear," says J. Gordon Leishman, a professor of aerospace engineering at the University of Maryland and author of a textbook on helicopter aerodynamics. The problem isn't the engine itself; that is actually relatively quiet. It's the rotors that make the classic "whup-whup" sound.
The simplest way to dampen a helicopter's noise is to increase the angle of the rotor's blades, also called the pitch, and slow them down. But there is a trade-off: raising the blades' pitch eventually causes a stall and decreases the lifting power. So there are limits to how much you can do that, Leishman says.
Another way to reduce the noise is to modify the tail rotor. One could make it larger, allowing it to spin more slowly. But a tail rotor can only get so large before it becomes impractical. Newer helicopter designs use a "fenestron," which is an enclosed tail rotor that has many more blades, and it is quieter.
But this, too, can affect performance, as it is heavier, though it uses less power to achieve a given thrust. This design has become common, though it doesn't look like the tail section in the pictures from the Bin Laden raid.
Then there's increasing the number of rotor blades. This helps because each additional blade carries less of the weight of the helicopter, and thus produces less noise. But there are limits to how many blades can be tacked onto the rotor's drive shaft, and once again weight becomes a consideration.
One approach for quiet helicopters that has appeared in McDonnell-Douglas designs is the No Tail Rotor, or NOTAR, which uses a jet of low-pressure air to provide thrust against the torque of the main rotor. Such helicopters are much quieter. As the pictures of the helicopter in the Bin Laden raid show the tail section, it's clear that this design wasn't used in the raid.
None of these noise-reduction methods relies on particularly new technologies, so why haven't stealth helicopters been more common? The answer lies in a combination of politics, budgets, and utility.
The first stealth helicopter was a Hughes OH-6A, a modified version of one called the "Loach" for its acronym -- LOH for light observation helicopter. Called "The Quiet One" It incorporated a number of changes that would be adopted later. Adding a rotor blade (it had five on the main rotor and four on the tail) and using a modified exhaust system were among them.
The problem was that modifying the copter was expensive, and there was only one mission for it -- to help secretly drop wiretapping equipment on a North Vietnamese communications cable. Only two were built and later dismantled.
Later, there was the Comanche program. Leishman notes that the Boeing/Sikorsky RAH-66 Comanche was the first helicopter designed from the "ground up" to be a stealth vehicle. It has odd angles (reminiscent of a stealth bomber) to reduce its radar cross-section, and was coated in radar-absorbing material. It also featured an enclosed tail rotor. The program was started in the late 1980s and the first models flew in 1996.
But the Comanche was designed for reconnaissance, and by 2004, when the program was canceled, the Army decided it could better use the funds on unmanned aerial vehicles (UAVs). By that time, $6.9 billion had been spent, plus another half-billion in contract cancellation fees to Boeing and Sikorsky.
Several reports have speculated that the U.S. helicopter wreckage in Bin Laden's compound is a modified Blackhawk. The helicopter's producer, Sikorsky, hasn't acknowledged that the helicopter is one of theirs, and the U.S. Army's Special Operations Command hasn't confirmed it either (and won't).
The U.S. government recently accused the Pakistanis of allowing Chinese military personnel access to it -- and possibly even pieces to take back and examine.
"If Beijing has had the opportunity to examine components and materials this will be of obvious interest in terms of materials technology for signature reduction," wrote Douglas Barrie, a fellow at the International Institute for Strategic Studies and an expert on the military aerospace industry in an email. "However copying or reverse engineering in this area is far from a simple task."