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RISMEDIA, May 12, 2011— (MCT)—Last week, the entire world was surprised to learn that the United States may very well own and operate stealth helicopters. Although the idea of a stealth helicopter has been around ever since the F-117 stealth fighter became well known in the 1990s, it seemed like the idea would be a long time in coming. And then, during the raid at Osama bin Laden’s compound, the idea was no longer farfetched. It seemed very real.

It brings up a good question: How could a helicopter, or any vehicle for that matter, become virtually invisible to radar and other advanced detection systems? Let’s take a look.

When you look at something like the F-117 stealth fighter or the B-2 bomber, the first thought you might have is, “how can an airplane like this fly?” That feeling is especially strong with the F-117. Instead of the plane being shaped in a smooth, aerodynamic way, it’s made up of flat surfaces and sharp angles. These flat surfaces are one key to radar invisibility.

Consider how a radar unit works. A directional antenna sends out a high power beam of radio waves, and then waits for the radio waves to reflect off of something and come back to the antenna. A reflection tells the radar unit two things. First, the radar unit learns how big the object is based on the amount of reflected energy that comes back. Second, the radar unit knows that radio waves travel about one foot per nanosecond. By counting the number of nanoseconds between sending and receiving the radio waves, the radar unit knows how far away the target is.

When the radio waves from the radar unit hit a normal airplane, some part of the plane’s curving body and wings will reflect the radio waves right back to the radar antenna. But when radio waves hit a flat panel that is angled away from the radar antenna, the reflection goes off into space. The radar antenna never sees any reflection, and the plane becomes invisible to radar. The only time the plane sends a reflection is in those cases where one of the flat panels happens to be faced right at the radar antenna. Those cases will be rare and short-lived.

There may also be special coatings on the flat panels to provide even more protection from radar. The B-2 Stealth Bomber, for example, has a special coating. Unfortunately the coating is fragile. It cannot, for example, handle rain.

There are other aspects to stealthiness as well. For example, an airplane usually makes a lot of noise with its jet engines. A stealth airplane tries to reduce the noise. The B-2 Stealth Bomber uses one simple technique to help in this regard—its engines are on top of its wings rather than below the wings. Most of the sound therefore gets blocked from reaching the ground as the plane is approaching. Airplanes can also be detected by the heat pouring out of the engines. A stealth airplane would try to reduce the heat signature of the engines.

Understanding all of these different factors, we can imagine some of the ideas that would go into making a stealth helicopter. The skin of the helicopter, and even the rotor blades, might be made of flat surfaces, and they might have special coatings. A normal helicopter typically has rather intricate couplings that allow the pilot to control the angle of the blades. These would usually provide a lot of radar reflections, but they may have been hidden by stealthy cover plates.

Helicopters are notoriously noisy. The noise comes from the engines, the main rotor blades and the tail rotor blades. The engines can be located on top of the helicopter and perhaps provided with extra plates that deflect noise toward the sky. The tail rotor, being small, can be completely enclosed in a fairing that eliminates most of its noise. The noise from the main rotor can be lessened with special blade shapes and by lowering the spin rate. The speed and fuel efficiency of the helicopter might be traded for noise when heading to the target.

It is unfortunate that the helicopter used in the Bin Laden raid had problems. If it had not, the world might have remained in the dark about stealth helicopters for several more years. Since its cover has been blown, however, we may get to see one of the invisible helicopters in the real world fairly soon.

For more information visit www.howstuffworks.com.

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