NOVA | Battle of the X-Planes | Designing for Stealth (non-Flash)

In October 2002, Boeing unveiled the Bird of Prey, a prototype that was used to demonstrate stealth features and advanced technologies that could be used on future combat aircraft. The ultra-secret project formally began in 1992, and Bird of Prey flew 38 test flights between 1996 and 1999.

How do you make an aircraft show up on a radar screen looking as small as a bird or even a large moth, and not like the lethal, 15-ton machine that it actually is? How do you prevent heat-seeking missiles from finding and destroying it?

To avoid helping potential enemies develop countermeasures, many details of stealth are classified. However, the basic principles are widely known. Select any of the categories below to find out how engineers can design planes to avoid detection.

Visual | Radar | Heat | Noise

Unlike the B-2 and F-117 in operation today, the light-gray Bird of Prey prototype was designed to be stealthy enough to operate in broad daylight. Stabilizers evenly distribute lift to minimize pressure disturbances and the moisture contrails they create. A white patch in front of the jet engine intake helps mask shadows.

Visual
While it may seem low-tech, the human eye is a powerful detector. In fact, in some circumstances a pilot may see an enemy plane before his onboard radar detector picks it up. Since the earliest days of aerial combat, planes have traditionally been painted in various types of camouflage in an attempt to blend into the natural background. Sometimes camouflage paint on the underside of an aircraft will be blue-gray, to make the plane appear to blend with the sky for anyone looking at it from below. For enemy pilots flying above, the top surface may be gray-green or gray-brown or sand-colored to match the ground (or sea) below.

In addition, designers can use brighter paint to minimize shadows cast on the plane by its own wings or other parts. They can also add specially placed lights to eliminate shadows or to illuminate the underside of a plane so it appears to those below to match the brightness of the surrounding sky. Finally, highly efficient engines can reduce smoke contrails from the engines, while moisture contrails can be diminished by inserting additives into the exhaust and by using specially designed aerodynamic surfaces to minimize pressure disturbances around the wing.

There are only six angles on the Bird of Prey, none of which is a right angle. The fuselage is rounded in order to scatter radar in many directions. The engine's air intake, a large radar target if not properly designed for stealth considerations, is on the top of the aircraft (to keep it hidden from ground-based radar) and behind the cockpit (to hide it from head-on enemy radar).

Radar
A stealthy plane's shape helps deflect echoes away from radar. Flat surfaces reflect radar most effectively, so aircraft designers avoid having any flat parts that are likely to face in the direction of threat radar on the ground or in planes up ahead. Right angles return a strong signal back to a radar detector, so these are avoided. Any angles—for example, where the wings join the fuselage—are greater or less than 90 degrees.

Radar hitting a surface such as a wing also creates electromagnetic waves that travel the length of the wing until they hit a sharp edge, or "discontinuity." At that point the waves scatter and may bounce back toward the radar detector. To send the scatter off in multiple directions and reduce the strength of the echo, edges are often saw-toothed. Saw-tooth shapes can be used throughout the plane wherever there might be discontinuities, such as along the trailing edge of a wing or at access panels or doors for landing gear or weapons.

The plane's surface can also help minimize radar echoes. Radar-absorbing materials, often made of fiberglass with carbon or ferrite particles, can be painted on or applied in composite skins. When radio waves hit these composites, much of their energy converts to heat. Sometimes designers add radar-absorbing structures beneath the skin. Honeycomb-shaped, these structures cause radar to bounce around inside the plane, dissipating energy with each bounce.

The Bird of Prey's engine intake is situated behind the cockpit, hiding its thermal signature from ground-launched missiles or thermal-imaging devices.

Heat
To avoid detection by thermal imaging devices or targeting by heat-seeking missiles, stealthy aircraft must minimize heat emissions. A major source of such infrared emissions is the engine exhaust. Engine intakes and exhausts can be mounted above the wings, lessening the chance that heat-seeking missiles shot from below will detect them. However, these masked features adversely affect aircraft maneuverability, so high-performance fighters like the F/A-22 and the X-35 JSF, which require excellent air combat capability, have their inlets and exhausts located more traditionally.

Engineers also design exhaust nozzles so that hot engine exhaust gases mix almost instantaneously with cold air surrounding the plane. Another source of heat is air friction caused by travel at very fast speeds, an effect that is particularly pronounced at the wing's leading edge. By pumping cool jet fuel inside the wing around the leading edge, this heat signature can be minimized. Sunlight glinting off the wings is another source of infrared radiation. It can be reduced through the use of IR-absorbing paints.

The Bird of Prey is a prototype stealth technology demonstrator designed mainly to avoid radar and visual detection. But it is also quieter than other fighters. Its maximum speed is only 300 miles per hour, well below the supersonic level.

Noise
Engines are an aircraft's major source of noise. Placing them on top of the airplane lessens the sound heard on the ground. In addition, great attention is paid to designing quiet engines. Traditional turbojets take a small amount of air and then shoot it out the back at very high speeds to create thrust, which is noisy. Both stealth aircraft and newer commercial planes, on the other hand, use a high bypass turbofan engine, which creates just as much if not more thrust by taking in a much higher volume of air but accelerating it less. The extra air also creates a cushion around the engine, which helps to dampen the sound. In addition, stealthy aircraft may be designed to fly only at subsonic speeds, so as to preclude the telltale sonic boom. However, higher performance stealth aircraft, like the F/A-22 and X-35, are designed to balance the tradeoffs between extreme stealth and excellent air combat performance (as measured by supersonic speed and exceptional maneuverability). These stealthy aircraft are therefore also capable of supersonic performance, and in the case of the F/A-22, of supercruise performance at very high altitudes.