Flying wings

Flight Journal, Oct 2003 by Tucker, Charles, Quinn, J J

For military airplanes, the flying wing has another advantage that is illustrated by the Northrop B-2 Spirit bomber. The smooth, blended shape of the flying wing reflects far less radar energy than the collection of surfaces and intersections on a conventional airplane. This makes the airplane much harder to detect, and that is a dramatic advantage for military missions.

The flying wing's radar stealth did not start with the B-2. The original Northrop jet flying wing, the YB-49, flew tests against the American air-defense radar net of the time. The YB-49 proved almost impossible to detect on those radar sets. Amazingly, the generals of the time did not appreciate the military implications of this and opted for the Convair B-36 instead of the B-49.

In Germany during WW II, the Horten brothers understood the advantages of low radar detectability. Their 1944 Horten IX jet-fighter prototype had double plywood skins with a layer of radar-absorbent material between them.

TECHNICAL CHALLENGES

While a flying wing may be the purest form of airplane, it is also one of the most difficult configurations to design well. The conventional "wing, body, tail" configuration exists for very good technical reasons. The major advantage of the conventional configuration is that each piece of the machine performs one major function. The wing provides lift and the fuselage carries the payload. The horizontal tail stabilizes the airplane in pitch and provides pitch control, and the vertical tail stabilizes the airplane directionally and provides yaw control.

The designer of a conventional airplane can affect specific airframe characteristics by adjusting the shape or size of one airframe component. Because the components' functions are relatively independent of one another, changing one component to affect one aspect of the design's aerodynamics or balance will not have a large effect on other characteristics.

On a flying wing, the designer has no such luxury. There is an old saying that any airplane is a bunch of compromises flying in formation. On a flying wing, however, the compromises talk to one another. The wing alone must perform all of the functions that the many components of the conventional airplane do. It must provide aerodynamic lift, stability, trim and control, while at the same time carrying the payload and fuel inside and keeping it all in balance. The coupling between effects further complicates the designers' problem. A change in the shape of the wing affects multiple characteristics of the airplane. The change you need to help in one area will likely hurt somewhere else. This makes it much harder to make everything come together at the same time to give an airplane with acceptable characteristics in all areas.

PITCH STABILITY AND TRIM

For an airplane to be stable in pitch, the center of gravity must be forward of the aerodynamic center of the configuration. The aerodynamic center is a point on the airplane at which the moments do not change with angle of attack. Any changes in lift caused by changes in angle of attack act through the aerodynamic center. On a conventional airplane, the designer can adjust the position of the wing on the fuselage and the size of the tail to make the airplane balance. With a flying wing, the only variables available to adjust stability are the wing's plan-form and the position of the major components and payload within the wing.