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'Flying Wing Boomerangs Into Favor'

July 30, 1988

The discussion of flying wings ("Flying Wing Boomerangs Into Favor," Part I, July 4) notes potential virtues of the configuration but was too brief to view the subject from the total perspective of alternative designs.

The comments ". . . Cargo planes or passenger planes would be dramatically more efficient. Planes with vast range and payload capacity could be smaller and cheaper," deserve fuller examination. The comments may prove misleading.

When an aeronautical engineer works toward a performance goal, and explores the trade-offs in various configurations by which the goal might be achieved, the result is usually conventional: tail in the rear.

The standard outcome is not because the designer is unaware of other configurations or because he is locked into a traditional configuration, but rather that conventional design emerges the winner after an objective weighing of the trade-offs. Because conventional usually wins, conventional is more common--hence conventional.

One common goal is to carry people or cargo long distances at high speed in an airplane that lands and takes off slowly and is economical to build and operate, and safe to fly. High lift devices (slats, flaps) permit slow speed takeoffs, then they are retracted for high speed flight.

In the slow speed, high lift mode, the associated large wing pitching movement can conveniently be balanced by the conventional tail. The tailless airplane does not have the stabilizing tail and so must either forgo high lift devices and use a larger wing (reducing the high speed flight capability) or obtain a partial fix by employing a modern, active control system (at the price of considerable complexity, meaning more cost and less reliability).

The versatility of birds demonstrates that flying wings, using active control, can handle a wide speed range. For some birds, tail size is almost negligible, and certain ones even fly well during molting when all tail feathers are gone.

With an airliner, the classic streamlined cigar-shaped fuselage is a low drag way of accommodating hundreds of passengers. As long as you have the fuselage, you might as well put the tail on the rear and, without the complexity of active control, conveniently utilize the best available high lift devices.

It is interesting to note that all high performance sailplanes have conventional configurations. The designs must be efficient and safe over a wide speed range. Do canards (horizontal stabilizer in front) and flying wing sailplanes work? Certainly. But in competitions, where top performance is the criterion for success and hence the evolutionary driving force for designs, conventional wins 100% of the time.

A stealth airplane has other operational goals than a wide speed range, safety, and economy, and it need not carry a large number of passengers; it does need a small radar cross section. Therefore, considering all the trade-offs, the flying wing appears to be an excellent design.

For a small two-person airplane, as discussed in your article, a satisfactory flying wing can be made, but it is the hard way to do an easy task and is unlikely to match the performance of a conventional configuration.

PAUL B. MacCREADY

Monrovia

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