Can tiny aircraft deliver the big picture?
- By Peter Garrison
- Air & Space magazine, May 2000
(Page 3 of 5)
The peculiar configuration of AeroVironment’s MAVs is the logical outcome of the six-inch size restraint. If you merely scaled a conventionally proportioned airplane down to a six-inch wingspan, its wings would have an area of only about .04 square foot. Flying at 30 mph—a higher speed would require too much power—such a wing could support only about three-quarters of an ounce at most, with no margin for maneuvering or gust response. But the weight of the entire aircraft, including powerplant and all the electronic and sensing equipment it is supposed to carry, would in reality be around two or three ounces.
It turns out that the best solution is simply to make the wing area as large as possible—essentially, to fill the entire six-inch DARPA circle with wing. This approach has other advantages as well: It provides a simple, stiff, voluminous structure with ample interior space for systems and payload. True, the circular planform lacks the characteristic usually associated with efficient airplanes: a fairly high aspect ratio. The most efficient airplanes have wings whose span from tip to tip is much greater than their chord—the distance from leading to trailing edges—and you don’t see a lot of airliners with circular wings.
But for an airplane of this size or smaller, a low aspect ratio may not be a hindrance. The very wingtip vortices that produce drag on conventional airplanes help produce lift instead on small, short-span wings operating at low Reynolds numbers (see “Mr. Reynolds, We’ve Got Your Number,” next page). In fact, recent research on insect flight suggests that the judicious use of tip and leading edge vortices keeps those notoriously small-winged bumblebees—the ones that, according to legend, myopic scientists have pronounced flightless—aloft. This is only one of the differences, fundamental to the creation of miniaturized aircraft, between full-scale and micro-scale aerodynamics. The behavior of air on micro-scale wings is only beginning to be understood.
Although most of the systems of a MAV are electronic and AeroVironment has concentrated on electrically powered airplanes, not everyone agrees that an electric motor is the best choice for a powerplant. Batteries have a low “power density”—that is, they pack little punch for their weight. (This is a problem for electric cars as well.) For some tasks, such as peering into upper-story windows or loitering inconspicuously, an aircraft that can hover is essential; at present, battery-powered electric motors don’t have the power to hover for long.
Those traveling the all-electric route look to future improvements in batteries, motors, and propellers, as well as to further miniaturization, for increases in power-to-weight ratio and efficiency—the fraction of the available power that goes into useful work—of tiny power plants. But gram for gram, chemical fuels like gasoline are much more energetic than batteries, and even though extremely tiny internal combustion engines, unlike tiny electric motors, are not available off the shelf, several programs are taking the internal combustion route instead.
MLB Company of Palo Alto, California, has flown several designs powered by small Cox model airplane engines. One of them takes off vertically. Stephen Moore of MLB says that at this scale the power requirement for vertical takeoff and hovering is not terribly different from that for agile maneuvering. Given the tremendous energy content of chemical fuel, a multi-mode tail-sitter craft that can both fly and hover becomes an attractive possibility.
A startling solution to the power problem is in the offing at the Massachusetts Institute of Technology in Cambridge, Massachusetts: a jet engine the size of a shirt button. Components of such engines have actually run in test beds. The baseline design involves a single centrifugal-flow compressor spinning at 2.5 million rpm on gas bearings. Combustion takes place in a doughnut-shaped chamber surrounding the engine, and the exhaust gas flows back inward toward the center through a turbine. A starter-generator is built into the case; if needed, the engine could serve as a tiny electrical generator, putting out 10 to 20, or perhaps as much as 100 watts, or it can be used as a jet engine with a thrust of up to a third of a pound.
The key to making such a device cheaply and in large numbers is a version of the same photolithographic manufacturing technique used at Caltech to make the wings of the Microbat. Engine parts would be etched in sheets of silicon, like microchips. (By the early 1990s, electric motors smaller than the point of a pin, invisible to the naked eye, had already been made by this technique.) Just one micro-engine would be sufficient to supply both the thrust and the electrical requirements of a present-day MAV.