In 1997 DARPA gave grants totaling several million dollars to several organizations to develop MAVs; AeroVironment, which had already begun attacking the problem on its own, was one of them. The company’s Simi Valley facility has produced a number of flying models, most of them of roughly circular planform, six inches in diameter, and powered by a single tractor propeller spinning at 20,000 rpm.
The most successful of AeroVironment’s models, nicknamed Black Widow, has remained aloft for more than 20 minutes flying at 35 mph. The ground operator launches it by compressed air from a telescoping rail, then controls it in flight by radio, like a model airplane—which, after all, it is. Unlike the typical radio-controlled flier, however, the Black Widow’s operator watches not the airplane itself, which is a mere speck darting in the sky, but the video picture sent back by its tiny television camera. The whole apparatus—airplane plus launch and control mechanisms—fits in a briefcase.
Fortuitously, Keennon says, various pieces of “COTS”—commercial off-the-shelf—hardware are available in the right size to fit on a six-inch flying disk. Flight controls, for example, are operated by tiny Swiss-made electric motors an eighth of an inch in diameter and 0.01 ounce in weight. The airplane’s “eye,” also an inexpensive item, is a 510- by 492-pixel color array like the ones used in home video cameras but stripped down to the size of a bean and the weight of 0.05 ounce.
AeroVironment’s current MAVs are skittish creatures, with high roll rates and low natural stability. They require skilled radio control operators. The next step in the program, which the company is currently pursuing with its own funds, independent of DARPA’s support, is to add electronic gyroscopes and autopilots that will keep the airplanes stable and upright. The operator would then need no special skill to fly one, and would be free to concentrate on the mission rather than on controlling the aircraft.
After adding stability, the next improvement will be GPS navigation, which would permit the MAV to fly a programmed mission without assistance from a human operator. The icing on the cake would be some kind of system using acoustic or optical sensing that would let it maneuver in an urban environment, avoiding obstacles on its own, just like a bird. That level of autonomy, however, is still far off.
The requirement that it send back usable video images puts an important lower limit on the size of a MAV, because each pixel in the imaging array must be considerably larger than the longest wavelength of visible light. This means that a video camera capable of sending back useful detail can’t be much smaller than the one Keennon’s team is now using. Another non-scalable item is the radio antenna. An antenna that fits within a six-inch space works efficiently only with short-wavelength, high-frequency radio waves. Unfortunately, high-frequency radio signals travel by line of sight—both antennas have to be able to “see” each other—and do not readily penetrate walls or travel around hills. A longer retractable trailing wire, however fine, would impose a severe drag penalty. Antenna size will also pose a problem for GPS reception, especially if future MAVs became significantly smaller than the current ones.
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.