Can tiny aircraft deliver the big picture?
- By Peter Garrison
- Air & Space magazine, May 2000
(Page 2 of 5)
In 1995 DARPA put out a specification for a small camera-carrying aircraft. Six inches—an arbitrary value, but one that has turned out to make practical sense—was the basic constraint: The entire aircraft had to fit within a six-inch sphere.
DARPA also specified a typical mission. The midget spyplane would fly one kilometer, just over half a mile, to a target; loiter there for half an hour in turbulent winds of up to 25 mph, perhaps maneuvering among obstacles such as buildings while repeatedly climbing to 350 feet and descending again; then return to its base. It had to be quiet and inconspicuous, its launching and control system had to be easily portable and operable by an unskilled person, and the whole system had to be both robust and cheap.
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.