Can tiny aircraft deliver the big picture?
- By Peter Garrison
- Air & Space magazine, May 2000
(Page 4 of 5)
At the Georgia Institute of Technology Aerospace Laboratory, Robert Michelson leads a project to develop and refine an entomopter, a machine that will not only fly like a bug but, if need be, crawl like one too. The entomopter has a “chemical nose” and other features to permit it to home in on certain kinds of targets. Its builders expect to provide it with navigation and obstacle-avoidance skills as well. But the present centerpiece of the project is its power plant, a device called a reciprocating chemical muscle.
The RCM is something like the piston and cylinder of a steam engine, except that the gas that drives it comes not from combustion but from a chemical reaction. The energy available from the chemical fuel is much greater than that available from current batteries. And the chemical reaction also has the advantage of versatility: Its waste heat can be converted into electricity to operate onboard sensors and transmitters, and spent gas can be vented over the wings to provide differential lift and, therefore, flight control.
By calling their prime mover a “muscle,” the Georgia researchers underscore their reliance on the guidance of Mother Nature. “Nothing in nature achieves sustained flight with fixed wings or with propellers,” observes Michelson. “All tiny creatures flap their wings continuously. Flies don’t glide.”
A similar project, called the Micromechanical Flying Insect, is under way at the University of California at Berkeley, where a team headed by biologist Michael Dickinson has shed light on how insects use their wings. To simulate the Reynolds number of insect flight, Dickinson and co-workers built and instrumented a pair of 10-inch wings driven by six separate actuators, and have observed them flapping in a tank of mineral oil. In addition to a new understanding of very-low-Reynolds-number aerodynamics, such work has spawned a new vocabulary for talking about flight phenomena, with terms like “delayed stall,” “rotational circulation,” and “wake capture.”
Wing flapping works in several ways to provide insects with a flying ability that would be the envy of any fighter pilot. To start with, the flapping of wings plays the same role as the spin of a helicopter’s rotor: It creates a relative wind over the lifting surface even while the vehicle—or bug—is standing still. But flapping also sets up tiny vortices that take the place of the cambered flying surfaces, high-lift devices, and moveable flight controls of fixed airplane wings. The eddies set up by their wings not only keep bugs aloft but also allow them to hover, fly backward or sideways, and turn on a dime (or the corresponding currency of the bug world).
Putting the new understanding to practical use is the next step, and not an easy one. The Berkeley team, with some sponsorship from DARPA, proposes to duplicate, in a mechanism about the size of a quarter, at least some of the abilities of a large, repulsive, carrion-eating fly called Calliphora. “You can’t build [robot insects] now based on known principles,” Dickinson has said. “You have to fundamentally rethink the problem.”
Most of the proposed uses for MAVs are military; the funding, after all, is coming from the Department of Defense. But some workers in the field propose broader applications for tiny flying robots. Georgia Tech’s Michelson has suggested sending robot “terminators” after real-life insect pests, but suspects that the largest potential outlet for small aerial robots might be the toy market. Stanford’s Ilan Kroo leads a team developing a “mesicopter,” a multi-rotor electric helicopter. Currently of centimeter size but potentially much smaller, the mesicopter is shaped like a thin, square wafer with a little rotor at each corner. Essentially a flying microchip, a mesicopter’s motors, sensors, guidance, and telemetry systems would be etched in place in a single completely automatic manufacturing operation. Kroo’s team envisions swarms of mesicopters investigating the interiors of storms or the atmosphere of Mars.
The word “swarm” is particularly significant. Of course, it suggests insects, and much of the more startling MAV research is headed in the direction of emulating those successful products of natural selection. But it also alludes to nature’s profligacy. Many creatures that live in hazardous environments reproduce in huge numbers so that just a few may survive to maturity. MEMS manufacturing techniques imply a similar approach to machines. Rather than launch a single costly, sophisticated, man-carrying device to do a job, you would launch hundreds of simple, cheap robots. If most of them fail, no matter—they are expendable. Only one needs to complete the task.