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Can tiny aircraft deliver the big picture?

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.

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.”

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