Small, smart and deadly, micro air vehicles swarm onto the battlefield.

Lead photo by Jerry Telfer/San Francisco Chronicle

The Berkeley team is one of about a dozen groups of engineers and biologists who are exploring the final frontier of flight: micro air vehicles (MAVs). By merging the aerodynamics of insects with GPS navigation and molecular electronics, they hope to initially create an arsenal of tiny reconnaissance tools. When perfected, Fearing’s stainless steel and Mylar robot flies will be able to flap their way into the most secret places on Earth—the bunkers where Saddam Hussein plans his genocidal campaigns, and where Chinese spymasters plot their raids on America’s nuclear weapons laboratories.

Looking to warfare in 2020 and beyond, some military strategists envision swarms of robot flies fluttering onto battlefields. Scout flies, equipped with miniature cameras, would do the work of reconnaissance teams by eavesdropping on tactical communications and sending back real-time video of enemy positions. Sniper flies would seek out field commanders, recognizing them by the iris patterns of their eyes. Then, they would become the 21st century incarnation of the tribesman’s poison dart as they hurled themselves into the carotid arteries of their targets.

Meanwhile, titanium-tipped robot flies too small to register on radar screens would gather in the weeds at the end of enemy runways. Then, rising as a swarm, they would allow themselves to be sucked into jet engine air intakes. The MAVs titanium bodies would fracture the whirling turbine blades and send a rain of red-hot fragments through thousands of pounds of jet fuel and ammunition. In Pentagon parlance, MAVs have the potential to become the ultimate force multiplier.

As futuristic as these scenarios may seem, the Pentagon’s four premier research funding organizations—the Army Research Office, the Office of Naval Research, the Air Force Office of Scientific Research and the Defense Advanced Research Projects Agency (DARPA)—are taking them seriously. Together, they have put upward of $50 million on the table to create the flapping-wing airframes, microscopic jet engines and molecule-size avionics packages needed to make MAVs a reality.

Learning To Flap

Building MAVs is more complicated than scaling down large aircraft. “Engineers say they can prove that a bumblebee can’t fly,” says Berkeley’s Michael Dickinson. “And if you apply the theory of fixed-wing aircraft to insects, you do calculate they can’t fly. You have to use something different.” This is where biologists like Dickinson come in, to develop a general theory of insect flight that can be mechanically duplicated with tiny robots.

Creating lift, the force that keeps flying machines aloft, and maintaining control once airborne, involves a host of aerodynamic factors. The most important characteristics can be summarized in what aerodynamics experts call a Reynolds Number. Large, fast-flying commercial aircraft—a Boeing 747, for example—have high Reynolds Numbers, well exceeding 100 million, says Thomas J. Mueller, Roth-Gibson Professor of Aerospace and Mechanical Engineering at the University of Notre Dame in Indiana. As planes shrink and move more slowly, their associated Reynolds Numbers decline. A fixed-wing, 6-in. MAV cruising at 30 mph might have a Reynolds Number of about 130,000, says Mueller. As a consequence, it becomes more difficult to control. Reducing an aircraft to the size of a large insect reduces the Reynolds Number to below 20,000. At a number this low, fixed-wing aircraft need turning radii that would be too large to make the kinds of sharp right-angle turns needed to navigate tight spaces like ventilation ducts.

To be a fly on the wall, MAVs will have to fly like flies—in other words, flap their wings.

“The size and the Reynolds Numbers of MAVs correspond to very small birds,” says Mueller. “We have very little information on the performance of these airfoils and wing shapes, but there has been a long history of natural flight studies with insects and small birds that may be helpful.” In June, Mueller invited the leading experts on bird and insect flight from around the world to meet with MAV designers for a conference titled “Fixed, Flapping and Rotary Wing Vehicles at Very Low Reynolds Numbers.” One of the most important messages for MAV designers was that even though insects flap their wings, they don’t fly anything at all like birds.

A Bug’s Flight

“You can’t fly like a bird if you’re the size of an insect,” says Berkeley’s Dickinson. “You have to fundamentally rethink the problem.” Bird wings might not appear to have much in common with plane wings, but the equations that describe flight are fundamentally the same for passenger planes or passenger pigeons. “Steady-state aerodynamics of airplanes works well for birds,” says Dickinson. “If you treat a bird wing like an airplane wing and at any given time calculate the speed and lift, then sum it up over the entire stroke, it explains how the bird can stay aloft. With insect flight it fails miserably.” About a half-dozen efforts at duplicating bug flight are now under way.

Small, flapping-wing MAVs can take advantage of a new type of motor that produces linear rather than rotator motion. It is called a piezoelectric motor and operates like the needle on a turntable, only in reverse. Piezoelectric materials are crystals that produce a tiny current when they are placed under pressure or otherwise mechanically deformed. Some of these materials respond to current by moving. Their high-power density means they are capable of high force output.

At the Center for Intelligent Mechatronics at Vanderbilt University in Nashville, Tenn., researchers have successfully applied this theory to build tiny piezoelectric actuators that can flap wings. At Auburn University in Auburn, Ala., researchers have created materials that change flight-control surfaces using the same principle.

Fly Power

The biggest challenge facing MAV designers is the same problem encountered by early pioneers of manned flight: finding a sufficiently lightweight powerplant. Piezoelectric motors are efficient, but still require electricity. Wings covered with photoelectric materials work to a degree, but ultimately reach the point at which they have too little light-capture area to produce enough power to flap wings or run electronics. Batteries can provide power for short flights, but are too heavy for missions extending beyond a few minutes.

The consensus among experts interviewed by POPULAR MECHANICS is that MAVs will need to produce their power on board. There are three possible technologies. The most powerful is the 13mm Microjet demonstrated by the British Defence Evaluation and Research Agency (DERA) at this year’s Farnborough International 2000 air show. “By mixing hydrogen peroxide with kerosene or a similar fuel, we’ve achieved flight duration times of up to 1 hour,” says a DERA spokesman. “Starting and stopping the engine is very simple and is achieved by a simple on/off value, making it reliable and simple to operate in the field.”

MAVs duplicate the aerodynamics of insects, nature’s best fliers. PHOTO BY JERRY TELFER/SAN FRANCISCO CHRONICLE

PHOTO BY VANDERBILT UNIVERSITY

Slow-motion wing flapping revealed the secret of insect flight. PHOTO BY UNIVERSITY OF CALIFORNIA AT BERKELEY

Vanderbilt’s Piezoelectric motors will give the robot bugs muscle. PHOTO BY VANDERBILT UNIVERSITY

Like a conventional jet engine, the MIT miniturbine would have a combustion chamber, turbine wheel and compressor wheel. Fuel burning in the combustion chamber would send exhaust gases through the blades of the turbine wheel, causing it to rotate and, in turn, drive the compressor wheel via a central shaft.

Small as they are, the DERA and MIT jet engines will be like Saturn V rockets compared to the motors being developed at Georgia Institute of Technology in Atlanta. “We are now being driven by fundamental, technological and economical considerations to explore and evaluate systems that are smaller and smaller,” says Uzi Landman, director of Georgia Tech’s Center for Computational Materials Science. To study tiny nanojets, Landman and collaborator Michael Moseler are using molecular dynamics simulations to observe the furthest frontiers of fuel economy—combustion involving as few as 200,000 propane molecules.

The GPS Connection

What might seem to be the most formidable challenge in developing MAVs—getting tiny robot fighters to find their targets—is actually less of a problem than one might think. “Microelectronics technology is the driving force behind shrinking systems. On-board computational capabilities per unit volume will continue to increase,” says Epstein. He points to shrinking avionics, which include GPS receivers weighing as little as 6 grams, roughly the weight of 12 aspirins.

Getting instructions into MAVs is also fairly straightforward. Infrared (IR) ports, like those on personal digital assistants, will allow MAVs to be programmed in the field. And, once in the battlefield, IR ports can be used to send coordinating instructions within a swarm.

A major breakthrough that will make these systems even smaller was recently reported by chemists at the University of California at Los Angeles (UCLA). They have coaxed ringlike groupings of rotaxane molecules to exhibit the on/off behavior of transistors. Industry experts who have examined this process say it has the potential to put the computing power of 10 Pentium processors in one-hundredth the space of one of these tiny chips. And, because rotaxane molecule transistors could be switched on and off using light, there would be no bulky wire interconnections needed. MAVs will someday carry the computing power of an F-22.

The merger of Georgia Tech’s nanojets with UCLA’s molecular transistors might bring about a newer, even smaller and smarter class of robot warriors: nano-MAVs so small they could hide on the wings of real flies.

The microturbine developed at MIT would be used to generate electricity for tiny motors. PHOTO BY C.C. LIN AND M.A. SCHMIDT/MIT

The roughly quarter-size Microjet, shown with fuel valve, is the most powerful engine. PHOTO BY DERA

Georgia Tech has created the tiniest jets. PHOTO BY GEORGIA INSTITUTE OF TECHNOLOGY

Auburn University uses smart materials to replace hinged control surfaces. ILLUSTRATION BY AUBURN UNIVERSITY