By J.R. Wilson
WASHINGTON - The small speck in the sky approaches in virtual silence, unnoticed by the large gathering of soldiers below. In flight, its tiny size and considerable agility evade all but happenstance recognition. After hovering for a few short seconds, it perches on a fifth floor windowsill, observing the flow of men and machines on the streets below.
Several kilometers away, the platoon leader watches the action on his wrist monitor. He sees his target and sends the signal. The tiny craft swoops down on the vehicle, alighting momentarily on the roof. It senses the trace of a suspected chemical agent and deploys a small tagging device, attaching it to the vehicle. Just seconds later it is back in the sky, vanishing down a narrow alley. Mission accomplished.
While it may sound like science fiction, this is a plausible mission of a future unmanned aircraft called a micro air vehicle (MAV), according to experts James McMichael, MAV program manager at the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., and former Defense Airborne Reconnaissance Office chief retired Air Force Col. Michael Francis.
Although a bumblebee, a scout, a sparrow, and a forward observer do not have much in common today, all will share a link early in the next century to a family of insect-like MAVs if scientists at DARPA are successful.
These future low-cost air vehicles will be small - no more than six inches in length, width, or height - and will be equipped with microsensors that enable them to perform a host of dangerous military missions. DARPA`s vision calls for individual soldiers at the platoon, company, or brigade level to carry MAVs into battle, and deploy them as needed for reconnaissance and surveillance, battle damage assessment, targeting, sensor emplacement, communications relays, or for sensing chemical, nuclear, or biological threats. Capable of real-time imaging, MAVs will be able to fly out to six miles at speeds as fast as 30 miles per hour for missions ranging from 20 minutes to 2 hours.
Simultaneous developments
It was impossible to even consider such vehicles until quite recently, when scientists developed a series of advances almost simultaneously in several micro-technologies. Chief among these was the rapid evolution of micro-electromechanical systems (MEMS), but also necessary to the success of an MAV are such micro systems as tiny charged-coupled-device-array cameras, equally small infrared sensors, and chip-sized hazardous substance detectors.
But being able to shrink the payload down to the size of a small bird is only half the battle. Now researchers are pushing for innovative technical solutions in aerodynamics, control, propulsion, power, navigation, and communications.
How do sparrows and bumblebees fit into this scenario?
Because of their size, MAVs will not be able to use conventional mechanical flight methodologies, such as propellers or jets. Instead, researchers are looking to nature`s approach to flight by attempting to mimic insects and small birds to resolve the old roadblock depicted by the Reynolds number (a measure of size multiplied by speed).
"While naturalists have seriously studied bird and insect flight for more than half a century, our basic understanding of the aerodynamics encountered here is very limited," McMichael explains. "Neither the range/payload performance of bees and wasps nor the agility of the dragonfly is predictable with more familiar high Reynolds number aerodynamics traditionally used in UAV [unmanned aerial vehicle] design. And if our understanding of low Reynolds number effects is limited, our ability to mechanize flight under these conditions has been even more elusive."
Designing tiny autonomous aircraft presents a serious systems-integration challenge in difficult conditions, he says. "With the small size of the MAV comes high surface-to-volume ratios and severely constrained weight and volume limitations. The technology challenge to develop and integrate all the physical elements and components necessary to sustain this new dimension in flight will require an unprecedented level of multifunctionality among the system components. The traditional "stuffing the shell" paradigm of conventional aircraft design is not likely to be workable for MAVs," McMichael says.
Innovative approaches
The weight of a 6-inch, fixed-wing MAV may be less than 2 ounces, yet it will need to remain airborne for as long as two hours while carrying a payload weighing less than an ounce to its 6-mile maximum range. It is unlikely a conventional, moderate-aspect-ratio, fixed-wing design would fit the bill. MAVs, therefore, may require more unusual configurations and approaches, ranging from low-aspect-ratio fixed wings to rotary wings or even more radical notions like flapping wings, note McMichael and Francis.
To fit everything into such a small package will require an unprecedented level of design integration in which many components serve multiple - and often diverse - functions. For example, the wings also may function as antennae or sensor apertures, while the power source may be integrated with the fuselage structure. Such small vehicles also will be far more susceptible to wind gusts, which heightens automatic flight- control challenges. This will also confront users with complex problems if they operate MAVs, as expected in urban "canyons."
In traditional design, the smaller the vehicle, the slower the speed and, usually, the higher the ratio of wing area to vehicle weight. Because MAVs will have limited wingspans so that soldiers can carry them in their packs, these tiny aircraft may need a flying wing design or even mimic the multiple, contoured wings of butterflies, according to McMichael and Francis.
"MAVs may have to cope with fully three-dimensional aerodynamics," McMichael and Francis wrote in an August 1997 DARPA paper. "Here, there are even less low-Reynolds number data available than there are for two-dimensional airfoils. To make matters worse, MAVs will experience highly unsteady flows due to the natural gustiness [turbulence] of the atmosphere. Interestingly, nature`s flyers of the same scale use another source of unsteady aerodynamics - flapping wings - to create both lift and propulsive thrust. For some applications, MAVs may ultimately have to do the same.
The answer to these problems may be new highly integrated flight-control systems with autonomous stabilization. "In confined areas like urban canyons and interior spaces, autonomous collision-avoidance systems will also be required," they wrote. "Small-scale propulsion systems will have to satisfy extraordinary requirements for high energy density and high power density. Acoustically quiet systems will also have to be developed to assure covertness."
Research contracts
On 12 December 1997, DARPA officials selected for negotiation six proposals to develop flight-enabling technologies for MAVs, with approximately $12 million allocated for that effort through the end of the century. The selected proposals, ranging from $650,000 to $3 million, are:
- Massachusetts Institute of Technology in Cambridge, Mass. - "MEMS-Based Micro-Gas Turbine Engines for Micro-Unmanned Aerial Vehicles;"
- D-STAR Engineering in Shelton, Conn. - "Low-Observable, Safe-Operation, Fuel Efficient, Light-Weight Propulsion, and Power System for Advanced Micro Air Vehicles;"
- Technology in Blacksburg Inc. of Blacksburg, Va. - "Thermoelectric-Based Advanced Micro-Air Vehicle;"
- SRI International in Menlo Park, Calif. - "Flapping-Wing Propulsion Using Electrostrictive Polymer Artificial Muscle Actuators;"
- Vanderbilt University in Nashville, Tenn. - "An Elasto-Dynamic Ornithoptic Flying Robotic Insect;" and
- California Institute of Technology in Pasadena, Calif. - "Micro Bat."
In addition, DARPA officials issued 2-year, $750,000 contracts to four small businesses under Phase II of the Small Business Innovation Research program to continue MAV-related research.
Engineers from IGR Inc. of Beechwood, Ohio, will complete development and demonstration of a very lightweight solid oxide fuel cell, tailored to the electrical power requirements of MAVs.
Designers from M-DOT Inc. in Phoenix, meanwhile, will continue to develop a gas turbine engine with 1.4 pounds of thrust; experts from AeroVironment Inc. in Simi Valley, Calif., will continue development and flight demonstration of an electric-powered, fixed-wing reconnaissance MAV; and engineers from Aerodyne Corp. of Billerica, Mass., will continue development of a hover vehicle that also will explore the capabilities of the mini-scale engine being developed at M-DOT.
DARPA leaders will issue additional MAV system development and demonstration contracts to companies in the near future to design and flight-test a 6-inch MAV. Overall, DARPA officials say they expect to spend about $35 million through 2000 on MAV enabling technologies and systems development, including initial flight units.
Urban warfare
"The resulting capability should be especially beneficial in the emerging urban warfighting environment, characterized by its complex topologies, confined spaces and areas [often inside buildings] and high civilian concentrations," McMichael says.
Reconnaissance is a primary application driver for the first generation of MAVs, as developers try to enhance situational awareness for small units or individual soldiers while simultaneously reducing their exposure to enemy fire.
That same application is a size driver - the MAV must be small enough individual soldiers can carry them easily and still have room for water, ammunition, and other necessities. Also pushing a compact design is the requirement for covert operation in most MAV missions.
The system also must be affordable enough to be discarded rather than repaired, but it also must be recoverable, if necessary. A ballpark unit flyaway cost mentioned in some DARPA literature is less than $1,000. Achieving that goal would lead to another primary MAV field operations goal: An almost non-existent logistics tail.
MAVs are not intended to replace either manned missions or traditional UAV missions, but to offer an increased level of flexibility and capability - eventually including operating within and relaying images from inside buildings - a capability not found in any other systems.
But the MAV concept has much broader implications and applications. An MAV could be built into ejection seats, for example, and automatically launched while the parachuting pilot is still in the air. This would provide a reconnaissance asset to the downed pilot as well as providing an aerial view to guide rescuers.
Commercial operators also could use them for everything from fire detection, border patrol, traffic monitoring, and power line inspection.
