The term, Micro Air Vehicle, may be somewhat misleading if interpreted too literally. We tend to think of flying model aircraft as "miniature", so the term "micro" now alludes to a class of significantly smaller vehicles. But MAVs are not small versions of larger aircraft. They are affordable, fully functional, militarily capable, small flight vehicles in a class of their own. The definition employed in DARPA's program limits these craft to a size less than 15 cm (about 6 inches) in length, width or height. This physical size puts this class of vehicle at least an order of magnitude smaller than any missionized UAV developed to date.
MAVs should be thought of as aerial robots, as six-degree-of-freedom machines whose mobility can deploy a useful micro payload to a remote or otherwise hazardous location where it may perform any of a variety of missions, including reconnaissance and surveillance, targeting, tagging and bio-chemical sensing.
Although the 15 cm limitation may appear somewhat arbitrary, it derives from both physics and technology considerations. To fully appreciate the scale implications, we can compare this class of vehicle with other familiar systems, as in Figure 1. This is a plot of vehicle gross weight vs Reynolds number. The Reynolds number (a measure of size multiplied by speed) is perhaps the most useful single parameter for characterizing the flight environment. The smallest current missionized UAV is the "Sender", developed and operated by the Naval Research Laboratory. Sender boasts a 4 foot wing span and weighs only 10 pounds - impressive specifications for its near 100 mile range capability. MAVs are an order of magnitude smaller and may display a wide variety of configurations, depending on specific mission requirements.
The Micro Air Vehicle Flight Regime Compared to Existing Flight Vehicles
The low Reynolds number regime is significant in that it projects a fundamental shift in physical behavior at MAV scales and speeds - an environment more common to the smallest birds and the largest insects. 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. 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 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.
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.
Yet to be developed, Micro Air Vehicles will be roughly one-tenth the scale of the Sender, and the weight of a six-inch, fixed-wing MAV may be only 50 grams or so, just one one-hundredth the weight of the Sender. Figure 2 illustrates the difference in size. Yet MAVs must be capable of staying aloft for perhaps 20 to 60 minutes while carrying a payload of 20 grams or less to a distance of perhaps 10 km. Finding high density sources of propulsion and power is a pivotal challenge. And while the Sender is a conventional, moderate aspect-ratio, fixed-wing aircraft, MAVs 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.
Size Comparison Between an MAV Concept Vehicle and a Small UAV
