1. Defense Advanced Research Projects Agency defined Micro air vehicles in 1992 as "affordable, fully functional, militarily capable, small flight vehicles which have a maximum size of 15 cm (about 6 inches) and which cater for special, limited duration military and civil missions" . Micro air vehicles were later considered as aerial robots with six-degrees-of-freedom and which can deploy a useful micro payload to a remote or otherwise hazardous location. Though the term "micro" is misleading it was used since model aircrafts were denoted as "miniature vehicles" and this new class of vehicles is significantly smaller than them. Further 15 cm size was arbitrarily fixed based on technology and physical considerations but now all smaller size (hand launched) unmanned aerial vehicles are grouped under this category.
Technical Challenges
2. Due to their small size, micro air vehicles face a number of technical challenges as illustrated below. A flight regime comparison of micro air vehicles with other flight vehicles is depicted in Fig.10, which shows clearly that they are substantially smaller than smallest unmanned aerial vehicle (Sender).
Aerodynamic issues. The conventional aircraft operates in high Reynolds number (measure of size and speed) regime in comparison to complicated low Reynolds number regime of micro air vehicles, which is common to insects/small birds.
Fig. 10. Flight regime of Micro Air Vehicles
Propulsion issues. Due to small size, miniaturised and lightweight power generation equipment, motor, battery, etc. are required.
Control issues. Smaller control surfaces along with smaller moment arms, requirement of miniaturised sensors/avionics due to space constraints, etc. complicates control issues.
Structural issues. Requirement of high surface to volume ratio, limitations of weight / volume, strong and lightweight structures, flexible wing designs, etc. exist.
Allied issues. In addition, ability to withstand nasty, treacherous military working environment, cope external flight issues like ambient temperatures, winds, moisture, launch system severity (like speed / launch angle) and handle shocks are important. Simple logistics trail, repair capability in field conditions and low costs are other considerations.
3. General Design Methodologies. Due to above constraints, designing micro air vehicles is a great challenging task to aerospace engineers. The conventional "stuffing the shell" type of aircraft design is not suitable and a highly integrated multifunctional system is required for these vehicles. Two design methodologies are followed for developing practical micro air vehicles. The most popular approach is conventional aircraft like configuration (where lift and thrust are provided separately) and second approach is direct mimicry or imitation of birds (where both lift and thrust are provided simultaneously).
Types of micro air vehicles
4. The micro air vehicles are broadly grouped into three main types
(Fig. 11. refers) as illustrated below.
(a) Fixed Wing (or) conventional design, where the flying wing produces lift and conventional propeller produces necessary thrust. Ex. WASP micro air vehicle, Black Widow, etc
(b) Rotary Wing, where both lift and thrust are produced by rotary wing. Ex. Microcraft, T Hawk.
(c) Flapping Wing that are direct mimicry of flying birds / insects where both lift and thrust are produced by flapping wing. Ex. Microbat, Entomopter
5. Each type has advantages and disadvantages. For example, rotary wing micro air vehicles can hover but has reduced loiter time, flapping wing micro air vehicles have complex control issues apart from high power consumption, etc. Hence, presently fixed wing conventional design is the most efficient micro air vehicle.
(a) WASP (b) Black Widow (c) Microcraft
(e) Microbat (f) Entomopter
Fig. 11. Types of Micro Air Vehicles
Components
6. A typical micro air vehicle system contains apart from micro air vehicle, a ruggedised tablet PC that serves as Operator Control Unit and a Ground Data Terminal for robust communication interface between Control Unit and micro air vehicle (Fig. 12. refers). These can all be carried and deployed by maximum of two soldiers. Based on the task/mission, micro air vehicle system can have a variety of adapted and integrated sensors/payloads (like optical, infrared, acoustic, bio-chemical, nuclear etc). The micro air vehicle can be launched from ground, from air using another unmanned aerial vehicle (or) aircraft and even from a ship.
Fig. 12. Components of Micro Air Vehicle system
Capabilities / Advantages of micro air vehicles
7. The advantages of micro air vehicles in comparison to unmanned air vehicles are enumerated below.
Compactness
Covertness
Transportability by single operator
Rapid deployment
Low radar cross section (stealth)
Low noise
Low production cost
Low weight
Maneuverability in a complex urban environment
Freedom from extensive logistics requirements
No logistical footprint and
Real time data acquisition
8. Due to these capabilities, it is a tremendous tactical and logistic asset to a soldier on the field. If their range, endurance, stealth, precision and mission versatility can be improved, their capability as a potent weapon system will increase. The covertness of micro air vehicle is due to their resemblance to small bird, whose radar signatures will be lost in clutter, airspeeds are below radar's minimum detectable velocity, low emitting power cannot be picked up by infrared sensors at higher ranges and minimal acoustic signature. Infact the above reasons can also contribute towards proliferation of micro air vehicles by other players as they can be developed using commercial off-the shelf products posing significant threats to air dominance.
Probable Applications
9. Military Applications. Some of the potential military applications/missions for micro air vehicles are as follows.
Intelligence, Surveillance and Reconnaissance
Target designation
Communication relay
Battle damage assessment
Air/Artillery spotting
Urban combat / Anti - terrorist operations
10. Civilian Applications. Some of the likely civilian applications/missions are as follows.
Monitoring hazardous environments and natural disasters like collapsed structures, chemical spills, caves, floods, fire, etc
Inspection of electrical transmission lines
Inspection of oil / gas pipelines
Biological/chemical sensing
GPS/telecommunication nodes
Traffic monitoring
Agricultural/environmental and crop monitoring
Wild life monitoring
Aerial imagery, Terrain mapping and Archeology
Police and security surveillance/interdiction and riot control
Hostage rescue
Border patrol (immigration/drug trafficking)
Civilian search and rescue missions including avalanche, earthquake survivor searches, etc.
Land and real estate management
