The US Defense Advanced Research Projects Agency (DARPA) Nano Air Vehicle (NAV) programme now has four competing concepts, as studies continue to develop the necessary aerodynamic design tools.
DARPA's goal is to demonstrate an indoor-flying unmanned vehicle with a maximum dimension in any axis not exceeding 7.5cm (2.95in), with a 0.01kg (0.02lb) maximum take-off weight, a payload of about 2g, a top speed of up to 1,720ft/min (10m/s), a 20min endurance and a range of 1km (0.54nm).
The four competing teams are Aerovironment, Draper Laboratories, Lockheed Martin, and California-based Micropropulsion. Two are flapping wing designs: Aerovironment's bird-like ornithopter and Micropropulsion's insect-like entomopter. Draper's is a coaxial rotorcraft and Lockheed's an unconventional maple seed-like vehicle.
While the aerodynamics of large manned and unmanned aircraft are well known, the conditions for air vehicles as small as a hummingbird or insect are largely unknown.
"We need new design tools. They need to be developed for the very low Reynolds numbers [at which an NAV will fly]," says former DARPA programme manager Darryll Pines, speaking at the Association for Unmanned Vehicle Systems International/US Air Force Research Laboratory-organised 2nd US-European MAV competition and workshop held in Fort Walton, Florida from 30 October-2 November.
The Reynolds number (Rn) is an aerodynamic term used to quantify the ratio of inertial to viscous forces in air and is related to the speed and scale of the air vehicle. Full-size aircraft have Reynolds numbers ranging from a few million to around 70 million for the Boeing 747.
A fruit fly has an Rn of around 500. For micro air vehicles the number is around 100,000. A NAV would fly at a Reynolds number of 5,000-15,000.