Except for the mechanical chatter, AeroVironment’s hummingbird micro-unmanned air vehicle (UAV) comes tantalisingly close to the ripping acceleration and acrobatic acumen of the real thing.
Cutting down the noise from the hummingbird's wings remains a challenge
Weighing in at 19g (0.67oz) with a 16.5cm (6.5in) wingspan, it has roughly twice the wingspan and is 6x heavier than the ruby-throated hummingbird, but it is smaller and lighter than South America’s giant hummingbird, which weighs up to 24g and has a wingspan of more than 20cm. Powered by an “off-the-shelf technology” battery, the so-called nano hummingbird has demonstrated an endurance of 11 minutes (without the hummingbird fairing) under full authority by an operator with a radio controller. Manoeuvrability includes not only vertical take-off and landing, but fore and aft flight to speeds as high as 11mph (18km/h), as well as side-to-side and rotational flight, the ability to handle side gusts up to 2m/s (6.6ft/s), and aerobatics, such as flips.
“We’re halfway there,” says Matt Keennon, AeroVironment’s nano air vehicle (NAV) project manager, of how close his company’s hummingbird is to the real thing. The lone biologically-inspired prototype is the output of the second phase of what was intended to be a three-phase Defense Advanced Research Projects Agency (Darpa) NAV programme, started in 2005. AeroVironment, based in Monrovia, California, was one of four companies with phase-one contracts. Two companies – Draper Labs and Lockheed Martin – built rotary-wing nanos, while AeroVironment and MicroPropulsion focused on flapping-wing aircraft. AeroVironment partnered with Duke University for wing shape and the University of Illinois (Urbana) for aerofoil research in the first phase.
Only AeroVironment was selected for a phase-two project under the NAV programme, which called for an “agile” aircraft that could “fit in one hand”. Keenon says the specifications called for a flapping-wing micro air vehicle that could fly 1km (0.54nm) outdoors and be able to “fly through a door, down hallways, and find a room with suspicious items”. Payload initially was to be 2g – an onboard camera, in this case – with a 7.6cm maximum dimension and 10g total weight.
“The final prototype achieves the noteworthy milestone of two-wing flapping, hovering and fast-forward flight with all power sources on the aircraft and all controls implemented through modulation of the wing strokes in a shape that resembles a real hummingbird, and carries an on-board camera that relays video to the pilot in real time,” says Darpa of the final phase-two product, unveiled to the public in February this year.
The agency has not revealed whether it will launch a third phase of the programme, in which AeroVironment would make the NAV smaller and lighter. “Technologies for batteries and different subsystems that were supposed to come along in parallel did not materialise,” says Keennon of the phase-two development.
Hidden beneath Darpa’s praise is decades of research and development on flapping-wing flight at AeroVironment, the most visible being company founder Paul MacCready’s work with the Smithsonian Institution on a half-scale flying Pterodactyl replica in the 1980s. The 5.5m-wingspan electric “aircraft” made 21 successful flights during the filming of the Smithsonian’s On the Wing, but crashed during a public display of the vehicle during the 1986 Armed Forces Day air show at the Andrews Air Force Base in Maryland. In the late 1990s, AeroVironment worked with the California Institute of Technology and the University of Southern California on a Darpa-funded project, the Microbat. The 23cm wingspan flapping micro air vehicle was controlled by the tail and was not meant to hover. Darpa had wanted a wingspan of 2.5cm, which Keennon says was “a bridge too far”. The final prototype, weighing 12.5g, featured open-loop “stick to surface” control and achieved a 22.75min endurance flight in 2002. Keennon says it was the “first remotely controlled, free-flying micro size flapping wing vehicle” of its kind.
Hovering and flying a tail-less vehicle that mimicked a hummingbird would be that much harder, particularly in the areas of servos and control systems. “It was extremely challenging and we didn’t know if it could even be done,” says Keennon of the 4.5-year project, adding that previous flapping-wing flight required using “huge” tail surfaces. “We were able to quickly find some high-speed video of hummingbirds in the lab environment” to study, says Keennon. The team noted that hummingbirds use “long wing strokes” covering nearly 360 degrees around the body. “With the two wings, it’s almost like a helicopter,” he says. Unlike a long-winged (high lift over drag) bird such as the albatross, which has a relatively flat wing stroke, Keennon says an aircraft mimicking the hummingbird requires a large-amplitude wing flapping for high thrust.
“There was never an effort to, say, emulate or copy the bone structure or feathers of the hummingbird,” he adds. “That’s far beyond our ability to do in a way that they would need to be to work properly. You either have to go all the way or go in a different direction.”
How AeroVironment ultimately built the micro hummingbird is spelled out in a patent application Keennon and his co-workers submitted in December 2009. Central to the success is a wing that features a membrane strung between a root-to-wingtip spar (or mast) and a root spar, or boom. Three-axis control is possible by a combination of varying the angle between the wing spar and root spar, limiting the range of root-spar rotation and changing the flapping speed of the wings.
Control algorithms must take into account fore and aft strokes. For example, to tip the NAV forward, the wings on the fore stroke (back to front) begin the sweep at high relative angle of attack (more lift) and end the stroke with lesser angle (less lift). On the aft stroke, conversely, the stroke starts producing lower thrust and ends with high thrust (higher angle of attack).
A TRICKY DYNAMIC
All of that “luffing” of the membrane has a negative consequence for noise as the membrane “snaps” between one position and another. Together, the result is two wing snaps twice per stroke. “It’s a tricky dynamic to make silent,” says Keennon.
“It’s louder than a hummingbird, there’s no getting around that,” he adds – but changes to wing design and stroke made the final prototype “a lot quieter” than earlier designs.
While AeroVironment holds out hope for phase-three funding, Keennon says the company is in “marketing mode” with the hummingbird and ground station, which includes the live video feed from the bird. He would not discuss other NAV projects underway, whether biologically inspired or not, except to say that “some are small and some are medium sized”. “We’re all pretty ecstatic at the progress we’ve made,” says Keennon of the progress to date on the nano hummingbird project. “We surprised ourselves.”