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For researchers building what could be the world’s smallest cyclocopter — a drone powered by two spinning paddle- wheel-like cyclorotors — every gram counts. The edges of each of the craft’s eight rotor blades were made by hand-laying hair-thin carbon fiber strands preimpregnated with epoxy into a Teflon mold, along with a 0.7-millimeter-thick carbon rod for the center of the frame. Then they were baked in an oven at about 175 degrees Celsius. The frames were wrapped in 5-micron-thick sheets of Mylar, the thinnest available, to create surface area.
Each rotor assembly had to weigh the equivalent of five Tic Tac breath mints or 2.5 grams, with each of the four blades on a rotor accounting for just .12 gram. The total vehicle weight came in at just 29 grams.
“Coming up with this fabrication technique and making this ultralight blade, that was a huge challenge,” says Moble Benedict, a Texas A&M University assistant professor who advised two of his aerospace engineering Ph.D. students, Carl Runco and David Coleman at the Advanced Vertical Flight Laboratory, in designing and building the drone. Building super-light blades “was the only way we were going to scale this thing down.”
Benedict and his teammates at Texas A&M started flying the miniature cyclocopter in December 2015 and have continued testing the design under a U.S. Army program. Benedict and collaborators at the University of Maryland had flown larger versions weighing 800, 500, 235, 210 and 60 grams over the preceding 10 years. Knowing that cyclocopters might not be the only answer for miniature flight, Benedict and Coleman also began flying a 62-gram flapping drone, dubbed the Robotic Hummingbird, for the Army in 2015.
They want to find the best solution to a deadly problem soldiers faced in Afghanistan, and one that’s likely to come up in other locations where urban canyons and forest canopies play the role of mountains. Soldiers in Afghanistan would get pinned down in a valley with limited knowledge of what might lie over the next ridge and with only a tenuous communications link to commanders. By pulling a cyclocopter or a flapping drone from a rucksack and letting it fly from the palm of his or her hand, a soldier of the future would gain a bird’s eye view with the aid of a 1- or 2-gram camera or the ability to relay communications via a network of other drones.
For aircraft technologists, questions abound, from which drone concept would work best in which circumstances to how cyclocopters and flapping drones would perform compared to conventional quadcopters and miniature helicopters, especially when faced with the great bane of all micro air vehicles: wind gusts.
The U.S. Army hopes to answer those questions under a program called MAST, short for Micro Autonomous Systems and Technology, which is based at the Army Research Laboratory in Adelphi, Md., and is set to wind down this year. MAST pays universities and companies to develop miniature ground and flying robots that could pay off in 10 to 30 years for soldiers in the field. Concepts funded under MAST include navigation sensors and joint robotic mapping schemes, perching mechanics and adhesives, alternative energy sources for micro vehicles, quadcopters, ducted-rotor aircraft, fixed-wing aircraft with wings that can lengthen or shrink, bee-sized flapping fliers and also Benedict’s cyclocopter and flapping-wing aircraft. The main purpose of MAST is to identify particular modes of propulsion worthy of further research, but the Army is also curious about the comparisons among the craft.
“Can these alternative configurations be more efficient; can they be more agile? You can start to ask questions like: ‘Is it useful to me to have it flapping because it looks like a bird; does that add some benefit?’” says Chris Kroninger, the microsystem mechanics team leader at the Army Research Lab.
The Army program ends in September though Benedict and his team plan to continue their research on micro aircraft.
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