The Air Force wants miniature drones that can perch and wait days or weeks for a target before attacking with an explosive warhead. Prototypes are already flying.

Battery power is the biggest limitation for small drones. Consumer quadrotors like the DJI Phantom can fly for half an hour or so. Fixed-wing craft like the military RQ-11B Raven stretch this to 90 minutes, but not nearly enough for prolonged surveillance. One way to get around this problem is to land—then only the camera and communications systems are drawing power. Perching has two other advantages: a stationary drone is less conspicuous than one circling overhead, and it has a stable vantage point close to the target. It's better to watch from a tree 100 feet away than a Predator at 10,000 feet.

Some drones already have "perch and stare" capability. AeroVironment's Qube , a quadrotor built for the police and military, can set down on any flat surface with its long landing legs. Quadrotors can set down easily enough, but landing is harder for fixed wing drones, especially when flat surfaces are not available. A truly useful drone would be able to perch as easily as a bird, taking advantage of streetlights or tree branches.

It's better to watch from a tree 100 feet away than a Predator at 10,000 feet.

The Air Foce has funded considerable research into this area, some of it under the Perching Micro Air Weapon project. This calls for a bird-sized drone able to land on structures—"power lines, trees, buildings, ground, etc"—send back video from three miles away, then take off again and perch elsewhere. The drone would carry a small warhead "on the order of half a pound of high energy explosive which will be used to target people and/or lightweight vehicles," similar to the existing SwitchBlade lethal drone .

Eye in the Sky Bleecker Street Media

If you saw the recent thriller Eye In The Sky starring Alan Rickman and Helen Mirren, that movie features not only a USAF MQ-9 Reaper watching from overhead, but two more exotic drones like the ones the Air Force wants to build. One resembles a bird, the other an insect. The drones in the movie have flapping wings, but the Air Force is taking the simpler approach of using fixed wings or rotors. The Perching Micro Air Weapon can do something the drones in the movie cannot: recharge itself by solar cells or other means. "The ability to harvest power/energy from external sources is critical to persistent surveillance," the specification says. This would allow a drone to wait days for the right moment to strike a target.

Perching is a surprisingly complicated manuver. Thankfully, birds perfected the technique millions of years ago and researchers have gleaned much valuable information from watching slow-motion video of them. Perching involves a well-controlled stall just above the perch, halting in mid-air so the bird just drops on to the surface. Even a clumsy flyer like a heron can land neatly on a fence, and drones ought to be able to do the same.

Bhargav Gajjar of Vishwa Robotics developed perching legs for drones based on a hawk. The spring-loaded claws have a powerful grip that should help make up for any residual speed that might otherwise cause the drone to bounce off its perch.

The Air Force didn't choose Gajjar's design, but two other projects made it to prototype stage. The version of the Perching Micro Air Weapon developed by Design Intelligence Incorporated with assistance from Oklahoma State University resembles a bird and could pass unnoticed almost anywhere. The drone has a modular design so that, like the Israeli ROTEM quadrotor , the payload can be switched from intelligence-gathering to lethal strike version. However they never actually fitted the drone with explosives. "The project was very successful, but the Air Force decided not to pursue it any further," James Grimsley, CEO of DII, told PM. DII has now moved on to developing small, solar-powered drones for non-military and commercial applications.

Aethermachines perching drone patent Aethermachines

The other prototype appears to have progressed further. The 3-lb. drone made by Aethermachines Inc is a more radical design that the company describes as "insect-like ". The 3D-printed drone has two shrouded rotors and takes off and lands vertically. The rotors act as wheels on the ground so the insect drone can scuttle along inside buildings and other closed spaces. It has a tiny ultrasonic sensor which, along with a set of accelerometers, allows it to perch.

Flight time is only 10 minutes per charge, but the insect can recharge itself in 40 minutes by perching on a power line and scavenging energy. This is its camouflage. Rather than pretending to be a monster insect, the drone is disguised to like any of the other anonymous black boxes hanging out near power equipment that nobody ever notices. Aethermachines's insect is a bomber rather than a kamikaze, with "the ability to aim and discharge the payload, allowing the MAV [Micro Air Vehicle] to return for reuse unless expendability is required by the mission." The payload can be either an explosive warhead or marking dye. With the latter, the target becomes a marked man easily identified later by security forces.

Perching drones might make good perimeter guards—like a minefield, but with human oversight and control.

Aethermachines patented an early design in 2010 that already had all these elements: the perching mechanism, power scavenging, rotor/wheels, color video camera, and "expellable offensive payload." The company claims it cannot discuss subsequent progress after the successful prototype. They may have passed it on to an industry partner for further development or production, as Aethermachines is not a manufacturer.

Meanwhile, there many new players in the field of perching drones. Some, like the aptly named GRASP Laboratory at Penn State, are working on claws that not only perch but also can pick up objects . Harvard's diminutive Robobees recently showed they can land on and stick to any surface using static electricity. Perching is unlikely to be a novelty for much longer.

Perching brings significant benefits, especially for military operations. Unlike other forms of airpower, perching drones can effectively occupy terrain, allowing the operators to see and engage any vehicles or people on foot in the area. Perching drones might make good perimeter guards—like a minefield, but with human oversight and control. They might move around to form ad-hoc unmanned checkpoints, reading the license plates of passing vehicles from roadside perches. Or they might, as the Air Force seems to imply, be used offensively, released from aircraft to find and attack specific individuals in urban environments, even inside buildings. The future of air war is sophisticated, small, and scary.

David Hambling's book Swarm Troopers: How small drones will conquer the world is out now.

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