Machine Design

Gliding high with servos

Flocks of unmanned paragliders steer themselves through dangerous missions using innovative servoactuators.

The Onyx platform in flight, here carrying an iRobot surveillance bot payload. The Onyx retracts and extends parafoil steering lines to maneuver its parafoil via capstans mounted to custom dc motors via planetary gearheads. Servos also control parafoil angle-of-attack for long or short glides.

Developed for Darpa, the Leapp paragliders are slow-flying, long-endurance UAVs for surveillance and delivering payloads. The MicroLeapp version (below) is small enough to be carried in a backpack. Its big brother can carry up to a 250-lb payload and is powered by a turbo diesel engine. For a ground launch, soldiers unfurl the paraglider wing behind the vehicle. Takeoff is in 100 ft or less and need not be from a paved runway.

It may sound like a low-tech way of flying military missions. But guided-parachute systems developed by Atair Aerospace Inc., Brooklyn, N.Y., are anything but.

The basic idea: Launch a small propeller-powered platform attached to a high-tech parafoil. Then give the platform enough smarts to fly itself, follow a flight plan, and manage fuel in such a way that it can hang around in the air for days on end.

The Long Endurance Autonomous Powered Paraglider, or Leapp, UAV is designed for special operations intelligence, surveillance, and reconnaissance. It can operate autonomously or be piloted by remote control via a portable base station. The MicroLeapp version is light enough to go in a backpack, flies for up to 8 hr, and can carry a 50-lb payload. It gets power from a gas engine not much different from those in model airplanes.

A bigger version of the Leapp UAV carries a turbo and supercharged diesel engine. It is designed to fly for up to 55 hr at greater than 35,000 ft using the largest elliptical paraglider wing ever built, with a wing span exceeding 112 ft. It can carry 2,400 lb, excluding fuel.

Another self-guiding system, called Onyx, is unpowered but is built to be thrown out the back of a cargo plane from as high as 35,000 ft. An onboard flight computer initially determines a heading using inputs from a GPS integrated inertial navigation system. Atair says the system can steer itself well enough to deliver a 1-ton pallet to within about 150 ft of its target from up to 30 miles away.

Onyx payloads steer themselves via swarming algorithms. Multiple Onyx systems link via RF peer-to-peer communication and execute moves as autonomous agents. The result looks like a flock of starings; each one independent yet flying without collision to the same place.

Each Onyx payload follows a path that aims for the target but keeps a minimum separation with others in the "flock." This lets paragliders head toward the same spot without colliding in midair. When the Onyx platform gets near a target, it descends in a spiral dive, then transitions from the parafoil to a landing parachute that brings it to terra firma.

Both Leapp and Onyx steer themselves with servoactuators that pull on the parafoil. The servos use custom-wound dc motors married to planetary gearheads and capstans with machined-in grooves. Plastic-coated steel cable coils into the grooves. The cables pull on the steering lines of the parafoil or on other parts of the chute to change its angle of attack.

Needle-roller bearings in the mechanism ensure that the cables retract and extend without coming out of their grooves, important for handling side-to-side loads on the capstan. The simplest systems carry a single servomotor that only steers the chute, pulling on one set of steering cables while extending those on the other side. Most systems, though, use two to four servos, two for steering, two more to adjust the glide slope. The additional servos permit maneuvers such as flat turns, which have low drag and thus consume little energy. This contrasts with ordinary high-drag turns made by manipulating the back edge of the chute that acts as an aileron. Changing the angle of attack also lets the systems boost forward speed by a factor of almost two for a quick dash.

The servos run from a custom-designed motor controller using a potentiometer as a position-feedback device. The motor controller gets commands from a navigation-control computer that, besides a GPS unit, also contains a barometric sensor, three-axis gyro, three-axis accelerometer, and three-axis magnetometer. The navigation controller can make dead-reckoning calculations if the GPS blacks out or there is jamming.

The parafoils on Atair UAVs must be small and light. One development that makes such chutes practical is special superlight but superstrong material.

Conventional parachutes typically have wing loadings of 1 to 2 lb/ft2. A 20ton tank would need a 40,000-ft2 parachute, nearly an acre of material. But Atair parafoils get nearly 5 lb/ft2 on wings that can generate positive lift in a flare maneuver, and 22 lb/ft2 on wings that don't need to lift.

The material that makes this possible is a composite fabric created by bonding high-strength Dyneema fibers between two layers of ultrathin polymer. The material is three times stronger and less than one third the weight of standard parachute nylon. It also stretches just one-sixth as much.

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