

Video: Bird-like glider Video: Bird-like glider

Miniature surveillance aircraft would never need to return to base if they could cling to overhead power lines to recharge their batteries. Now engineers at the Massachusetts Institute of Technology are close to perfecting the trick – taking inspiration from birds.

It is no easy feat for an aircraft to slow down quickly in mid-air to hook onto high-voltage cables with enough precision to avoid bridging and shorting neighbouring wires. The MIT engineers’ answer is to send their 30-centimetre-wide micro air vehicle (MAV) into a controlled stall, pointing its nose up at just the right point in its trajectory to collide with and hook onto the cable.

In straight and level flight, air flows smoothly over aircraft wings because layers of streamlined air stick to the wings’ upper skin. But when a wing bites into the air at ever greater angles to the horizontal, a point is reached where the airflow detaches from it, creating turbulence and eliminating aerodynamic lift.

In aircraft with people on board, the loss of lift from a stall can be fatal. A bird, however, uses stalling deliberately to brake when it is coming in to land, losing lift just before it wants to touch down. It does this by flaring its wing feathers, angling them against the airflow and increasing drag, and simultaneously doing the same beneath its body by lowering its tail feathers.

Now Russ Tedrake and Rick Cory at MIT have developed a flight algorithm that lets a glider enter a stall at just the right point to perch on a cable, just as a bird does.

Clever glider

They took a styrofoam glider and fitted it with a motor able to control an elevator – a hinged flap to the rear of the tailplane.

“The heart of the system is a library of trajectories,” says Tedrake. “Wall-mounted cameras report on the glider’s position and the control system looks up a trajectory that will take it to the perch.”

The pair catapulted the glider across their lab and allowed the algorithm to steer it to a stalling point beside a cable. At just the right point, the elevator swung the nose hard upward, stalling the plane gently against the cable. Their results are encouraging (see video, above).

The Air Force Research Laboratory in Dayton, Ohio, which is also developing power-line planes, is impressed. “This work is unique,” says Gregory Reich of the laboratory. “I don’t think anyone else is addressing the flight control problem in nearly as much detail.”