MIT researchers have demonstrated a new control system that allows a foam glider with only a single motor on its tail to land on a perch, just like a pet parakeet.

The work could have important implications for the design of robotic planes, greatly improving their maneuverability and potentially allowing them to recharge their batteries simply by alighting on power lines.

Everyone knows what it's like for an airplane to land: the slow maneuvering into an approach pattern, the long descent, and the brakes slamming on as soon as the plane touches down, which seems to just barely bring it to a rest a mile later.

Birds, however, can switch from barreling forward at full speed to lightly touching down on a target as narrow as a telephone wire. Our feathered friends can land so precisely because they take advantage of a complicated physical phenomenon called "stall."

Even when a commercial airplane is changing altitude or banking, its wings are never more than a few degrees away from level. Within that narrow range of angles, the airflow over the plane's wings is smooth and regular, like the flow of water around a small, smooth stone in a creek bed.

A bird approaching its perch, however, will tilt its wings back at a much sharper angle. The airflow over the wings becomes turbulent and large vortices – whirlwinds – form behind the wings. The effects of the vortices are hard to predict: If a plane tilts its wings back too far, it can fall out of the sky, hence the name "stall."

Math landing

The smooth airflow over the wings of a normally operating plane is well-understood mathematically; as a consequence, engineers are highly confident that a commercial airliner will respond to the pilot's commands as intended.

But stall is a much more complicated phenomenon, and even the best descriptions of it are time-consuming to compute.

To design their control system, the MIT researchers first developed their own mathematical model of a glider in stall. For a range of launch conditions, they used the model to calculate sequences of instructions intended to guide the glider to its perch.

The researchers also developed a set of error-correction controls that could nudge the glider back onto its trajectory when location sensors determined that it had deviated.

Military use

For some time, the United States Air Force has been interested in the possibility of unmanned aerial vehicles that could land in confined spaces and has been funding and monitoring research in the area.

"What [the MIT] team is doing is unique," said Gregory Reich of the Air Force Research Laboratory in Ohio. "I don't think anyone else is addressing the flight control problem in nearly as much detail."

Reich pointed out, however, that in their experiments, the MIT researchers used data from wall-mounted cameras to gauge the glider's position, and the control algorithms ran on a computer on the ground, which transmitted instructions to the glider. "The computational power that you may have on board a vehicle of this size is really, really limited," Reich says.

In other words, even though the MIT researchers' course correction algorithms are simple, they may not be simple enough.

MIT associate professor Russ Tedrake who worked on the bird-lander believes, however, that computer processors powerful enough to handle the control algorithms are only a few years off. His lab has already begun to address the problem of moving the glider's location sensors onboard.

Meanwhile, Rick Cory, a MIT PhD student who worked with Tedrake on the project, will be moving to California to take a job researching advanced robotics techniques for Disney, and he noted some parallels in the institutions' goals.

"I visited the Air Force, and I visited Disney, and they actually have a lot in common," Cory said. "The Air Force wants an airplane that can land on a power line, and Disney wants a flying Tinker Bell that can land on a lantern. But the technology's similar."

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