Airplane landings can be less than graceful.
The aircraft slowly maneuvers into an approach pattern, begins a long descent, and then slams on the brakes as soon as it touches down, which barely seems to 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. MIT researchers have now given a foam glider this same ability using a new control system that could have important implications for robotic planes, greatly improving their maneuverability and potentially allowing them to recharge their batteries simply by alighting on power lines.
Birds 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.”
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: Even the best descriptions of it are time-consuming to compute.
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