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The Perching Glider

A smoke visualization still of the actual vortex wake behind our glider during a free-flight high angle of attack landing.

Image Courtesy of Jason Dorfman (MIT/CSAIL).

News

Introduction

Project Overview video available here.

Birds routinely execute post-stall maneuvers with a speed and precision far beyond the capabilities of our best aircraft control systems. One remarkable example is a bird exploiting post-stall pressure drag in order to rapidly decelerate to land on a perch. While it is tempting to attribute this agility to the intricate morphology of the wings, tail, feathers and overall sensory motor system of the animal, it turns out that even a simple fixed-wing foam glider made out of rigid flat plates and controlled by a single motor at the tail, is capable of executing a highly-dynamic and accurate bird-like perching maneuver. Moreover, because it can take advantage of post-stall pressure drag to stop, it doesn't require a propeller at all.

Landing Like A Bird

Bird landings are a fantastic demonstration of agility and accuracy. During a perching maneuver a bird will flare its wings and tail as it orients its entire body to an extremely high angle of attack. This intentional transition to "post-stall flight" causes the airflow around the wings to separate, meaning the air fails to smoothly follow the contour of the wing, detaching at the leading edge and creating unsteady, low-pressure pockets of air immediately behind the wings. These post-stall aerodynamics induce a strong pressure drag, which, in combination with viscous drag forces, create a powerful set of aerodynamic brakes for the bird allowing it to rapidly decelerate and execute impressive short-landings. Even more impressive is the fact that most birds can land with extreme precision at low air speeds, despite inevitable disturbances from the wind.

This video beautifully captures a Eurasian Eagle owl landing on a perch (shot at 1000 fps). Notice the ruffling of the feathers as it approaches the perch, indicating the airflow is anything but smooth and steady.

A Simple Fixed-Wing Glider

In order to understand the fundamentals of a perching maneuver, we use a simple glider (i.e. no propeller) with only a single motor for the tail. It is made out of RC foam and carries a small battery, radio receiver, and a few small motion capture markers, and weighs approximately 90 grams total. Although our glider has nowhere near the mechanical and sensing capabilities of a real bird, it allows us to strip down the problem to its most basic level. We ask the question: Can this simple fixed-wing glider execute a short point landing by exploiting post-stall pressure drag, similar to a bird-like perching maneuver?

The Perching Maneuver

A cartoon of a basic perching maneuver for our glider is shown above. The glider is launched at a random initial speed that ranges anywhere from 6.0 to 8.5 meters per second (13.5-19 mph) and begins 3.5 meters (12 ft) away from the perch. It must then quickly decelerate to a near stop before making the point landing, by attaching a small hook under its belly to the perch. In order to slow down fast enough, the glider must orient its entire body to a high angle of attack, allowing it to exploit both viscous and pressure drag for braking. The entire maneuver last just a fraction of a second and is computer-controlled by varying the angle of the tail.

Below is a high-speed video of our computer controlled glider landing on a suspended string perch. The autopilot (designed through an optimal control procedure) is consistently able to accurately execute the point landing to within a few centimeters, from a fairly large set of initial launching speeds. Here, the glider is launched at approximately 7 meters per second and the video is slowed down approximately 15 times. The second video is a different glider with a slightly different trajectory slowed down approx. 11 times. The third video is a head on shot of the perching maneuver slowed down 33 times. (Click images to load video, or download videos here: perching1, perching2, perching3)

Flow Visualization

Using our own free-flight wind tunnel (shown below) we were able to capture beautiful images of our glider's actual vortex wake during the high angle of attack phase of the perching maneuver. These images clearly demonstrate that our glider is influenced by very complicated aerodynamics before landing.

Images Courtesy of Jason Dorfman. (click to see larger images)

Publications

Rick Cory and Russ Tedrake. Landing on a dime: Control of bird-inspired perching maneuvers for fixed-wing aircraft. Submitted to Bioinspiration & Biomimetics, Special Issue on Bioinspired Flight. Under Review, 2010.

Rick Cory. Supermaneuverable Perching. PhD Thesis, MIT, June 2010. [ pdf ]

John W. Roberts, Rick Cory, and Russ Tedrake. On the controllability of fixed-wing perching. In Proceedings of the American Control Conference (ACC), 2009. [ pdf ]

Rick Cory and Russ Tedrake. Experiments in fixed-wing UAV perching. In Proceedings of the AIAA Guidance, Navigation, and Control Conference. AIAA, 2008. [ pdf ]

Russ Tedrake, Ian R. Manchester, Mark M. Tobenkin, and John W. Roberts. LQR-Trees: Feedback motion planning via sums of squares verification. To appear in the International Journal of Robotics Research, 2010. [ pdf ]

Project Contact Info

Rick Cory
Postdoctoral Associate, MIT CSAIL
Robot Locomotion Group
rcory [at] csail.mit.edu

Russ Tedrake
Associate Professor, MIT EECS
Robot Locomotion Group
russt [at] mit.edu