To gather the data that direct the helicopter during automated flight, Shterenberg and Gavrilets outfitted the craft with a custom-built data-acquisition box. Remote-control helicopter pilot Raja Bortcosh led the chopper through myriad maneuvers, and the box recorded his commands and the helicopter's sensor outputs. "Using that data," Gavrilets says, "we were able to reconstruct the way the pilot directs the helicopter to perform the maneuvers. And then we were able to build the first dynamic model-mathematical model-of the helicopter in aerobatic flight, which had never been done before." Once they had reconstructed the flight commands as mathematical equations, the researchers-joined by postdoctoral associate Bernard Mettler, aeronautics and astronautics graduate student Ioannis Martinos, and electrical engineering and computer science graduate student Kara Sprague '01, MEng '02-programmed the information into Mr. Chopper's computer, allowing the machine to duplicate the maneuvers on its own.

Mettler, whose doctoral research at Carnegie Mellon University focused on modeling and control techniques for miniature helicopters, notes that it takes years to become proficient at controlling a helicopter, so it's not easy to duplicate these skills with a computer. "The MIT autonomous helicopter group, by successfully executing aerobatic maneuvers, achieved a new state of the art in flight performance under computer control," he says.

To test their mathematical models, the researchers built a flight simulator with a duplicate avionics box and interface. Through the simulator, they can watch a 3-D image of a helicopter move wherever their models direct it. The team tries to eliminate all mathematical inaccuracies on the simulator before it tests the commands with the helicopter.

On the field, with the selected mathematical algorithm programmed into the helicopter, the pilot controls the chopper as it takes off from the ground. Directing it to hover at a specific location and altitude, the pilot flips a switch that puts the helicopter on autopilot. The helicopter follows the instructions of the algorithm and performs the maneuver within prescribed parameters of altitude, speed, and distance, and all the while, the researchers monitor its flight data on the ground computer. After completing the maneuver, the chopper returns to its hovering position, and the pilot takes manual control and lands the chopper.

Thanks to the simulator and the researchers' ability to test the mathematical models before taking to the skies, the helicopter has crashed only twice in its four-year history, despite the fact that, as Feron says, "the windows for making mistakes and recovering from them are extremely narrow." Both crashes were attributable to hardware failures, not numerical errors.