Snowbird Flies on Flapping Wings - and Human Power
6:12 PM on Sep 23, 2010
Students at the University of Toronto have achieved a long-sought aeronautical feat - sustained human-powered, flapping-wing flight. On August 2, the team's Snowbird ornithopter maintained both altitude and airspeed for 19.3s, covering a distance of 145m at an average speed of 25.6km/h - all on the leg power of Todd Reichert, one of the two-man development team. An official record claim has been filed.
Video and pictures: Ornithopter Project
Watch the video and you will see the Snowbird did not take off under human power - it was towed aloft by a car, like a glider. Once airborne, the tow rope is released and the wing begins flapping to maintain altitude and speed.
Cameron Roberston, chief structural engineer and the other half of the development team, says data from the first half of the flight shows the aircraft climbing while maintaining speed - indicating a power margin. But he says the flights showed there is a "hard limit" on the pilot's physical endurance of 15-20 flapping strokes.
The key to the Snowbird's success is a ultra-lightweight airframe and aeroelastically tailored wing. The aircraft has a span of 105ft, but weighs just 94lb. The wing has to generate both lift and thrust as it flaps, and to sustain flight it has to generate more lift and thrust on the downstroke than its does lift and drag on the upstroke.
To accomplish this, the Snowbird's wing structure twists passively and automatically so that the angle of attack relative to the local airflow is lower on the upstroke than the downstroke, and the net result is positive lift and thrust.
Adapted from a Dutch bicycle design, the propulsion system works like a leg press, says Robertson. When the pilot's feet go out, the mechanism pulls down on the wing at mid semi-span, caussing it to flap. On of the reasons the fuselage is set so low, he says, is to give the steepest pull-down angle possible.