Design Philosophy for a Human-Powered Aircraft
The power required to fly any aircraft mounts rapidly with increases in flight speed, so to achieve flight on the limited power of the human engine, the aircraft must be designed to fly quite slowly. At these slow speeds, the wing must be incredibly large to produce the required lift, and the structure must be incredibly light. To help support the light structure, external wire bracing is typically used, and although these wires add drag, the weight savings in the structure is significant. This wire-braced structure favours a hanging fuselage design, which is typical for human-powered aircraft. In the case of a human-powered ornithopter, the bracing wires are additionally used to pull the wing down during the thrusting portion of the stroke.
Flapping Wing Theory
In an ornithopter the wings must produce both the lift to counteract the weight of the aircraft, and the thrust to counteract the body drag. Lift is produced in the conventional way, with the oncoming air striking the wing at a positive angle of attack; thus no feathers, valves or folding of the wing is required to produce lift. The key is to produce enough thrust with the wing to keep the aircraft flying at the required forward velocity. This thrust is produced by placing the wing at a lower angle of attack, relative to the local flow velocity, on the upstroke, and at a higher angle of attack on the downstroke. It can be seen in the figure below that this results in a large amount of lift and thrust on the downstroke and a small amount of lift and drag on the upstroke. The net result is positive lift and positive thrust.
Throughout the stroke the wing must twist with the proper magnitude and phase to produce the proper angles of attack. This is accomplished passively by designing the structure in such a way that the aerodynamic and inertial forces produce the proper twist. Achieving the right twist along the entirety of the span is one of the more difficult problems in building a successful ornithopter and has formed the basis of much of the past research at the University of Toronto. For more information about various aerodynamic and structural models, as well as several predecessor ornithopter projects at UTIAS please visit http://www.ornithopter.net/research_e.html.