What makes human powered flight so exceptional and rare? The fact that the human body as engine delivers only a low amount power for its weight. A healthy adult amateur cyclist can deliver 250-300 Watt for several minutes. Compare that to a car engine, which easily provides 100 times more power, and at higher speed! The pilot as engine needs to lift his/her own weight, plus the weight of the aircraft. Flying cost a lot of energy, and flying on human power is on the verge of what is physically possible. Only with extreme lightweight engineering and clever designs can we pedal through the sky!
The graph below shows how long a trained cyclists can deliver a certain constant power. If you want to fly for 5 minutes or more, your HPA needs to be very efficient, requiring less than 250 Watt. Depending on several design choices, the flight speed typically ranges between 25 to 40 km/h. Don’t be fooled by the speed values; it is definitely harder to fly at this speed compared to riding on the road at the same speed! A further challenge is that the pilot’s attention is needed for flying the aircraft, so he/she cannot concentrate fully on pedaling. Very challenging, very fun! It is definitely the most healthy form of flying!
Maneuverability and control of HPAs is an important topic. Flying straight is often possible with rudder/elevator only, but flying turns is a real challenge. It takes more power, and roll control. Flying a figure-of-eight is exceptionally difficult for the pilot: it takes good piloting skills and good power output simultaneously. The figure-of-eigth can be considered the ‘holy grail’ of human powered flight!
Design: aerodynamics and structure
With so little power available, the flight speed needs to be low. But a slow aircraft needs a large wing to stay in the air. This results in a large structure, and increased weight. Most HPA designs converge to wingspans ranging from 30 to 30 meter, en weigh between 30 and 60 kg. With the right selection of lightweight materials and efficient structural engineering it is possible to fly solely on human power. Most HPAs have a structure made from carbon tubing, with one or more bracing wires. The aerodynamic shape is made from foam and balsa wood. The skin of the aircraft is a thin polyester film, held in place by double-sided tape: lightweight, and definitely strong enough for its task!