HPAs are aircraft belonging to the class of vehicles known as human-powered vehicles. Early attempts at human-powered flight were unsuccessful because of the difficulty of achieving the high power-to-weight ratio. Prototypes often used ornithopter principles which were not only too heavy to meet this requirement but aerodynamically unsatisfactory. Human-powered aircraft have been successfully flown over considerable distances. However, they are still primarily constructed as engineering challenges rather than for any kind of recreational or utilitarian purpose.