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To overcome the objection of forward motion, several other machines have been proposed in which the surfaces are moved in relation to the body, as well as the air, thus making it possible for the device to stand stationary while the revolving or reciprocating surfaces still continue in motion in regard to the air. One type of the moving surface machine, the "Helicopter," is provided with revolving surfaces arranged in the form of vertical air screws or propellers, the blades of the propellers being inclined so that they drive down a continuous stream of air and produce the continuous upward reaction that supports the machine. While such machines have succeeded in raising themselves off the ground they are not yet practical flying devices. The "ornithopter" or "orthopter" is a flapping wing machine that maintains flight after the manner of the bird (Ornis). Like the helicopter, the ornithopter has not yet proved successful.

Fig. 1. Comparison Between the Kite and Aeroplane; Fig. 2, Showing the Lift and Drag Forces Produced by the Air Stream. The Propeller (P), Acts in a Manner Similar to the Kite String (S) in Producing Relative Motion Between the Air and the Lifting Surfaces.