Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/64

§ 30 more advanced considerations than can be entered into at present.

A further interesting example is found in the aerial tourbillion (Fig. 23), in which the rotor K is a stick of segmental section mounted to revolve freely about the axis L. The plane face of the rotor is set truly at right angles to the axis of rotation. If this apparatus be held in a current of air with the plane face fronting the wind, as, for instance, by holding it outside the window of a railway carriage in motion, the rotor evinces no tendency to go round in the one direction or the other. If, however, a considerable initial spin be imparted in either direction, the wind will suddenly get a bite, so to speak, and the rotor will gather speed



and spin at an enormous rate, as if it were furnished with sails like a well-designed windmill.

Referring to Fig. 24, we have at a the type of flow illustrated to which the blade of the rotor will give rise when its motion is normal to the air; b similarly indicates the form of flow when the rotor is going round slowly, not fast enough for the air to take hold. In both these figures we have the flow independent of the "rear body form," and the rotor behaves just as if it were a flat plate. Now, let us suppose that the rotor be given a sufficient initial spin to bring about the state of things represented at c.