The surface of discontinuity that ordinarily springs from the leading edge has got so close to the rear body of the rotor as to have ejected the "dead-water" on that side, and the resulting form of flow will be something like that illustrated in Fig. 25.
Fig. 25.
Here the pressure on the left-hand side (as shown) will be that of the "dead-water," which is, as we know, somewhat less than that of hydrostatic head, while that on the right-hand side will, owing to the centrifugal component of the stream, be very much lower; that is to say, the rotor will experience a force acting from left to right which is in the direction of the initial spin, so that the motion will be accelerated and will continue. The fact that the propelling force only comes into existence when the initial spin is sufficient to eject the dead water from the leading side of the rotor blade fully explains the observed fact that a very considerable initial spin is necessary.
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