Abstract:
The present paper discusses the effects on the performance of a contra-rotating airscrew pair of the oscillatory nature of the flow round the blades. The blade sections at a representative radius are developed into two infinite cascades in a plane, and the two-dimensional flow in this plane is discussed : for simplicity the blade sections are replaced by vortices with strengths equal to tb.e circulations round the blades. On this basis, it is shown that if the two screws are to absorb equal powers at equal rotational speeds, the mean circulations round the blades must be equal ; however, this implies, for similar sections and equal chords, a coarser pitch setting for the front screw than for the rear. For this condition, the slipstream velocity has an oscillatory rotational component ; its mean rotation is, however, zero. In designing a contra-rotating airscrew pair; the most obvious way of assessing mean values for the local wind speed and direction is to imagine the number of blades to become infinite, while the blade settings and solidities are maintained; the slipstreams are then uniform. In the numerical example given it is shown that this method is quite good enough ; although the local thrust variations are of the order of ±20 per cent. from their mean values, the latter are less than 0.5 per cent. different from those given by the assumption of an infinite number of blades. No account has been taken in the present paper of the vortices shed by the blades as the circulation changes; it may be anticipated that their effect will be to reduce the magnitude of the oscillatory variations in thrust, etc., to a degree defending on the value of the frequency parameter.
