Month in the Patent Office

The specification contains a detailed mathematical analysis of the lift and lift‐moment about the flapping axis of a helicopter rotor blade to demonstrate that vibration (due to variation of lift‐moment) can be materially reduced by providing a minimum of five blades per rotor. Figs.1a, 1b and 1c are diagrams showing in arbitrary units the moment variation for two, three, four, five and six bladed rotors, the total lift being assumed constant, and the forward speed 0.4 of the blade tip speed. Curve A, fig. 1a shows the trefoil pattern for the moment variation of a two‐bladed rotor, two of these curves phased 90 deg. apart giving the circle B for four blades. Three blades give an approximate circle C, fig. 1b, while fig. 1c shows the much smaller variation occurring in the case of five and six bladed rotors as indicated by the curves D and E respectively.