Month in the Patent Office

A blade for a helicopter or like rotor carries a propulsive duct of the continuous flow or intermittent type at its outer extremity, the centre line of the duct being inwardly and forwardly inclined to the direction tangential to the path swept by the duct. Fig. 1 shows an arrangement in which the inclined centre line 11—11 of a circular duct 7 is straight, while fig. 4 shows a rectangular‐section duct 7a of which the centre line 11a—11a is curved.