MESSRS. ROTOL AIRSCREWS, LTD., have now completed the development, to the production stage, of a constant speed contra–rotating airscrew, the principle of which was mentioned in NATURE of May 17, 1941, p. 602. This model consists of two three–bladed airscrews mounted on the same centre line, normally the engine hub, rotating in opposite directions. The aerodynamic efficiency of this device is not appreciable at flight speeds of less than three hundred miles per hour, but above this it is worth while, and at five hundred miles per hour it gives an increase of about 7 per cent. One particular example weighs 497 lb., compared with 450 lb., for the normal airscrew. Metal or wooden blades of any detachable type can be used equally well. The de Havilland Aircraft Co., Ltd., and the Fairey Aviation Co., Ltd., have also announced the production of contra–rotating airscrews.