From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Study on the aerodynamic efficiency of fixed and variable pitch aerial propellers.
| Identifier | ExFiles\Box 10\3\ 03-page006 | |
| Date | 28th January 1937 guessed | |
| AERIAL PROPELLERS. The following conclusions were arrived at as a result of a study of the report by Mr. F.{Mr Friese} W. Lanchester on 'The Screw Propeller' and Major Green's paper on the same subject. The blade of a propeller may be regarded as an aeroplane wing moving in a helical path, and its efficiency depends on the ratio of lift to drag of the section. This ratio of lift to drag of an aeroplane wing section has been found both by experiment and theory to be a maximum at a small angle of incidence (about 3° or 4°) to the relative wind. It would, therefore, appear that to get the maximum efficiency out of a propeller it should be so designed that every section of the blade is working at the angle of maximum lift/drag to the helical path which that section traverses. This condition cannot be fully met as the speed of advance of the aeroplane varies, but it could be designed for one particular speed of advance, say, the climbing speed. With a fixed pitch propeller designed on this basis, at any other speed of advance other than that for which it is designed, the angle of incidence will be greater or less than that giving maximum lift/drag, and the efficiency will fall off. With a variable pitch propeller, the pitch could be altered for different speeds of advance so that the blade sections are always working at their most efficient angle. It is not known what is the best compromise with a fixed pitch propeller. Mr. Lanchester suggests that the pitch should be correct for climbing speed, but on the other hand, the French appear to design for maximum speed. | ||