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).
Letter from Ogilvie & Partners providing recommendations for improving the fan design on the 40-50 HP chassis.
| Identifier | ExFiles\Box 75\3\ scan0010 | |
| Date | 19th September 1924 | |
| COPY. OGILVIE & PARTNERS, LTD. 104, High Holborn, LONDON, W.C.R. 19th. September, 1924. A.{Mr Adams} J.{Mr Johnson W.M.} Rowledge, Esq., Messrs. Rolls-Royce Ltd., Nightingale Rd., Derby. Dear Rowledge, I have considered the fan for your 40-50 H.P. chassis. Since I have no data concerning the power to be absorbed or the quantity of air to be delivered, the design in so far as the size and number of blades are concerned must be a pure guess, and I shall therefore assume that the leading dimensions of your existing fan are correct. My further remarks can be grouped under two headings:- (1) Improvement possible on existing fan in order to secure greater efficiency. (2) Possible means of decreasing the rate of increase of power absorbed with speed. (1)...a. The angle of the blades. This is given as 45°. It is too high. The blades are probably working under conditions similar to those of an aeroplane wing when stalled. The fan is therefore very inefficient. If a constant blade angle is used, make it 30°, not 45°. But it would be preferable to give it the true pitch angles which I show on the attached sheet. (1)...b. The section of the blades. As far as I can measure it you are using a strip of 10G plate 2-1/4" wide, bent into the arc of a circle. The ratio of height of arc to chord of arc being 1 in 8.5. This ratio is too high. It should lie between 1/50 and 1/20. I should recommend one in twenty. One in fifty, though better aerodynamically, would make the blade very whippy. It would further be an improvement to fine down the leading and trailing edge to approximate as near as possible to a good streamline section (maximum thickness about 0.3 - 0.35 of chord from leading edge) bent with its centre line a circular arc of 1 in 20. (2) Rate of increase of power with speed. The speed of the car and the speed of the fan increase at the same rate. It therefore follows that the power absorbed by the fan will increase as the cube of the speed of the fan, and the efficiency of the fan will remain constant. To make the power increase at any less rate involves either:- | ||