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).
The merits of high versus low speed engine fans and their drive ratios.
| Identifier | ExFiles\Box 75\3\ scan0211 | |
| Date | 1st February 1925 guessed | |
| EFC1/T9.2.25. -4- Contd. whether one wants the fan to be a high speed or a slow speed fan (of similar efficiency). It is probable that a rather lower speed fan is more generally satisfactory, driving with a larger fan pulley, because then the driving belt has less to do. Having selected a high efficiency fan of suitable speed-thrust characteristic, it would appear that the drive ratio should then be arranged that for the nearly fastest speed of the engine there would not be an undue amount of belt slip, though probably some amount would be allowed. It is of little use arranging the fan for better efficiency at low chassis speeds, if this means belt slip at high speeds, with the resulting loss of efficiency and consequent wear of belt. The non-slip (ideal) speed of the fan with the 20 HP. chassis running at 50 m.p.h. on top gear, with the 11/50 back axle ratio, 31.4" effective tyre diameter, and present standard fan driving and driven pulleys, is 3200 r.p.m. the fan drive ratio being 1.316. We know that to try to drive the fan at this speed involves considerable belt slip, so, with this higher efficiency fan we might use a somewhat smaller ratio with the advantage of less slip and consequently less wear of belt. It would, however, appear that we do not want to lower the fan speed ratio to such an extent that at moderate speeds we get less <strikethrough>slipandconsequentlylesswearofbelt</strikethrough> thrust (assumed to represent the cooling effect) for a given | ||