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).
Summary of the reasons for the effectiveness of aluminium drums in dissipating heat.
| Identifier | ExFiles\Box 85\1\ scan0031 | |
| Date | 4th March 1936 | |
| -2- Hs{Lord Ernest Hives - Chair}/AFM.{Anthony F. Martindale}22/KW.4.3.36. The reason for the effectiveness of the aluminium drum in dissipating heat can be summarised as follows :- (1) For the same weight, it is possible to construct an aluminium drum having a much greater cooling surface than a C.I. drum. (2) For various forms of drums of different materials we have been able to show that the rate of decrease of temperature at a given temperature above air, and at a given rotational speed, varies as the cooling surface area, provided that the depth of the ribs is not too great in proportion to the distance between them. When this ratio becomes considerable, as it does in the case of our aluminium drums, the "cooling efficiency" begins to fall off. Comparing a standard Bentley rear drum and our aluminium rear drum, the "cooling efficiency" of the latter is 78% of the former, but its area is 2.25 times as great: therefore the ratio of maximum temperatures attained should be 1.75 : 1.0. This is remarkably well borne out in practice, as the ratio of maximum temperatures on the graph attached is 350/200 = 1.75. (3) The heat capacity of the composite drum is much greater than a C.I. drum of the same weight, owing to the high specific heat of aluminium. In the case of the two drums quoted above, the ratio is 1 : 2.45. This means that in a given stop, the temperature rise of each drum will be inversely proportional to these figures. This is very important. Its chief advantage lies in the fact that it reduces the expansion lag of shoes behind drum, which we have experienced in an acute form on Ph.III. We have measured the relative stiffness of the two drums quoted above, and the aluminium drum is twice as stiff as the C.I. drum. | ||