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).
Page showing a table and formula for calculating horsepower required to overcome air resistance.
| Identifier | ExFiles\Box 161\1\ scan0020 | |
| Date | 31th May 1936 guessed | |
| 5. The following table, Fig. 9, is offered as a convenience to engineers for figuring H.P. required to overcome air resistance. Variations from 100 sq. ft. of frontal area can easily be calculated and the power required for various degrees of efficiency of streamlining, as represented by the various air resistance coefficients, is quickly obtained throughout the speed range. AIR RESISTANCE H.P. PER 100 SQ. FT. FRONTAL AREA M.P.H. K=0040 K=0036 K=0032 K=0028 K=0024 K=0020 K=0016 K=0012 K=0008 K=0006 K=0004 30 28.80 25.38 23.04 21.76 17.28 14.40 11.52 8.64 5.76 4.32 2.88 40 68.00 61.20 54.40 47.60 40.80 34.00 27.20 20.40 13.60 10.20 6.80 50 133.60 120.24 106.88 93.52 80.16 66.80 53.44 40.08 26.72 20.04 13.36 60 230.40 207.36 184.32 161.28 138.24 115.20 92.16 69.12 46.08 34.56 23.04 70 366.40 329.76 293.12 256.48 219.84 183.20 146.56 109.92 73.28 54.96 36.64 80 544.00 489.60 435.20 380.80 326.40 272.00 217.60 163.20 108.80 81.60 54.40 90 776.00 698.40 620.80 543.20 465.60 388.00 310.40 232.80 155.20 116.40 77.60 100 1066.66 959.99 855.55 746.66 639.99 533.33 426.66 319.99 213.33 154.99 106.66 Fig. 9 The following equation has been found sufficiently accurate for all practical purposes and by using it the cost of overcoming air resistance for any equipment for any period may be determined, with due allowance for a proper value for the air speed: H P Ha = H C ( K A V^3 / 375 ) where: H P Ha is the horsepower hours to overcome air resistance for the period. H is the number of hours of operation (from schedule). C is a constant which represents the error which would exist if the equation were used with V, the average speed for the period, without consideration of hours of operation at various speeds. For all practical purposes a value of 1.33 may be used for C. K is the air resistance coefficient of the equipment. A is the frontal or projected area of the equipment. V is the average speed of operation (from schedule). In the following table, Fig. 10, three of the examples used in determining a value for C for use in the above equation are shown. | ||