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).
Comparing wind tunnel and full-scale test results for a Bentley.
| Identifier | ExFiles\Box 160\2\ scan0136 | |
| Date | 6th July 1933 | |
| To Ev.{Ivan Evernden - coachwork} from Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} c. to E.{Mr Elliott - Chief Engineer} Hs{Lord Ernest Hives - Chair}/Rn.{Mr Robinson}9/KT.6.7.33. WIND TUNNEL TESTS - BENTLEY. We have been comparing the results obtained in the wind tunnel with the full scale tests. We have not been able to repeat the lap 89 M.P.H. that we obtained on B.l., but can consistently lap at 86.5 M.P.H. with 90 road wheel H.P. Your tunnel drag for 1/8 scale model at 70 ft/sec. is apparently 1.3055 lbs.(please confirm) to give 86.75 M.P.H. Full scale at 130 ft/sec. this becomes, we presume, 295 lbs. drag. From the attached calculation you will see that our full scale drag on the track calculated from the road wheel H.P. figures is about 335, which compares closely with the figure of 341 that you have assumed. Therefore the scale effect gives a figure for the drag which is about 12% too low in the wind tunnel. This is worth keeping in mind, to see if the same percentage holds for other models of cars, which we may make with different finesse ratios. We might also ask Vickers if they have had a similar discrepancy in other 1/8 scale car models they have tested. With regard to the K.{Mr Kilner} used by Mr.Lay in his S.A.E. paper. The actual frontal area of Bentley l from the G.A. seems to be 28.38 sq.ft. With 345 lbs. drag at 86 M.P.H. this makes K = .00165 compared with Mr Lay's figure of .00155 for a typical Sedan. This is somewhat disconcerting as it seems reasonable that with an eddy free front and a clean back we should be slightly better than a conventional American Sedan. If, however, the American wind tunnel has the same scale effect as the one we are using, their full scale K.{Mr Kilner} for the American Sedan becomes .00174, which makes our close coupled Bentley 5 1/2% its superior. Hs{Lord Ernest Hives - Chair}/Rm.{William Robotham - Chief Engineer} | ||