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).
Wind tunnel testing methods, air resistance, and the aerodynamic benefits of streamlined vehicle forms.
| Identifier | ExFiles\Box 161\1\ scan0023 | |
| Date | 31th May 1936 guessed | |
| 8. 2. Ground effect must be reproduced in the wind tunnel but the usual ground board gives results which are within 5% of actual road conditions. A moving belt, free from whip, travelling under the model at the same speed as the air, reproduces full scale operating ground effect. When such a belt is used the differences between readings with it and without it but with the ordinary ground board are not important. 3. Variations in wind direction and wind velocity in actual operation average out over any extended period. However, the effect of side winds does not average out. As shown above, the effect of the average side wind is actually beneficial on the streamlined forms. 4. It is true that the use of the average speed for determining horsepower hours required to overcome air resistance over any given period introduces an error. However this error favors streamlining instead of penalizing it. When speeds over any given period are calculated accurately in accordance with their extent and duration, it is found that the total horsepower hours used for overcoming air resistance are at least 33% more than when the figure representing the average speed is used. Thus, in making comparisons, the conventional equipment as well as the streamlined equipment must be so penalized. When this is done the result shows an increase in the cost of overcoming air resistance for the period which is much larger for the conventional than for the streamlined form. 5. It is assumed that wind tunnel models are made sufficiently accurately to reproduce the shape or form of the hull or shell of the full scale equipment. If this is done details may be omitted unless the air resistance of the details themselves is sought. The resistance of any equipment is so largely controlled by its form that, at least in the present state of the art, little attention need be given to resistance caused by details. For example, the effect of permitting the air to flow through the radiator and engine chambers of a model of a conventional car, instead of closing this air passage as is done in the usual wind tunnel model, was found by Jaray in 1921 to decrease the total resistance slightly less than 1%. The effect of permitting it to flow through ducts which take the air at the point of highest dynamic pressure and guide it smoothly to an exhaust at the rear was to reduce the total resistance by about 3%. Note that each of these details involves decrease, rather than increase, in the total resistance -- just the opposite of what is usually claimed by the skeptics. The effect of placing exact reproductions of bumpers, ornaments and all understructure, as it exists in standard production, as compared with the model without ornaments and accessories and with the bottom completely covered and faired in, on a certain well-known make of streamlined car, was to increase the total resistance only 11.1%. To summarize, it may be said that, with ordinary technical skill and attention to detail, the sum of all the errors in wind tunnel testing, with the ground board method, will not exceed 20% of the reported resistance readings. Air resistance of current conventional forms in the heavy transportation field exceeds that of efficiently streamlined forms by amounts ranging from 100% to 400%. It will be time enough to worry about this 20% when the differences between the conventional and the ideal forms are not so great. | ||