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 effects of side winds on air resistance and the limitations of wind tunnel testing for vehicles.
| Identifier | ExFiles\Box 161\1\ scan0022 | |
| Date | 31th May 1936 guessed | |
| 7. Neglect of the effect of side winds has caused an error in determin-ing air resistance coefficients for earth bound vehicles up to the present time. The effect of side winds on R in the equation K = R / AV^2 is shown in Fig. 11. It is seen that R in this equation must equal RH{R. Hollingworth} + RS{Sir Henry Royce's Secretary}, where RH{R. Hollingworth} is the direct head wind and RS{Sir Henry Royce's Secretary} is the effect of the side wind on the forward motion. Correction, for the effect of side winds, of the air coefficients reported in this paper have not been made. If this were done the streamlined forms would show still greater savings over the conventional forms. ----- The subject matter of this paper precludes any adequate discussion of wind tunnel testing technique. However, a word in passing, must be said regarding the opinion, sometimes expressed, that information obtained in the wind tunnel on land vehicles is too unreliable to be of any value. This opinion is usually based on one or more of the following five allegations: 1. The full scale model will vary materially in its air resistance from the wind tunnel model even though similarly shaped. 2. Road conditions cannot be reproduced in a wind tunnel, particularly as regards ground effect. 3. Variations in wind direction and velocity in actual operation nullify wind tunnel calculations. 4. Since resistance varies as the square of the speed and horsepower as the cube of the speed, errors are introduced in the resistance or power equations by the use of average speeds of operation. 5. Wind tunnel models are not sufficiently accurate to warrant de-termination from them of reliable conclusions, particularly as to the air flow in the region of the radiators, engines, undersurfaces and revolving wheels. Brief comments on each of these items follows: 1. The scale effect, that is, the Reynolds number correction, is of no importance on conventional equipment whose resistance follows the V squared law so closely. For streamlined equipment, by systematically varying the speeds of the tunnel test, the exponent n can be obtained by logarithmic plotting and full scale values can be accurately determined. It should also be noted that if no scale correction is applied to the values obtained from tunnel test for the streamlined equipment, the calculations of resistance will be ultra-conservative since the resistance of the streamlined equipment varies less rapidly than V squared. So that computations on the V squared law will always give resistance and horsepower values for the streamlined equipment greater than the actual values. A busy engineer who evaluates the resistance of streamlined equipment on the simple V squared law is certain to receive a pleasant surprise in the form of less resistance and horsepower in actual operation. | ||