NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS I5 



A and C is precise to o.i per cent. The density of the solution was 

 taken on a Westfall balance to the same degree of precision. 



Since the gage is sensitive to less than o.i per cent for heads of 

 more than 0.3 inch, the measurement of velocity depends on the pre- 

 cision of the Pitot tube. The latter is good to probably 0.25 per cent 

 in velocity. However, the air current always has some fluctuation at 

 high speeds so that in the end the velocity measurement is limited in 

 precision by the closeness with which such fluctuations can be aver- 

 aged. In a very steady current, such as our wind tunnel, it was found 

 that the error in estimating velocity was less than 0.5 per cent. The 

 average of a number of observations is of course better than this. 



Change in density of the salt solution is I per cent for a change 

 of 60 degrees F. in temperature. A temperature correction is ordi- 

 narily unnecessary. 



An alcohol gage is a sensitive and consistent instrument, but 

 requires calibration to eliminate errors due to viscosity and capil- 

 larity. The question of its suitability for precise work will be dis- 

 cussed later in another paper. It has the great advantage over the 

 Chattock gage in that it requires no delicate manipulation to -get a 

 balance, no cross-wire and microscope, and with it it is possible to 

 estimate the mean of fluctuations. The alcohol gage has been suc- 

 cessfully used to measure air speeds as low as two miles per hour. 

 It is shown with the Chattock gage in plate 5. 



II. NOTES ON THE DIMENSIONAL THEORY OF 



WIND TUNNEL EXPERIMENTS 



By EDGAR BUCKINGHAM 



UNITED STATES BUREAU OF STANDARDS 



Introduction 



The forces which will act between a solid body and a fluid in con- 

 tact with it in consequence of a relative motion of the two, cannot, 

 except in a few of the simplest cases, be predicted by computation 

 from the size and shape of the body, the relative velocity, and the 

 physical properties of the fluid : the information can be obtained only 

 from experiment. Such experiments may be expensive or impracti- 

 cable, and it often appears desirable to get the required information, 

 in advance of the final decision on the design of a structure which is 

 to be subject to aerodynamic or hydrodynamic forces, by making 

 preliminary experiments on a small model of the proposed structure. 



