Table 1 58 197 



CORRECTIONS TO ROBINSON CUP ANEMOMETERS 



The official Weather Bureau instrument used for measuring speed of natural winds is a 

 Robinson type cup anemometer. Before January 1, 1928, a four-cup driving unit was used; 

 after that date a three-cup unit, because of the large errors of the older type at high speeds. 

 The table gives the speeds indicated by the old and new instruments at various true speeds. 



Note. — Values above a true speed of 75 miles per hour are extrapolated. 



It must be borne in mind that problems in aerodynamics can not be idealized as easily as 

 many problems in mechanics. The side of a building may not be regarded as a thin flat 

 plate in computing the force of the wind, and data for a cylinder of a given length can not 

 be directly applied for the wind force on a cylinder of some other length. Further, objects 

 nearby exert an appreciable influence. 



These complications limit the strict application of a test on a particular object to geo- 

 metrically similar objects in similar surroundings. They also cause apparent discrepancies 

 among the results of different experimenters which are to be attributed to departure from 

 geometrical similarity of the models, to the effects of the relative size of the body and the 

 air stream, of the supports or other nearby objects, and to differences in the fine structure 

 (turbulence) of the approximately steady air streams rather than to errors of measuring. 



The data here given are intended to apply to the ideal condition of an isolated body of 

 exactly the shape specified in a uniform, steady air stream of infinite extent. 



Example of tables : Take the problem of the resistance of a sphere 1.5 cm. diam. moving 

 at a speed 35 m/sec. (3500 cm/sec.) through still air of density 0.00 10 g/cm 3 and viscosity 

 0.000173 g/cm sec. The Reynolds number is 3500X1-5X0.0010/0.000173 or 30,347; 

 logioR is 4.482; whence from Table 162 C is 0.50. From Table 157 the value of q for std. 

 density is 76.56 kg (force) /m 2 . The ratio of the actual to std. density is 0.0010/0.0012255 or 

 0.816. The resistance is therefore \ 0.50 \ \ n/4 }■ ] 1.5/100 y\ 76.56 }■ i 0.816 [> =0.00552 

 kg (force) = 5413 dynes. 



Smithsonian Tables 



