TABLE 1 18. 



AERODYNAMICS. 



The pressure on a plane surface normal to the wind is for ordinary wind velocities expressed by 



where k is a constant depending on the units employed, w the mass of unit volume of the air, 

 a the area of the surface and v the velocity of the wind.* Engineers generally use the table of 

 values of P given by Smeaton in 1759. This table was calculated from the formula 



P==. 00492 z/' 2 



and gives the pressure in pounds per square foot when v is expressed in miles per hour. The 

 corresponding formula when v is expressed in feet per second is 



/ ) =.OO228z/ 2 . 



Later determinations do not agree well together, but give on the average somewhat lower 

 values for the coefficient. The value of w depends, of course, on the temperature and the baro- 

 metric pressure. Langley'st experiments give kw=. 00166 at ordinary barometric pressure and 

 10 C. temperature. 



For planes inclined at an angle a less than 90 to the direction of the wind the pressure may 

 be expressed as P a = F a Pw 



Table 118, founded on the experiments of Langley, gives the value of f a for different values of 

 a. The word aspect, in the headings, is used by him to define the position of the plane relative to 

 the direction of motion. The numerical value of the aspect is the ratio of the linear dimension 

 transverse to the direction of motion to the linear dimension, a vertical plane through which is 

 parallel to the direction of motion. 



TABLE 118. Values of F a In Equation P a = P a P9o. 



* The pressure on a spherical surface is approximately 0.36 that on a plane circular surface of the same diameter 

 as the sphere ; on a cylindrical surface with axis normal to the wind, about 0.5 that on a rectangular surface of length 

 equal to the length, and breadth equal to the diameter of the cylinder. 



t The data here given on Professor Langley's authority were communicated by him to the author. 



SMITHSONIAN TABLES. 



108 



