124 



Table 108. 



AERODYNAMICS. 



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



P =^ kwav^ 

 where ^ 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 /'given by Smeaton in 1759. This table was calculated from the formula 



>p== .004922''^ 

 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 



/>=. 00228 5^2. 



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's experiments give /67« = .ooi66 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 /'a = FaP^^- 



Table 108, founded on the experiments of Langley, gives the value of Fa. 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 108. —Values of Fa In Eanatlon Pa = FaP9o. 



* The following pressures in pounds per square inch show roughly the influence of the shape and sire of the resist- 

 ing surface (Dines' results). The wind velocity was 20.9 miles per hour. The flat plates were | in. thick. 



Square, sides 4 in 1.51 Plate, 6 in. diam. 90° cone at back 1.49 



Circle, same area 1.51 



Rectangle, 16 in. by 1 i 70 



Sguare, 12 in. sides 1.57 



Circle, same area i 55 



Rectangle, 24 in. by 6 1.59 



Square, sides 16 in 1.52 



Plate, 6 in. diam. 4I thick 1.45 



Ditto, curved side to wind 0.92 



Sphere, 6 in. diam 0.67 



Same, cone in front 0.98 



" sharp 30° cone at back 1.54 



" cone in front 0.60 



5 in. Robinson cup on 8J in. of \ in. rod .... 1.68 



Same, with back to wind 0.73 



9 in. cup on 6^ in. of § in. rod 1.75 



Same, with back to wind 0.60 



22 in. cup on 9! in. of \ in. rod 2.60 



Same, with back to wind 1.04 



Smithsonian Tables. 



