NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 2/ 



III. THE PITOT TUBE AND THE INCLINED MANOMETER 

 By J. C. HUNSAKER 



For aeronautical purposes the absolute measurement of velocity is 

 of less importance than the measurement of impactual pressure. For 

 this reason, an anemometer from which the velocity may be deduced 

 from a pressure measurement is preferable to any of the vane ane- 

 mometers which measure velocity directly, and require a somewhat 

 laborious calculation of the density of the air before the effect of the 

 wind can be evaluated. 



The most common as well as the most convenient form of pressure 

 anemometer is the double Pitot tube. Reference may be made to the 

 papers of Taylor ' and Zahm,^ in which it is shown that the equation 

 to a stream line in any perfect gas may be simplified in the case of air 

 by considering- the air incompressible for velocities below lOO feet per 

 second. The simplified expression connecting pressure and velocity 

 in moving air is then Bernoulli's equation as used in hydraulics : 



where v.^ and p.^ are velocity and pressure at any point, and v., and 

 p2 are corresponding values for some other point in the same stream 

 line. 



Let us choose the point where Vo is zero, then 



OCT ^ ^ ' - 



In air this is the barometric pressure. Let us change the notation 

 so that '^ +/' = /'o = barometric pressure, a constant. 



p is now the pressure in the unchecked stream, the " static " pres- 

 sure, and pQ is the pressure in the impact tube where the current is 

 brought to rest. This is called " dynamic " pressure. 



The Pitot tube is a device for transmitting the pressure difference, 



Po — p= ^'^ ; ) from which the velocity may readily be calculated. The 



2g 



quantity '^ -- is commonly called "velocity" pressure. 



^ Experiments with Ventilating Fans and Pipes, by D. W. Taylor, Naval 

 Constructor, U. S. Navy, Trans. Soc. Naval Architects and Marine Engineers, 

 IQOS- 



^ Measurement of Air Velocity and Pressure, A. F. Zahm, Ph. D., Physical 

 Review, December, 1903. 



