NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 3 



The wind tunnel of the Institute of Technology was built in 

 accordance with the English plans, with the exception of several 

 changes of an engineering nature introduced with a view to a more 

 economical use of power and an increase of the maximum wind speed 

 from 34 to 40 miles per hour. 



Upon completion of the tunnel an investigation of the steadiness 

 of flow and the precision of measurements was made in which it 

 appeared that the equipment had lost none of its excellence in its 

 reproduction in the United States. 



As will be shown below, the current is steady both in time and 

 across a cross-section within about i per cent in velocity. Aleasure- 

 ments of velocity by means of the calibrated Pitot tube presented by 

 the National Physical Laboratory are precise to one-half of I per cent. 

 Force and couple measurements on the balance are precise to one- 

 half of 1 per cent for ordinary magnitudes. Calculated coefficients 

 which involve several measurements of force, moment, velocity. 

 angle, area, and distance, as well as one or more assumptions, can be 

 considered as precise to within 2 per cent. It is believed that it is 

 not practicable to increase the precision of the observations to such 

 an extent that the possible cumulative error shall be materially less 

 than the above. 



Description of Wind Tunnel 



A shed 20 by 25 by 66 feet houses the wind tunnel proper, 16 

 square feet in section, and some 53 feet in length (pi. i). Air 

 is drawn through an entrance nozzle and through the square tunnel 

 by a four-bladed propeller, driven Ijy a 10 H. P. motor. Models 

 under test are mounted in the center of the square trunk on the 

 vertical arm of the balance to be described later. 



The air entering the mouth passes through a honeycomb made up 

 of a nest of 3-inch metal conduit pipes 2 feet 6 inches in length. 

 This honeycomb has an important efifect in straightening the flow 

 and preventing swirl. 



Passing through the square trunk and past the model, the air is 

 drawn past a star-shaped longitudinal baffle into an expanding cone. 

 In this the plans of the National Physical Laljoratory were departed 

 from by expanding in a length of 11 feet to a cylinder of 7 feet diam- 

 eter. This cone expands to 6 feet in the English tunnel. M. Eifl:'el 

 affirms that the working of a fan is much improved by expanding the 

 suction pipe in such a manner as to reduce the velocity and so raise 

 the static pressure of the air. Since the fan must discharge into the 



