XO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS 5 



The iiuluclion motor is directly connected to a 12 H. P. direct cur- 

 rent generator, which is turned at constant speed and which gener- 

 ates, therefore, a constant direct current voltage for given load. 



By change of the generator field rheostat and motor field rheostat 

 the propeller speed can be regulated to hold any wind velocity from 

 4 to 40 miles per hour. The control is very sensitive. Left to itself, 

 the speed of the wind in the tunnel will vary by 2 per cent in 2 or 3 

 minutes. This variation is so slow that by manipulation of the rheo- 

 stats the flow can be kept constant within ^ per cent. The cause of 

 the surging of the air is not understood, but is probably due to hunt- 

 ing of the governor of the prime mover in the Cambridge power 

 house causing changes in frequency too small to be apparent. The 

 gustiness of outdoor winds seems to have no effect, although the 

 building is not air-tight. 



Aerodynamical Balance 



The aerodynamical balance (pi. 3) was constructed by the Cam- 

 bridge Scientific Instrument Company, England, to the plans and pat- 

 terns of the National Physical Laboratory. The balance is described 

 in detail by Mr. L. Bairstow in the Technical Report of the Advisory 

 Committee for Aeronautics, London, 1912-13. For details of oper- 

 ation and the precision of measurements reference may be made to 

 the original article. 



In general, the balance consists of three arms mutually at right 

 angles representing the axes of coordinates in space about and along 

 which couples and forces are to be measured. The model is mounted 

 on the upper end of the vertical arm which projects through an oil 

 seal in the bottom of the tunnel. 



The entire balance rests on a steel point, bearing in a steel cone. 

 The point is supported on a cast-iron standard secured to a concrete 

 pillar, which in turn rests on a large concrete slab. The balance is 

 then quite free from vibration of the floor, building, or tunnel. 



The balance is normally free to rock about its pivot in any direction. 

 When wind blows against the model, the components of the force 

 exerted are measured by determining what weights must be hung on 

 the two horizontal arms to hold the model in position. Likewise the 

 balance is free to rotate about a vertical axis through the pivot. The 

 moment producing this rotation is balanced by a calibrated wire with 

 graduated torsion head. 



Force in the vertical axis is measured by means of a fourth arm. 

 The model for this measurement is mounted on a vertical rod which 



