NO. 4 WIND TUNNEL EXPERIMENTS IN AERODYNAMICS / 



inserted under the base plate of the balance standard, and the whole 

 balance raised to its proper height. It was now necessary to rectify 

 the vertical axis of the balance. 



To bring the axis of the balance more nearly vertical by more 

 sensitive means the following method was employed: The small 

 torsion wire, used in aerodynamical measurements with the balance, 

 was set in place. The lower pivot of the balance was engaged in its 

 bearing, leaving the balance free to rotate about its vertical axis, but 

 constrained from tipping laterally. 



The torsion wire was adjusted by means of the micrometer head 

 until the cross-hair on the fixed telescope coincided with that on the 

 mirror attached to the balance proper. 



A weight of 0.4 pound was placed on one balance arm. The 

 micrometer head was again turned until the cross-hairs were coinci- 

 dent. By setting up on the holding-down bolts, the balance axis was 

 adjusted until placing a weight on either of the arms required no 

 further rotation of the torsion head to maintain coincidence of the 

 cross-hairs. In such case the axis of the balance is vertical. The 

 final adjustment admits of a possible error of less than 1/400 inch- 

 pound on the torsion wire. The angularity of the balance axis 

 remaining uncorrected may be computed as follows : 



Let 



F = force hung on arm. 



/? = angle of balance axis to vertical. 



Then, taking moments about the vertical axis 



Fsin^Xi8" = o.4Xsin|8x i8"= 1/400 

 or/? = o°025. 



Determination of Wind Direction in the Horizontal Plane 



As a first approximation, the wind was assumed parallel to the axis 

 of the tunnel. A vertical flat plate was mounted on the balance arm 

 and carefully set parallel to a line drawn on the floor of the tunnel in 

 the direction of its axis. The plate was inclined 8 degrees to right 

 and left of this position and the transverse force measured on the 

 balance. The observations were repeated for 6 degrees and for a 

 second plate to eliminate errors due to irregularities in the plates. 

 The transverse force on one side was greater than on the other, indi- 

 cating an error in the assumed wind direction. A new line was drawn 

 on the tunnel floor making an angle of 0.3 degrees with the original 

 line. The observations for transverse force were repeated. It was 

 then found that the average of transverse force readings taken for 



