PRECISION DRAG BALANCE OF ONE COMPONENT

Investigations were conducted to design a drag balance of 1 component which is capable of yielding precise measurements. A balance was designed which eliminates static friction of all moving parts, including the sting, by supporting them on a system of rotating bearings. The axial force of the model applied to the sting is transmitted to a precision spring of iso-elastic material whose resulting stretch is measured with an accuracy of 0.00002 in. by means of a linear variable differential transformer and associated electronic equipment. Since the full-scale spring stretch is 0.100 in., the full-scale reading accuracy of the measurement is 0.02. Because of the slight unsteadiness of the moving surfaces supporting the sting, the accuracy reliably achieved is slightly better than 0.1. The sting has a 316-in. -diam hole running its full length, which permits air at model-base pressure to equalize the pressure in the box on the downstream end of the balance. This box contains the working parts of the balance. Since the after end of the sting is in the box which is at base pressure, no base-pressure correction need be made for the sting cross section. The balance was used in projects on skin friction and base pressure.