Miller 



Fig. 5 - Simplified 

 drawing of balance 



Dynamic forces and moments were applied to the assembled system with 

 electrodynamic shakers and measured with piezoelectric force gages. The gages 

 were attached to a metal block on the end of the balance in positions determined 

 by the force or moment to be applied. The force was transmitted to the gage 

 through a thin rod in order to avoid any lateral excitation which had been found 

 to affect the calibration of the force gages. Care had to be taken to distinguish 

 resonances in the driving rod from those of the dynamometer. Driving-rod 

 resonances were eliminated by changing its length or by clamping weights to the 

 rod. Preliminary calibrations were made in air, but after it was found that the 

 shakers would operate in water and the force gages were waterproofed, the final 

 calibrations were made with the tunnel filled with water. The results are shown 

 in Figs. 8-11. Figure 8 includes a calibration made in air, which shows very 

 little difference from the one made in water. The calibrations showed good 

 repeatability except for the horizontal bending moment, where three separate 

 calibrations showed considerable spread at the lower frequencies, although the 

 side-force calibrations obtained with the same loading showed excellent agree- 

 ment. These frequency response curves are in good agreement with the theo- 

 retical curves that were computed for the final design, except for the small 

 resonances in bending moment and side force at approximately 230 Hz. The 

 cause of these resonances has not been determined, and attempts to eliminate 

 them have been unsuccessful. The outputs shown in these figures are in rela- 

 tive units. The values used for reducing the test data were derived from these 

 curves at blade frequency. The calibration was put into the computer program 

 in the form of a 6x6 matrix. When the instantaneous readings in millivolts of 

 the six channels are multiplied by this matrix, the thrust, torque, side forces, 

 and bending moments are obtained in pounds or pound-feet. 



A typical calibration for one propeller position follows as a coefficient ma- 

 trix of a set of equations giving the forces and moments as functions of the mil- 

 livolt output of the six strain-gage bridges: 



T = 1.75 et + 0.029 e„ + 



Q= 

 Fi = 

 Ml = 

 F2 = 

 Mo = 



+ 0.076 eq + 



+ 



+ 



+ 



+ 



+ 



+ 0, 



+ - 1.96 Of + 0.137 e„ + + , 



+ + 0.072 ef J + 0.086 e^ +0 +0, 



+ +0 +0 - 1.89 Bf + 0.098 e , 



+ +0 +0 - 0.069 er - 0.085 e . 



r 2 mj 



260 



