Miller 



The final assembly is shown in Fig. 4. The flywheel, sting, and balance as- 

 sembly weighs 160 pounds and is supported by rubber -mounted bearings on 

 either side of the tungsten flywheel. This assembly is connected to the main 

 tunnel drive shaft by a rubber coupling. Electrical signals from the balance are 

 transmitted through a cable in the shaft and the unsteady signals are amplified 

 before passing through the sliprings and brushes to the analysis and recording 

 instrumentation. 



A simplified drawing of the balance is shown in Fig. 5. It has the form of a 

 steel cylinder with two thin-walled sections on which semiconductor strain gages 

 are mounted. The balance fits a taper socket in the end of the sting and the pro- 

 peller mounts on the taper on the other end. Figure 6 shows a developed dia- 

 gram of the gaging and wiring diagrams of the bridges for measuring the six 

 components of force and moment. After gaging, the balance was waterproofed 

 using wax covered with a protective coating of soft epoxy. The completed bal- 

 ance is shown in the photograph of Fig. 7. 



Although semiconductor gages are more temperature-sensitive than metal- 

 lic gages, no difficulties have been experienced with them. This is probably due 

 to the relatively small changes in temperature in the laboratory and the tunnel 

 water and the four-arm bridge arrangement of the gages. There is a consider- 

 able drift in the zero readings, but this would only affect the steady measure- 

 ments and they are made immediately after the bridges are balanced. 



CALIBRATIONS 



Both static and dynamic calibrations were made for the completed dyna- 

 mometer. The static calibrations were made for the balance alone outside the 

 tunnel. The dynamic calibrations were made with the balance on the flywheel 

 and sting assembly and supported on rubber mounts. The calibrations were also 

 repeated after installation in the tunnel and with the tunnel filled with water. 



For the static calibrations, pure torque and thrust were applied in a con- 

 ventional manner. Since the axial position of the side force determines its in- 

 fluence on the bending-moment reading and an applied pure moment produced 

 some side force reading, it was necessary to obtain the side force and moment 

 calibrations for the particular axial position that the center of the propeller 

 would occupy. Instead of making individual calibrations for each propeller posi- 

 tion, a general calibration was obtained by fitting a cylindrical sleeve to the 

 propeller end of the balance and hanging weights at several measured axial lo- 

 cations. This is a method commonly used for calibrating sting-mounted wind- 

 tunnel balances. This procedures yields enough information to determine side 

 force and bending moment for any axial position of the propeller. The results 

 of this calibration showed that, aside from the effect of side forces on the 

 bending-moment readings inherent in this type of balance design, the only sig- 

 nificant interactions were a small effect of torque on the thrust readings and 

 some effect of bending moment on the output of the side-force gages in the same 

 plane . 



258 



