Miller 



basin. It uses a six-component strain-gaged balance similar to the string- 

 mounted balances used for wind-tunnel testing. Its design, construction, and 

 calibration are described in detail in Ref. (8). A third dynamometer was devel- 

 oped for use with propellers in the 24-inch water tunnel. Its design, operation, 

 and some typical results will be described here. 



DESIGN OF THE WATER-TUNNEL DYNAMOMETER 



In order to be useful over the range of test conditions used in the water tun- 

 nel, a dynamometer must have a relatively flat frequency response extending 

 from the lowest shaft frequency to several times the highest propeller blade 

 frequency. This range extends from about 10 Hz to at least 400 Hz. It should 

 also be able to measure the steady components of the forces and moments. To 

 simplify calibration, it would be desirable to have a flat response through the 

 working range and extending continuously to zero. The system must be isolated 

 from the vibrations of the tunnel and must be small enough so as not to cause 

 too much disturbance of the tunnel flow. A preliminary examination of these 

 requirements showed that it would be impossible to avoid some resonances well 

 below the upper limit of the desired working range. For isolation, it was de- 

 cided to mount the propeller and measuring elements on a stiff, heavy shaft ro- 

 tating in soft mounted bearings and driven through a soft coupling. For each of 

 the six components to be measured, at low frequencies, this propeller, balance, 

 and shaft assembly can be considered as a one-degree-of-freedom system with 

 the natural frequency being determined by the stiffness of the supports and cou- 

 pling and the mass or moment of inertia of the system. Simple calculations 

 showed that it would be possible to keep the axial, torsional, heaving, and pitch- 

 ing resonances below 8 Hz. In order to reduce the diameter without increasing 

 the torsional frequency, it was decided to construct a substantial portion of this 

 assembly of tungsten. A schematic of this system is shown in Fig. 1. It can be 

 contained in a housing about 4 feet long with a maximum diameter of 7 inches 

 tapering to 2 inches at the propeller. 



Measuring Spring 



Drive Coupling-^L. 



•Mass of Propeller 



Fig. 1 - Simple schematic of two-degree- 

 of-freedom system 



The next problem was to make all the higher vibrational modes fall at fre- 

 quencies above the operating range. The lowest of these is determined by the 

 mass or moment of inertia of the propeller and the stiffness of the measuring 



256 



