Lehman and Kaplan 



Strain Gages 



I Members Holding 



Strain Gage Beam 

 Under Tension 



Face Which 

 Deflects 

 Under Load 



Fixed Face 



Fig. 4 - Pretensioned strain-gaged flexure element 



each other when a force is applied normal to the thin sides of the unit, the strain 

 gaged strip of metal permits only as much motion as the force is capable of ad- 

 ditionally stretching the metal strip. Thus deflections are very small, and the 

 unit has a relatively high resonant frequency. The wiring of the strain gage 

 bridges is shown in Fig. 5. 



Any number of such basic elements as shown in Fig. 4 can be fastened to- 

 gether so that forces in the desired directions can be sensed (and determined). 

 The interaction of such systems is extremely low, somewhat under 0.5% for all 

 forces or moments except those about an axis perpendicular to both stiff plates, 

 where the interaction is approximately 1.0% (9). 



Appendage- Balance Arrangement 



The initial propeller appendage investigations (10) had the appendage com- 

 pletely spanning the test section. In this arrangement the appendage was not as 

 stiff in the transverse direction as might be desired for obtaining data at fre- 

 quencies above the blade rate. The appendage design was therefore modified, 

 based on the suggestions of Dr. Murray Strasberg of the Naval Ship Research 

 and Development Center. This design, used for most of the tests reported here, 

 consisted of having the test appendage perpendicular to the support members. 

 A sketch of this test arrangement with a two-bladed propeller is shown in Fig. 6. 

 It can be noted that the appendage was held in place by two support bars extend- 

 ing through the holes in the mounting windows and then fastened into the dynamic 

 balances. Using the pretensioned strain- gaged force balances discussed, the 

 motion of the support bar under an induced force loading is less than 0.001 in.; 

 thus radial-lip shaft seals can be employed without introducing significant seal 

 reaction forces on the support bars. Consequently the forces introduced on the 

 appendage by propeller action are not affected by seal dynamics. The support 



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