Propeller -Induced Appendage Forces 





Fig. 3 - Four-bladed test pro- 

 pellers; the BT-1 blades are at 

 the right, and the BT-2 blades 

 are at the left 



techniques permits the motion of one plate relative to the other to be calibrated 

 as a function of applied force. While this approach is satisfactory for most 

 steady- state force measurements, it is of little value in sensing dynamic force 

 fluctuations as the deflection of one plate relative to the other is quite large and 

 thus the resonant frequency is quite low. 



To overcome this defect a system was employed using the advantage of a 

 flexure plate sensing element while limiting the deflections to extremely small 

 increments, thereby producing a system which is quite stiff. This technique 

 uses pretensioned strain- gaged beams. In this application a thin strip of metal 

 having four strain gages mounted upon it is attached to an opening in one plate of 

 the element (Fig. 4). This opening (as well as the strain-gaged strip of metal) is 

 located perpendicular to the weak sides of the element. This strain-gaged strip 

 of metal is then placed in tension between its end- clamping members by stretch- 

 ing it before tightening the end clamps. To the bottom plate of the sensing ele- 

 ment is attached a very rigid massive member, which in turn is then clamped to 

 the center of the strain- gaged strip of metal. In this manner, while the two stiff 

 plates of the element still retain their capability of easily moving relative to 



173 



