Experimental Determination of Unsteady Propeller Forces 



elements and to some extent, particularly for lateral modes, by the characteris- 

 tics of the sting. Starting with a preliminary design, a number of progressive 

 changes were made in the sting and balance until the system was free of reso- 

 nances within the working range. In order to evaluate these design changes, the 

 system was represented by lumped parameters as shown in Fig. 2, The values 

 of these parameters were put into a computer program (9) to calculate the 

 strains in the gaged elements for a unit force applied to the propeller. Very 

 little trouble was experienced in obtaining a good frequency response for thrust 

 and torque. However, 36 modifications had to be made and computed before an 

 acceptable response was obtained for lateral excitation. The computed response 

 of the final design for lateral excitation is shown in Fig. 3. The resonance at 

 about 6 Hz is due to the sting and balance assembly vibrating as a rigid mass on 

 the soft bearing supports. The next resonance, at 465 Hz, is the first bending 

 mode. The region between these resonances is the useful working range. The 

 response curves for thrust and torque were similar except that the useful range 

 extended to higher frequencies for thrust. 



2 |V\H "3 h^^ 

 k-, 



Fig. 2 - Lumped constant system 



Frequency 

 Fig. 3 - Computed response to lateral excitation 



257 



