Besoh and Liu 



were not insignificant. 



Flutter characteristics calculated for several other torsion- 

 type struts using three-dimensional loading were similar to those of 

 Model 2T. Flutter invariably occurred in mode 2 . Calculated flutter 

 speeds, which are compared with experimental values in Figure 14, 

 ranged from 59 percent conservative for very light pods to 36 percent 

 nonconservative for very heavy pods. Frequency predictions showed 

 good agreement with measured values at the experimental flutter 

 speeds. Flutter mode shapes were predicted to be first torsion, with 

 occasionally significant amounts of first bending or second bending. 

 These mode shape predictions agreed with visually observed mode 

 shapes. 



Examples of predicted flutter characteristics of both bending - 

 type and torsion-type struts, as well as a strut in the transition region 

 (pod configuration B), are shown in Figure 3. The increasingly conser- 

 vative torsional flutter speed predictions are evident as pod inertia 

 decreases, until bending flutter occurs with pod configuration A. In 

 view of the good correlation between theoretical and experimental fre- 

 quency and mode shape in the torsional flutter region, it is concluded 

 that torsional flutter is an instability of hydroelastic mode 2 for torsion- 

 type struts. 



A new mode similar to that of Model 2 and Model 2T also ap- 

 pears in the hydroelastic modes of other torsion-type struts. This 

 mode appears at lower speeds for struts with lighter pods. The stabi- 

 lity of the new mode decreases as strut pods become lighter and strut 

 configurations shift from torsion-type to bending -type. Bending flutter 

 appears to originate when the new mode becomes unstable at a lower 

 speed than the mode which is unstable in torsional flutter. A shift in 

 the mode shapes of the second and third modes occurs as part of this 

 transition. It is perhaps not coincidental that the mode which is un- 

 stable in the torsional region, and the mode which is incorrectly pre- 

 dicted to be unstable in the bending region, both originate as first 

 torsion modes. 



Calculations made for struts with large pods included an appro- 

 ximate correction for hydrodynamic forces acting on the pod. The 

 correction, added to the tip of the strut, consisted of the linearized 

 lift and moment due to the unsteady motion of a pod-sized slender body, 

 and is described on page 417 of Reference 12. This correction pro- 

 duced much lower flutter speeds than using pod added mass alone, 

 particularly for heavy pods. 



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