Flutter of Flexible Hydrofoil Struts 



This was the basis for dividing flutter phenomena into two regions. 

 Flutter in one region occur ed in a predominantly bending mode shape, 

 and will be referred to as bending flutter. Flutter in the other region 

 occurred in a predominantly torsional mode shape, and will be referr- 

 ed to as torsional flutter. 



It appears that all hydrofoil struts, including those with pods 

 and foils attached, undergo either bending flutter or torsional flutter. 

 The type of flutter characteristic of a given strut can be determined by 

 examining its vibration modes, except in a transition region where 

 strong coupling of structural modes occurs. Most available data can be 

 readily placed into the appropriate flutter regions. 



Experimental results from each flutter region were examined 

 separately. The two flutter regions corresponded to two ranges of ge- 

 neralized mass ratio. In the bending flutter region, struts had low va- 

 lues of generalized mass ratio, while struts in the torsional flutter 

 region had high values of generalized mass ratio. Flutter speed varied 

 differently in each region as a function of mass ratio or strut submer- 

 gence, a related parameter. 



Calculated flutter characteristics show substantial qualitative 

 agreement with observed characteristics. Flutter was found to occur 

 in a different hydroelastic mode in each flutter region. Predicted flutt- 

 er inception speeds for torsional flutter were conservative for most 

 struts, with many predictions being overconservative. Unfortunately, 

 flutter speed predictions for bending flutter were not usable because 

 two flutter modes were often predicted to be unstable in the bending 

 flutter region, with the wrong mode predicted to be the least stable. 

 This discrepancy was related to an extreme sensitivity of the flutter 

 calculation to hydrodynamic loading modification in the bending flutter 

 region. 



II. EXPERIMENTAL FLUTTER CHARACTERISTICS 



II. 1. Bending-Type and Torsion-Type Struts 



The flutter mode of a strut is strongly correlated with the nature 

 of the vibration modes of the strut in air or in water. It is therefore 

 convenient to define a method for classifying struts according to im- 

 portant differences in vibration modes. Strut mode shapes are those of 

 a cantilever beam, with bending displacements perpendicular to the 

 plane of the strut and torsional rotations about a spanwise elastic axis. 

 Mode shapes are designated by their similarity to the uncoupled mode 

 shapes of a cantilever beam. Some uncoupled mode shapes are shown 

 in Figure 2, numbered in order of increasing frequency. 



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