BENDING FLUTTER AND TORSIONAL FLUTTER 

 OF FLEXIBLE HYDROFOIL STRUTS 



Peter K. Besch and Yuan-Ning Liu 



Naval Ship Research and Development Center 

 Bethesda, Maryland, U.S.A. 



ABSTRACT 



A large body of experimental and theoretical flutter 

 results for hydrofoil struts were analyzed to deter- 

 mine significant characteristics. Flutter was found 

 to occur in two different structural mode shapes, cor- 

 responding to a predominantly bending mode and a pre- 

 dominantly torsional mode, respectively. The flutter 

 mode shape was related to the vibration mode shapes 

 and the generalized mass ratio of the strut at zero 

 speed. The behavior of the hydroelastic modes of ty- 

 pical struts as a function of speed was investigated 

 using a strip theory with three-dimensional loading mo - 

 difications. Flutter predictions for struts which under- 

 went flutter in the tor sional mode were usually conser- 

 vative and predicted the correct mode shape. However, 

 flutter predictions for struts which underwent flutter 

 in the bending mode were unreliable in predicting the 

 mode of flutter because of an extreme sensitivity to 

 the loading modification used. Strut-foil systems of 

 the inverted-T configuration typical of full-scale hydro- 

 foil craft appear to undergo either bending flutter or 

 torsional flutter, depending on pod and foil character- 

 istics. 



I. INTRODUCTION 



The high speeds associated with many unconventional ships will 

 require a better understanding of flutter and other hydroelastic phe- 

 nomena than has been available for design of existing ships. 



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