Flutter of Flexible Hydrofoil Strut i 



ferences in mode shape were visually observed. Differences in flutt- 

 er mode shapes were reported by Huang |_2j as a result of flutter test- 

 ing a strut with and without a heavy pod. The bending amplitude of the 

 strut alone was reported to be considerably larger than the torsional 

 amplitude which was also present. When the pod was added, the tor- 

 sional amplitude became larger than bending. 



A similar result was obtained in an experiment performed at 

 the Naval Ship Research and Development Center (NSRDC) in which 

 a bending -type strut and a torsion-type strut were flutter tested. 

 Both struts had been previously tested but mode shapes were not re- 

 ported. Motions of the struts were visually observed and recorded 

 on video tape. The bending-type strut, Model A of Reference 3, under- 

 went large first bending oscillations with little evident twisting. In 

 constrast, the torsion-type strut, Model 2T of Reference 4, displayed 

 first torsion oscillations with no visible bending. 



In addition to a change in mode shape, a change in frequency 

 would be expected when the flutter mode changes. Several pod confi- 

 gurations for Model 2T were flutter testedj^4] , and a significant change 

 in flutter frequency occured when the strut changed from bending- 

 type to torsion-type. Flutter data for this strut are plotted in Figure 3. 

 As the pod mass was increased and the pod center of gravity was 

 moved aft, an abrupt increase in frequency occured between pod con- 

 figurations A and B. Vibration modes calculated in water identify pod 

 configuration A as a bending-type model, while pod configuration B 

 gives strongly coupled second bending and first torsion modes for 

 both its second and third modes and therefore falls in the transition 

 region between bending-type and torsion-type struts. Pod configur- 

 ation C was a torsion-type strut. 



Although mode shapes have been observed in only a small 

 number of cases, other aspects of flutter data exhibit a dual nature 

 corresponding to differences in mode shape. The effects of generaliz- 

 ed mass ratio and of strut submergence vary according to the flutter 

 region. These effects will be discussed below. 



II. 3. Generalized Mass Ratio 



Generalized mass ratio is a parameter which indicates the 

 relative importance of structural and fluid inertia in determining the 

 motion of a strut. Both structural and fluid inertia are related to the 

 vibration mode shape (and therefore to the elastic properties) of the 

 strut. This relationship is included in the most general form of the 

 parameter, which can be expressed in terms of matrix elements as 



347 



