Flutter of Flexible Hydrofoil Struts 



III. 5. Struts with Foils 



Successful flutter analysis of strut-foil systems is of consider- 

 able practical importance, because struts will generally be used in 

 combination with load-bearing foils. Only inverted-T strut-foil confi- 

 gurations have been considered in the present work, in view of the in- 

 terest of the U. S. Navy in such configurations. It is clear that for such 

 systems foils have a sizable effect on flutter characteristics. Flutter 

 speeds obtained experimentally by Huang [_Z~\ showed an increase of as 

 much as 146 percent when a pod was replaced by a pod and foil combi- 

 nation of equal mass. The parameters governing the effects of foils on 

 flutter characteristics have only begun to be investigated. An early 

 discovery has been that foil angle of attack is an important flutter 

 parameter \Z~\ . 



While experimental results are relatively scarce, much can be 

 deduced about the flutter characteristics of struts with foils by con- 

 sidering the structural effects of adding foils. A strut with no tip at- 

 tachment will usually be a bending -type strut, and a heavily tip- 

 weighted strut will be a torsion-type strut. Therefore struts with foils 

 will vary from bending -type to torsion-type, with many configurations 

 being in the transition region, according to the size and weight of the 

 foils. Other parameters will be important to the extent that they pro- 

 duce bending -type or torsion-type characteristics. The rotational 

 inertia of the foils will affect the coupling between the second and third 

 vibration modes, so that large or high aspect ratio foils will produce 

 struts in the transition region. Large or heavy pods tend to produce 

 torsion-type struts. These effects are related to the generalized mass 

 ratio of the strut. 



Flutter characteristics calculated for a strut with foils were 

 consistent with these deductions. The strut had a large pod and full- 

 sized foils. The calculated instability occurred in hydroelastic 

 mode 2, the unstable mode in torsional flutter. The flutter speed pre- 

 diction was overconservative. The second and third vibration modes 

 at zero speed were composed equally of second bending and first tor- 

 sion mode shapes, indicating that the strut was in the transition re- 

 gion. 



The flutter analysis performed on the strut-foil model [2] will 

 be described in detail to permit comparison with previous results. 

 Structural characteristics of the model are given in the Appendix. 

 Several approximations were made in obtaining a theoretical repre- 

 sentation for the pod and foils. Structural properties of the pod and 

 the foils were represented by adding equivalent masses and moments 

 of inertia to the tip of the strut. Hydrodynamic loading on the pod and 



361 



