A Parameter Survey Of Hydrofoil Strut Flutter

In order to explore the fundamental nature of hydrofoil flutter, a hydroelastic instability which is a potential cause of structural failure of lifting surfaces operating at high speeds in water, a series of hydrofoil models was tested in a towing basin. Flutter data were obtained from four surface-piercing hydrofoil strut models with various pod and foil configurations. Flutter occurred in two independent hydroelastic modes having predominantly first bending and first torsion mode shapes, respectively. The unstable modes were found to exist simultaneously in at least one strut. Flutter inception speed of the bending flutter mode was affected by strut submergence and the presence or absence of a foil. Flutter inception speed of the torsional flutter mode was dependent on strut cavitation pattern, strut submergence, pod moment of inertia, pod length, and the presence or absence of a foil. (Author).

Experimental flutter-test results for some flexible cantilever-wing models partly submerged in water are presented. The results tend to corroborate theoretical predictions of a stable region at low ratios of structural mass to fluid mass. Theoretical studies indicate this dynamic stability to be closely associated with the presence of fluid damping forces. An explicit expression is presented which defines the lowest mass ratios for which representative section (flexure-torsion) flutter solutions are obtainable as a function of the various structural parameters. Classical flutter of hydrofoils is possible, especially for heavy sections with the center of gravity located far aft. Theoretical indications are that the flutter boundary at low mass ratios is very sensitive to small differences in the various theories. Inclusion of sweep in a modal analysis resulted in flutter solutions in the low mass-ratio range where none had previously been obtained. (Author).

A series of surface piercing hydrofoil struts were designed, fabricated, and flutter tested. Classical flutter was obtained at mass-densityratios as low as 0.1 and limited experimental parameter studies were completed. Selected experimental results are compared in det il with existing aerodynamic flutter theory and it is seen that a model type of analysis appears to hold considerable promise for predicting lowmass-density-ratio flutter. (Author).