A Parametric 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).

The report presents design procedures and parametric trends that can be used to avoid flutter and divergence of hydrofoil strut-foil systems. Flutter stability of conventional T-foils can be achieved by scaling existing prototype systems which are stable; scaled flutter models must be used to determine the stability of new designs. Several stable T-foil designs are available for craft with displacements of up to 300 tons at subcavitating speeds (below 50 knots). The availability of higher speed designs for ships of practical size is limited to one system that is stable to 62 knots. Divergence stability of subcavitating systems can be ensured by theoretical analysis using an available computer program. (Author).