Aeroelastic Stability of Lifting Surfaces in High-Density Fluids

Abstract

The large increases anticipated in speeds of vehicles towed or propelled underwater suggests a re-examination of the problem of stability of flexible lifting surfaces mounted thereon. Experimental and theoretical evidence is assembled which suggests that oscillatory aeroelastic instability (flutter) is very unlikely at the structural-to-fluid mass ratios typical of hydrodynamic operation. It is shown that static instability (divergence) is the more important practical problem but that its occurrence can be predicted with greater confidence. Flutter data obtained in high-density fluids are reviewed, and various sources of inaccuracy in their theoretical prediction are analyzed. The need is expressed for more precise means of analytically representing both dynamic-elastic systems and three-dimensional unsteady hydrodynamic loads. For a simple hydrofoil with single degrees of freedom in bending and torsion, the theoretical influence of several significant parameters on high-density flutter is calculated and discussed. Recommendations are made for refinements to existing techniques of analysis to include the presence of channel boundaries, free surfaces, cavitation or separated flow.

Flutter of a finite rectangular Wing’D mill in pitch and heave

“…G nearly matches the 1D decline but is ¼ lower at the peak. The G peak lowers and shifts up in k due to the dominant 1/ kA 2 reduction of Im{I t } in eqn (11).…”

“…The ratio of foil mass to the virtual mass m of the circumscribing fluid cylinder is <<1 with the 700x water density whereas aircraft wing mass ratios are >>1. Pitch and heave flutter calculations of typical hydrofoils by eminent aeroelasticians 11 backed up their empirical lower flutter limit of roughly unit ratio. An upside down version of Figure 1 with two opposed ferrocement blades could be not be adjusted to flutter when towed through water.…”