Low Mass Ratio Hydrofoil Flutter Experimental Model Design Procedure

The present paper describes the design concept and specifications of a hydrofoil model to be actually tested for flutter experimental analysis at CNR-INM Institute of Marine Engineering towing tank in Rome. The design procedure is the result of concurrent application of numerical and analytical approaches: CAD models are used for geometrical modelling and mass properties calculations, FEM is employed to calculate model stiffness, natural frequencies and verify model strength, and Theodorsen analytical approach is implemented to predict flutter velocity. Theodorsen approach allows calculating the flutter condition as a function of physical parameters as geometries, mass and stiffness, assuming two-dimensional, incompressible aerodynamic coefficients and sinusoidal harmonic motion at flutter instability condition (zero damping condition). As first step, the authors built a broad literature review upon past flutter experimental experiences in both air and water flow focusing on the troubles linked to the increase of flow density and viscosity, the technical issues to be considered when designing the flutter model and setting up experimental campaigns. Most of the flutter experimental campaigns reported in the literature deal with high mass ratio models as aerofoils operating in light, low viscosity fluids; less common are experimental reports about low mass ratio models as light hydrofoils. The sample design process started by a dynamical scale of a hydrofoil model, flutter-tested in 1971, chosen as main reference. The model is designed to encounter flutter at a speed compatible with the range of velocity imposed by the water tank facilities. The combination of design parameters is optimised to meet facilities speed range, construction issues and Theodorsen approach application field.