Development and Modal Characterization of a Scaled Underwater Kite Wing

This chapter presents the development and modal characterization of a compliant wing for an underwater marine hydrokinetic (MHK) kite. MHK kites are systems engineered to efficiently harness current and tidal energy from bodies of water, employing cross-current flight patterns to augment velocity and, thus, power production. Given the substantial fluid dynamic loading that MHK kites undergo, which scales with the square of the apparent flow speed at the kite, comprehensive structural analysis is critical for the success of the system design. The speed-dependent loading of the wing can result in wing bending and twisting, which alters the flight dynamics of the kite and requires adaptive control strategies to maintain performance across all flow regimes. The previous North Carolina State University (NCSU) MHK scaled experimental vehicle features a rigid wing, fabricated using aluminum ribs and spars and carbon-fiber laminate skin, which does not appreciably flex during underwater flight. However, when sized for full-scale deployment, MHK kite wings typically cannot remain rigid without excessive internal structural material resulting in high vehicle weight, or without decreasing the wing aspect ratio, which negatively impacts kite performance. To observe and demonstrate wing flexure and twist under small-scale experimental flight conditions, a new compliant wing is devised and built, making use of a foam core and a thin fiberglass skin. To integrate with the NCSU experimental vehicle, the wing is fabricated as two individual left- and right-hand mirrored halves and is approximately of the same wingspan of 0.842 meter. The damped natural frequency of the first bending mode of one compliant half-wing is measured using a laser Doppler vibrometer, and the wing’s effective elasticity modulus was identified. The characterized compliant wing was then simulated in the dynamic model of a small-scale MHK kite and was predicted to experience a maximum tip deflection of 4.05% of the kite wingspan during cross-current flight. Such modeling could prove useful for optimal kite flight controller tuning and appropriate design of the kite wing for structural fatigue life considerations and efficient material usage.