Evaluation of Dynamic Lift Coefficients of High Aspect Ratio Rudders and Control Surfaces

Abstract : The goal of this Trident Project was the determination of the maximum dynamic lift coefficient of high aspect ratio control surfaces to reduce the uncertainty in design. The effective design of rudders, diving planes and other control surfaces with high aspect ratios, i.e. a larger depth in proportion to chord length, is hindered by uncertainty in the dynamic coefficient of lift of the control surface as it passes through a transient fluid flow affected by the fluid free surface. This unsteady flow creates momentary spikes in loading, which if large enough, can cause immediate or fatigue-induced failure. High aspect ratio control surfaces are more efficient (higher lift-to-drag ratios) than traditional designs and generate greater lift at smaller angles of attack to the incoming flow, however their increasing length produces significantly greater bending moments in the shaft. This has led to a number of in-service failures. As high aspect ratio lifting surfaces become more commonplace on surface vessels and submarines, more precise knowledge of loading conditions is required to avoid more failures. The dynamic lift coefficient is a dimensionless load factor that can be applied to geometrically similar bodies. As the dynamic coefficient of lift is dimensionless, by employing the principles of similitude, the testing of large bodies such as ships hulls or wing sections can be successfully analyzed through the use of scale models tested in wind tunnels or tow tanks. Data collection took place in the David Taylor Model Basin at the Naval Surface Warfare Center. Two 1/5-scale rudders were tested on a donated 15-foot vessel model. Strain gages were attached to the rudder and shaft to determine the strain distribution and overall deformation.