Wind tunnel testing of two-dimensional multi-element wing sails

Abstract

This thesis describes just part of the design and optimisation of wing sails for multihull yachts. This is a broad and wide ranging subject with a growing interest. A lot of research has started in academic departments to help with the design of wing sails and to fill the gap left by secretive sailing syndicates in the public knowledge. The first part of this thesis describes general multihull yacht stability. This is followed by a comprehensive and critical study of the history of wing sail design and current trends in racing, commercial/industrial and pleasure craft markets. It goes on to describe how several wing sails were made for the purpose of 2-dimensional wind tunnel testing at the University of Auckland’s Twisted Flow Wind Tunnel. The final two models consisted of a NACA 0025 and a NACA 0009 wings of equal chord length which were surface pressure tapped using the Yacht Research Unit’s pressure tap system. In total 128 pressure taps were used to test the wings up to a Reynolds number of 900,000 in the high speed section of the Twisted Flow Wind Tunnel. Testing was done investigating the effects of pivot point location, gap size, camber (aft element deflection), Reynolds number and angle of attack. It was discovered that as expected, camber and angle of attach have the largest effect on the lift and drag output of a wing. The initial gap size had only a small effect (approx. 3%) on the performance of the wings for a pivot location of 80% with the best result being an initial gap size between 0-1percent of the leading elements chord length. When the pivot location was moved forward on the leading element to 40%, there was a decrease in performance for negative gap sizes. The models were tested at more than five hundred unique configurations and still more could be done before a complete understanding of their performance could be achieved. This was without changing the variables of profile section, chord ratio or number of elements. Without the tight constraints of a strict yacht design class, there is so much variation and initial results show little gain in the two-dimensional aerodynamic performance if the rule of having a small to none initial gap size and a pivot location 10-20% forward from the trailing edge of the leading element. For multihull racing yachts, the current practice of setting the wing up on shore for initial gap and pivot point based on the predicted wind speed (i.e. Reynolds number) and having the wing sail trimmers adjust only camber and angle of attack during a race would be enough unless course conditions changed dramatically. The recent Americas Cup racing has indicated that boat control and lift producing dagger boards are where the largest performance gains are. Until these are closed down, the small gains from adjusting slot geometry while sailing doesn’t justify the extra crew work and complexity in control.