Opposite page: day 14 of AC34 and the USA defenders demonstrate how they have successfully adopted Emirates TNZ’s devastating AC72 roll-tacking technique. Of equal significance, note the perfectly fair twist in the wing to drive acceleration. Left: RANS high-lift AC72 simulations with original sections (far left) and optimised sections (left). AC72 model in Auckland’s twisted flow wind tunnel (bottom left) and Emirates TNZ trim terminology (below)
‘By this time it was clear that the boats would be foiling downwind and that the boats were impressively fast, which had led to the apparent wind angle range downwind tightening from 25-45° AWA to 20-30° AWA. Downwind wing trims were becoming a lot more like upwind with less camber required and the wing twisted in as little as 10kt TWS. ‘Consequently, we were more interested in designing Wing Two as a more upwind optimised wing.’
A single-element wing was again explored but discarded. The efficiency gained upwind was offset by seriously compromised performance downwind and the engineering involved in developing sufficient twist to lower the centre of effort would have been challenging.
Different chord combinations were also examined and laminar flow refinements were investigated. ‘In the end the potential gains from these refinements of the designs were relatively small, especially compared to the expected gains we were finding in the development of the hydrofoils and platform aerodynamics.
came out somewhere between the two. In late 2011 two identical test wings were built for the team’s SL33 catamarans. These were scaled versions of their AC72 0.9 design, which was very close to the final design for Wing One. The SL33 wings were partly about gaining experi- ence in building wings and testing control systems, but also offered an opportunity to validate aerodynamic assumptions. They did reveal some surprises.
‘The VPP had indicated that, for opti- mal VMG, the wing should be depowered downwind above 8kt TWS. This was viewed with some suspicion and questions remained whether or not it would be better to power up and sail lower and slower. ‘Two-boat testing confirmed that the predicted target trims from the VPP were accurate. The boats were easily able to sail very deep true wind angles but there is diminishing return for sailing lower; it is more advantageous to depower and sail the yacht fast.’
Main element twist, identified in the theoretical model as beneficial downwind, was difficult to evaluate in the early stages of foiling development. ‘However, a surprise came in using W1 twist upwind, where the gain rate was higher than what had been predicted by the VPP. ‘It was felt that this was a dynamic effect. With the slot between the wing and jib better set up, the yacht was able to build speed and accelerate more efficiently. There was an obvious cost to implement- ing W1 twist in the structure and control system; however, after its success in the SL33 upwind it was hard to discount it.’ Laminar flow tests were also conducted on the SL33 wings.
With the design for Wing One signed off in November 2011, attention turned to Wing Two. At this stage the team was able to observe the work of rival teams with both Artemis and Oracle showing more upwind orientation in their designs due to their smaller flap-chord ratios.
‘Additionally, there was a cost benefit in reusing the existing tooling from Wing One. There was also a desire to have the two wings close in design in case we had to swap wings or wing parts mid-regatta. ‘As a result, the two wings were essen- tially identical, just with the main element spar and rib shapes optimised more for laminar flow in the top half of Wing Two.’ Finally, since it had been established that high lift was no longer such a priority, it was decided to remove the tab on the trailing edge of the main element in both wings to achieve some weight saving. ‘Without the tab present to condition the flow onto the flap, stall would occur at large flap angles. To offset this, the hinge point for the flap was moved aft to reduce the slot opening.’ Removing the tab reduced the weight by about 40kg and the shift in the hinge point largely mitigated the stall implications.
The authors conclude that the ETNZ wing design programme involved elements of discovery, refinement and compromise. ‘The computational methods and tools used for the aerodynamic simulations and many of the design trade-offs in the wing development proved essential to the programme success.
‘While wing aerodynamics was not the game-changer that was originally expected, a lot of analysis was required to establish the correct compromises with the structural and control system design.’ q
SEAHORSE 29
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