tspeer
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- Feb 26, 2002
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- Port Gamble, Washington, USA
The flap on the OTUSA AC72 could twist 40 degrees between the foot and head. That is the aerodynamic equivalent to twisting the entire wing by 32 degrees, as one degree of flap deflection produces the same change in lift as a 0.8 degree change in angle of attack. So getting enough twist with current wingsail designs is not a problem.
The structural demands of a wingsail should not be underestimated. OTUSA tested a twisting main element on an AC45 wing that used a tubular spar with a shell around it. The spar was much larger in comparison with the thickness of the main element than the proposed configuration and did not have any joints in it, so it was likely much stiffer. It was found the twisting main element did not perform as well as the rigid D-spar wing. The ability to twist the main element did not make up for the reduction in stiffness. ETNZ build their first AC72 wing with a twisting main element, but reverted to a rigid main element for their second wingsail. Cogito's win in the C-class Challenge Trophy has been attributed to its ability to twist the entire wingsail. That makes two examples where twist was not found to be important and one where it was, so the anecdotal case for wingsail twist is somewhat mixed.
I think the most important thing is to achieve the intended spanwise lift distribution given the operating conditions. This can be done with a combination of planform shaping, camber, and twist. At a given spanwise station, the desired lift can be obtained from a range of angles of attack and flap deflections, with the best combination being the one that results in the least profile drag for the specified lift. I've attached some MSES-generated section data for a wingsail. In the first set of data boundary layer transition is fixed near the stagnation point so the boundary layer is fully turbulent. In the second set, the boundary layer transition was free to occur at the location where the boundary layer was predicted to become unstable. This section was designed to benefit from a significant (but not extreme) amount of laminar flow, and these two cases represent pessimistic and optimistic assumptions regarding the ambient conditions. Most other sections will probably be more like the fully turbulent case. The black dot-dashed line is a locus of combinations of angle of attack (determined by wing trim and main element twist) and flap deflection that would give near optimum performance. For the free transition case, this turned out to be where the stagnation point was just to windward of the leading edge. For the fully turbulent case, it turned out to be at almost the same angle of attack over much of the lift range.
But in both cases, there wasn't much difference in profile drag for different combinations of angle of attack and flap deflection at the same lift, provided the combination wasn't wildly different from the optimum. This is why twisting the main element of a wingsail may not be worth the complication and compromise in other characteristics. Provided there is some way of getting to the right lift for the section, the payoff is not very high for doing it with both twist and flap deflection compared to doing it with flap deflection alone. A rigid D spar in the main element and a torsionally flexible flap has proven to be an effective way of satisfying both the structural requirements and the the aerodynamic need to twist to match the shear in the apparent wind direction. The rigid D spar and twisting flap approach was chosen for the 2010 America's Cup trimaran when the design was completely free and there were no restrictions whatever on the wingsail design. The Design Rules written for subsequent America's Cup matches were based on that experience.
Airbus worked with OTUSA in the last two America's Cup campaigns to develop pressure sensors that could be applied to a wingsail to measure the pressure distributions on both sides at several spanwise stations. However, the pressure data have not proven to be very useful as a guide for how to best trim the wingsail. For all but the lightest wind conditions, the wingsail can produce more lift than the righting moment from the hulls can stand. So maximizing the lift by itself is not a very useful approach. What matters more is minimizing the drag and getting the right balance of lift and heeling moment to maximize the performance of the whole boat.
As for the benefit of a continuous surface instead of a slotted flap, this has been explored in landyacht wingsails for several years. The single element wingsails can achieve the same top speeds as the slotted flap wingsails, but their acceleration is low due to their lower maximum lift. I've seen single element landyachts require nearly the whole first leg of a windward-leeward race course to finally get up to speed, by which time the slotted flap yachts were well down the leeward leg. For the 33rd America's Cup match, OTUSA sailed their trimaran with both a wingmast-sail combination and a rigid wingsail with slotted flap. The rigid wingsail section could match the continuous soft sail section at its best lift/drag ratio point, while also having much less drag at off-optimum conditions, as shown in the third attachment.
I'm not saying the current wingsails can't be improved. However, in order to have a better approach, one needs to address the actual deficiencies and requirements of current wingsails.
The structural demands of a wingsail should not be underestimated. OTUSA tested a twisting main element on an AC45 wing that used a tubular spar with a shell around it. The spar was much larger in comparison with the thickness of the main element than the proposed configuration and did not have any joints in it, so it was likely much stiffer. It was found the twisting main element did not perform as well as the rigid D-spar wing. The ability to twist the main element did not make up for the reduction in stiffness. ETNZ build their first AC72 wing with a twisting main element, but reverted to a rigid main element for their second wingsail. Cogito's win in the C-class Challenge Trophy has been attributed to its ability to twist the entire wingsail. That makes two examples where twist was not found to be important and one where it was, so the anecdotal case for wingsail twist is somewhat mixed.
I think the most important thing is to achieve the intended spanwise lift distribution given the operating conditions. This can be done with a combination of planform shaping, camber, and twist. At a given spanwise station, the desired lift can be obtained from a range of angles of attack and flap deflections, with the best combination being the one that results in the least profile drag for the specified lift. I've attached some MSES-generated section data for a wingsail. In the first set of data boundary layer transition is fixed near the stagnation point so the boundary layer is fully turbulent. In the second set, the boundary layer transition was free to occur at the location where the boundary layer was predicted to become unstable. This section was designed to benefit from a significant (but not extreme) amount of laminar flow, and these two cases represent pessimistic and optimistic assumptions regarding the ambient conditions. Most other sections will probably be more like the fully turbulent case. The black dot-dashed line is a locus of combinations of angle of attack (determined by wing trim and main element twist) and flap deflection that would give near optimum performance. For the free transition case, this turned out to be where the stagnation point was just to windward of the leading edge. For the fully turbulent case, it turned out to be at almost the same angle of attack over much of the lift range.
But in both cases, there wasn't much difference in profile drag for different combinations of angle of attack and flap deflection at the same lift, provided the combination wasn't wildly different from the optimum. This is why twisting the main element of a wingsail may not be worth the complication and compromise in other characteristics. Provided there is some way of getting to the right lift for the section, the payoff is not very high for doing it with both twist and flap deflection compared to doing it with flap deflection alone. A rigid D spar in the main element and a torsionally flexible flap has proven to be an effective way of satisfying both the structural requirements and the the aerodynamic need to twist to match the shear in the apparent wind direction. The rigid D spar and twisting flap approach was chosen for the 2010 America's Cup trimaran when the design was completely free and there were no restrictions whatever on the wingsail design. The Design Rules written for subsequent America's Cup matches were based on that experience.
Airbus worked with OTUSA in the last two America's Cup campaigns to develop pressure sensors that could be applied to a wingsail to measure the pressure distributions on both sides at several spanwise stations. However, the pressure data have not proven to be very useful as a guide for how to best trim the wingsail. For all but the lightest wind conditions, the wingsail can produce more lift than the righting moment from the hulls can stand. So maximizing the lift by itself is not a very useful approach. What matters more is minimizing the drag and getting the right balance of lift and heeling moment to maximize the performance of the whole boat.
As for the benefit of a continuous surface instead of a slotted flap, this has been explored in landyacht wingsails for several years. The single element wingsails can achieve the same top speeds as the slotted flap wingsails, but their acceleration is low due to their lower maximum lift. I've seen single element landyachts require nearly the whole first leg of a windward-leeward race course to finally get up to speed, by which time the slotted flap yachts were well down the leeward leg. For the 33rd America's Cup match, OTUSA sailed their trimaran with both a wingmast-sail combination and a rigid wingsail with slotted flap. The rigid wingsail section could match the continuous soft sail section at its best lift/drag ratio point, while also having much less drag at off-optimum conditions, as shown in the third attachment.
I'm not saying the current wingsails can't be improved. However, in order to have a better approach, one needs to address the actual deficiencies and requirements of current wingsails.
