Oracle Team USA in flight
Yes, the racing, the athletic sailors sprinting across flying platform, and the capsizes are fascinating for the casual America’s Cup fan, but the true fascination is embedded deep in the science forged by each team’s technology partners, of which Oracle Racing Team is flush, including BMW and aeronautics giant Airbus. On the morning of the first races of the Louis Vuitton Cup World Series Chicago, Airbus’ Executive Vice President Engineering Charles Champion and Oracle Racing Team’s Ian Burns delved into the tools the two teams, as well as SoftBank Team Japan are using to develop their AC50s for next year’s match. Here’s what we learned from Oracle’s veteran director of performance.
About foil control systems: We have up to 100,000 lines of code to program the control systems. It’s a level of complexity that is one of the areas of the competition that we’ve been given free range within certain parameters on automation.
About stable flight: It’s a real challenge because we’re limited by the length of the daggerboard and rudders to 2 meters. Stability is slow. The more stable the foil package is the more energy that’s slowing the boat down, so having a good control system and having a fast control system and a smarter control system as we can get is important.
About structural components: If you look at the wings of the Airbus A350 and the shape of the daggerboards, there are parallels. We have similar composite problems in making it strong enough to withstand the loads. The whole weight of the boat is sitting on the foil, and in some cases only a small part of it, so it needs to be strong but made as thin as it possibly can.
About foil efficiency: The easiest solution is usually the slowest and the highest drag solution. The new thing for us is we go to speed fast enough to boil the water on the daggerboard and rudder, which is cavitation. The easiest way to reduce that is to make the foil thinner. The engineers want to make it thicker, so it’s a balance. A 10-percent reduction in the thickness of the foil, which is a large reduction in strength, can give you 1 to 3 knots boost in top speed before you cavitate. On the first reach that difference will get you to the mark in front. The margins are small, but you don’t need much to get an edge, a cross or an overlap that will make a difference.
About using Airbus facilities for structural load testing of the foils: We had a beautiful look at the board breaking, and used tomography, which is a process that Airbus owns a machine, which allows us to do this. You can actually see inside the object and trace the failure right to the source, which is a unique opportunity.
About hydraulic efficiency: We brought the allowable pressure in the rule down to 5,000 psi to try and get the relationship of automotive and aeronautical companies that are using the same type of equipment. We used to be at 10,000 psi and very few companies and parts are made at that level. It’s very expensive for small gains in performance.
The guys who grind put out about 300 to 400 watts continuously and they do it for 30 minutes. It’s about half a horsepower, so ultimately we have very little power, which makes it a game of efficiency; how you make it from the handle right to the actual thing you’re moving as efficient as possible.
The actual speed and efficiency of the system, where Jimmy Spithill sees something like the bows coming down or coming up he has a reaction time of .2, .3 of a second, which is fast, but the time from then—when he realizes a change needs to happen—to when the actual daggerboard gets into position has a large bearing on how far you have to move it and also whether the boat will fly stable. We have two Airbus engineers working in our department and their main focus is measuring what our current system is and reducing the delivery time. We started at about half a second. We’ve halved that time and we hope to halve it again before the America’s cup.
About Computational Fluid Dynamics: On these high-efficiency boats we’re at about 30 times the power to weight ratio of the old boats. The limiting thing on how fast you can go is the lift-to-drag ratio of the bits below and above the water. Those two ratios determine how close to the wind and how fast you go. The only thing we can do is to reduce drag. On these boats, it’s all about the drag and the drag prediction models we use with Airbus are 10 times as large and take 10 times as long to run so they are a huge step in accuracy.
About daggerboard structural analysis: The boards move a lot under load. If you see them under the water they move as much as 1 to 1.5 feet. Some of our thinner boards move as much as 2 feet while we’re sailing. They flex a lot, and that means the mold shape we analyzed in the mold is different than what we’re sailing with so the fluid structural interaction program—which is a big deal in airplanes. The first thing is we get a realistic model of what we’re sailing with and the second thing is taking advantage of that and make [the foil] better when it’s deformed. That’s a difficult science. In this flexural world there are opportunities.
About Micro-Electro-Mechanical Systems (MEMs): With five or six of the [aerodynamic pressure sensor] stations placed up and down the wing, we end up with a perfect map of the pressure around the wing. The guys go sailing and come in with 350 pressure-channel data records of temperature and pressure accurately calibrated.
About why the boats keep a bow-down angle of attack: It’s a righting-moment advantage to get the weight forward and unload the rudders. There’s a slight aerodynamic benefit, perhaps, but it’s mainly about righting moment and rocking the boat over to windward. You see the guys trying to run with the windward bow just inches above the water. When the wing is over the windward hull you can pull it in tighter and get more force from it. Any percent or two of righting moment is a big difference.