Question About America's Cup Hydrofoils

Does anyone know the mechanism by which they maintained height when on foils? Unlike small sailing hydrofoil classes like the moth, which utilize a wand connected to a mechanical flap on the main lifting T-foil, a similar arrangement not possible with the big AC cats.

The main lifting foils on the AC cats are daggerboard J-foils, which have no flaps or other moving parts. A see two possibilities. One is that height is controlled by changing the angle of attack of the aft T-foil on the rudders. That seems like the easiest solution, but I think I read that this was not permitted by the rules. The second possibility is that the main J-foils are pivoted at the daggerboard trunk, and small changes in angle of attack provided the height control. It appears that ultrasonic sensors were used. https://senix.com/toughsonic-ultrasonic-sensors-in-americas-cup/. It is still not clear how the foils were adjusted to regulate lift.

Curious if anyone can explain this. Thanks.

 

AC Foil Control

The foils are adjusted in rake by the skipper which changes the angle of incidence of the main foil. The following is a description of how the foil design itself can regulate lift which was one of the breakthru's in AC34. The new boats, however, have the lift controlled directly by the skipper using much refined systems that allow quick response of the foil so that the boat is essentially flown by the skipper. The problem with this type system is that it requires the crew to grind almost 100% of the time to provide enough hydraulic power for constant, quick movement of the foil by the skipper. So part of the ongoing R & D is looking at combinations of the original uptip* foil and the new extremely quick control system. The manual(hydraulic) flight control allows use of a very low drag foil with the drawback just mentioned. Electronic control of the foil is not allowed under the rules. Sensors can provide information for the skipper, but cannot be enabled to provide hydraulic or electro-mechanical control inputs that result in foil movement under the AC rules.

*Note: the foil described as an "L" foil here was described by its inventors as an "Up-tip" foil (see below) and can allow automatic altitude control with no moving parts at the cost of some drag.

From Tom Speer:

The curved part of the vertical foil produces essentially the same lift as it rises. This is necessary to counter the side force from the sail rig, which does not change as the height changes.

Because the horizontal lift is constant but the vertical area is reduced as the boat rises, the leeway angle increases. It is the coupling of leeway with heave that is exploited by the L foil to provide vertical static stability.

The dihedral angle of the horizontal wing is set so that the angle of attack of the wing is reduced as the leeway angle increases. This satisfies the static stability condition that the vertical lift decrease as the heave increases.

Because the same horizontal lift is produced over a reduced vertical span, the sideways wash in the wake is also greater and the trailing vortices are more intense. This causes a coupling with the horizontal wing that increases the vertical lift, because the horizontal wing acts as a winglet for the vertical part of the foil (and vice versa). The dihedral angle required for vertical stability is greater than what one might expect by looking at the wing alone because it must overcome this wake-coupled influence. The result is there is a range of dihedral angles that provide positive vertical stability and a range of dihedral angles that are destabilizing in heave because of the coupling with the shed vorticity of the vertical part of the foil.

Although there are times when the foil tip has broached the surface, this is not the normal mechanism for providing heave stability in L foils. The best performance is obtained with the hull just above the wavetops and the wing submerged well below the surface. The leeway-modulated heave stability is still effective in this condition, and the induced drag is minimized.

Canting the foil inboard has the effect of increasing the dihedral angle of the wing, which enhances the heave stability. The vertical lift is spread over a greater span because the curved part of the foil is oriented to provide more vertical component of the force. This reduces the induced drag due to the vertical force. However, the induced drag of the horizontal force would be increased, so cant is typically used off the wind when the side force from the rig is less and the side force produced by the foils is correspondingly less. The foils still have to support the weight of the boat, so the vertical force is not lessened, but the relative proportions of vertical and horizontal force are changed, making the canted foil better suited to the operating condition. Cant allows the leeway-modulated heave stability to be increased an an acceptable penalty in the induced drag because of the lower side force and the higher speeds, which also reduce induced drag.

Upwind, the foils are canted to their vertical position to minimize the induced drag from the high side force and reduced speeds. The reduction in horizontal wing dihedral angle with vertical cant impacts the leeway-modulated heave stability, which is why it is much more difficult to achieve stable flight upwind than downwind. The crew had to be more active in trimming the wing and foil to deal with the reduction in natural heave stability, which was very hard on the grinders when flyng upwind.

Whether canted or upright, the mechanism for providing natural heave stability was still the coupling between heave and leeway, which led to a reduction in vertical lift because of the designed-in coupling between leeway and vertical lift by virtue of the wing dihedral. Reduction in horizontal/vertical-lifting area due to the foil tip broaching the surface was not part of this primary source of heave stability. Allowing the tip to broach the surface had big penalties in terms of induced drag and increased leeway due to insufficient vertical span.
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Uptip Foils:

Link to Part 1 and Part 2: http://www.cupinfo.com/en/featuresindex.php

Quote from the article,Part 1:

When we were working on the rule, we knew you wanted to get as much lift as possible when you were going fast downwind,” Melvin says. "For instance, in the 2010 America’s Cup, sailed on giant multihulls, the maximum amount of lift we thought we could get was about 50% of the weight of the boat. At that time, we were still relying on the hull to provide pitch control, so what’s come out of this is the boats all now have elevators (the horizontal foils on the rudders).

“At Team New Zealand, we developed a new type of foil that allows you to keep your height above the water more or less steady. No one had been able to do that before, at least not on a course-racing boat that was not going downwind. We developed that mostly on our SL33 test boats -- they came with the stock constant curvature “C” foils and with those kinds of foils, you can generate 50% boat weight lift before they get unstable. But we noticed that when we could get one boat up fully foiling for a few seconds it would really accelerate away from the other boat – and that got the wheels turning. How, with such a huge potential benefit, can we achieve stable flight downwind? So our design team came up with the “up-tip” type of boards. We refined those on the 33s and our 72 is designed to do that and fortunately it worked right of the box.”

 

Thanks for the explanation. The mechanism for allowing rake adjustment of the main foil is probably pretty complicated. Some sort of pivot mechanism on the entire trunk that the daggerboard runs through, connected to hydraulic actuators. There will need to be a wateright seal included as well.