ETNZ's New Wing: the Banana foil! 

That’s clever. I hadn’t thought of that

That’s very clever thinking. Another option is they’ve come up with some clever aerolastic warping design that’ll allow operation of a curved flap surface

No quite. Wing fences try to do that. Winglets try to take energy back from the vortex rather than stop it.

Yes, they do all of that, but aerodynamically and structurally it is ALWAYS better to do the same thing with simply the same amount of extra horizontal span instead of winglets. It is only external constraints (class rules, airport terminal sizes, etc) that ever make winglets the right option.

@@BruceHoult interesting - thanks for your response

@@BruceHoult As far as i know winglets are built in order to separate the low air pressure area above the wing from the high air pressure area below it. This prevents the air below the the wing from following the pressure gradient by flowing sideways around the tip of the wing, which would result in spiral shaped turbulence as the wing moves forward through the air during the flight/movement. This turbulence has to get it's kinetic energy from somewhere, which can be noticed as additional drag on the wing. I can't see how to achieve this effect by extending the wing horizontally, which would just move the problem further outward, but the drag would still be the same. Did I get anything wrong?

@@noel913 yes, you did. It is only the total length of the wing that counts for stopping the fluid going around the end. The air/water does not care whether the wing is straight or bends up or down. In fact bends create additional drag at the bend itself. All winglets do is "move the problem further outward", except part of the "out" might be "up".

@@BruceHoult no a winglet doesn't just move the problem up instead of outward, since there is no horizontal air pressure gradient, so there's no force "pulling the air inwards" such as there is a gradient between above and below the wing. Why would an extension of the wing length solve the problem, if at the new wing tip would still be the difference in air pressure above and below the wing. As far as I can see this, the air doesn't care if it's 10 or 15 metres distant from the hull. It just follows the gradient and if it does so by flowing around the tip "sideways", it creates a vortex. I'm curious for further explanation.

Lol

Not 100% sure but I think the rules dictate that the flaps have to be adjusted together so you can't use the outboard part of the foil more than the inboard part. Since the foils are symmetric, the foil's centre of lift is in line with the strut (and the lift vector is parallel to the strut) regardless of whether or not it's anhedral. T foils actually produce a bit more righting moment for the reasons mozzy gave.

@@robertohaubold752 The centre of the vertical component of the lift, ie the part that counteracts the effect of gravity on the mass of the yacht does not act through the central support however. It moves in any system with anhedral or dihedral. This means that with anhedral, as the boat leans to leeward, the angle of attack of the wing must be increased to remain at the same height above the water and the centre of lift moves closer to the yacht centreline REDUCING righting moment exactly when you would want it to INCREASE.

Flap differential was one of the mysteries of the previous cup. I've never heard any on the flight controllers talk about it in the off boat communications or interviews. It's a bit of a mind exercise to u derstnad how it would help and when one would use flap differential

Generally the shallower the angle with which a foil pierces the water, the more the ventilation, so I would expect a curve like this to make ventilation worse, not better, at least for the case where the tip of the foil just peaks above the water

@@Dovorans i hear you but there are already a lot of foils out in the wild that are actively surviving a partial vent such as Imoca, Kite foiling rigs and some Surfing foils like Armstrong and Axis that i know of , i think that element of the tech is already there for the taking

@@CarkeekW the AC75, and for that matter, the AC40 also partially vent. You'll frequently see the outboard tip a couple inches out of the water. The point stands that a steeper angle is better if you want the flow to reattach because the thinner the film of water over the top of the wing the easier it is for the leading edge to throw that film up into the air.

@@weatheranddarkness it's going to be an interesting few months 😉

Cheers!

More like the other way around lol 😂

I thought that this was more of an A-shape than a V-shape so you'd expect reduced stability. With an A shape, as the boat heels right (relative to the foil), the left side of the foil becomes closer to horizontal and makes a greater proportion of its lift in the vertical direction.

No worries!

yeah, the more you delve in to the reasons for anhedral in aircraft, the more the analogies to these boats fall down. The side slip I think is an interesting point, as we don't really know what the AoA is on AC 'wings' / foils. In a yacht / dinghy the keel / centreboard will see a fairly narrow angle of attack which is also called leeway. It may be that these fall are always experiencing some side slip, or leeway. Or it may be that their cant angle means the angle of attack contains no side slip. I think one of the big reasons for the pitot tube on the INEOS test boat is to answer accurately some of these questions about flow velocity, but also direction

Thanks Robert!

Thanks, I try my best, not always the most 'pro' but hopefully conveys the message

It certainly is. Awesome to get this amount of access to the full design process and watch it all unfold

Winglets are only ever a good idea under certain circumstances even in aviation. Essentially the higher percentage of time spent in the cruise at high altitude the better they are. This video explains quite well in the “disadvantages” section. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-zWjrEGkpqiE.html

Not just off the hull…..but on the actual foil arm.

I guess I should’ve finished watching the video before asking questions. I need to rewatch those older videos. I still want some underwater footage.

@@zlm001 the teams have it internally. Who knows if it'll ever be public.

I've heard that not only are they having new rudders, but the F50's will start using T foils rather than L foils for their main lifting surface. This will make them a lot closer to the AC75s that we see in the cut in terms of foil technology

I don't think you can add them here. But you could post on Sailing Anarchy maybe? Or host them on a image sharing site like Flickr and share a link here?

@@MozzySails docs.google.com/document/d/1-knAqvitg3ir0azH_4yjYWkOA-6_W8TpvXIjbgmr0zI/edit

I think there will be... should I pop up a video on the implications?

@@MozzySailsWhat is adequately robust in one design classes generally? Does INEOS simply wear the loss of time for the retrofit and cross their fingers that the class design team got their calculations right this time?

Oops

if they are being towed, they use both foils. But if taking off under sail, then they need to raise the windward foil to get the righting moment to pull the sails in and build speed.

I suspect the drag may be prohibitive too for the sail power to overcome.

It’s an interesting train of thought. Given the yacht weight is fixed and stipulated we know one foil produces a corresponding amount of lift most of the time. How then does the boat remain at constant fly height when the second foil enters the water? The foil flaps have to provide a means to halve the lift of each foil.

@@robingimblett2171 The Angle of attack of the foils are variable. They have to be to provide a varying amount of lift as the speed changes otherwise they would launch you airborne as you accelerate.

A basic calculation of drag will take the drag coefficient for any given section shape and multiply by the frontal area. We don't know the section shapes, but they will be around 0.04 for foil shape. The one thing we can more accurately comment on is the frontal area, both in terms of the length of appendage they have the in water (both the wing and the strut / arm) and the thickness. All else being similar (thickness and section shape) then a Y foil will have less frontal area as you need less vertical struct / foil arm.

Yes, that's basically it. As the you start to roll, the centre of lift relative to your centre of gravity moves to high wing and thus you roll further / quicker. But, on a AC75 the lif is already way out to one side of where the CoG is, so effect is very minor... the boat is already trying to roll to windward, that's what the driving force of the sail is balanced against.

think you might be right on both counts

The AC40 comes supplied with two one design foils. If teams convert it to an LEQ12 test boat they can put a further two foils on

@@MozzySails and once the ac75 is built? 3 sets?

Overall yacht weight is significantly less for AC37.

The CoG is important. The dihedral only bring the plane level if the CoG is within the fuselage. AoA on the lower wing increase and levels the aircraft. However, the CoG of a foiling monohull is several meters away from the 'fuselage' or bulb. The centre of lift is already massively offset from the CoG: to create leverage for the power of the sails. The very slight balance of lift between the two halfs of the foil wing is immaterial to the overall balance / stability: hence it's not a good analogy.

@@MozzySails No, as long as the CoG is offset from the centre of the the vertical component of the lift produced, then there is a couple formed and a moment arm. The CoG could be well outside the fuselage and this would still have an effect. For example, an Airbus can fly happily with one wing full of fuel and one wing empty. This brings the centre of gravity outside the fuselage. With a relatively small anhedral on these foils, the effect won’t be huge, but it WILL still exist, and will be unwanted. In the case of these boats, the fuselage size once foil borne is an irrelevance since it is not in the water. However, the CoG of these boats is definitely well INSIDE the fuselage. There is almost no weight off to the sides. I strongly suspect that the anhedral disappears once foil borne in the same way that a 787 with carbon wings the wings bend upon dramatically on takeoff. Deliberately allowing wing bend through flexibility smooths the ride in an airliner and may very well be the desired effect with these foils. Rigid wings provide a very hard ride which might make control more difficult. You can feel the difference if you compare an Embraer to a 787. I very much respect your knowledge of sailing, since there I am an amateur, but equally, when it comes to wings, lift, CofG, aviation etc I’m afraid you are in my area of expertise.😂

@@21142317 maybe you can help me then! I thought the stability from dihedral comes from the sideslip when an aircraft rolls. The reason I say the CoG is important was because I thought it was the position of the CoG relative to the lift that causes this side slip. But will these wings sideslip when they roll? Unlike a plane they are not only holding the mass of the boat up, but they are also preventing leeway so I am unsure how the AoA on each of the wing sections (either side of foil arm) changes as the boat rolls). But there are two things I am sure of which helped me reach my conclusion: 1) The CoG of the yacht is about 6m from the wing. So a shift in the CoL within the wing of 10's of cm is unlikely to dramatically off balance the yacht. 2) The boats do roll the wings to initiate a turn in the same way an aircraft rolls to bank in to a turn. So even if I am wrong on the above, I don't see how it would be linked to increased manoeuvrability.

@@MozzySails Let me first say that I agree with your conclusion that anhedral is not helpful for manoeuvrability in a boat in the same way that it is in an aircraft. My argument is purely about your reasoning. In an agile aircraft, you want instability to enable a fast roll rate because you don’t turn until you have rolled to the correct angle. Lift perpendicular to the wings creates the turn towards the centre of the circle, not yaw, so once at the correct angle of bank for the turn, the instability is of no benefit. Anything that slows this roll reduces your agility. In a foiling boat the turn is more like yawing an aircraft in a skidding manner, so roll rate is an irrelevance. Modern fighters are so unstable that humans are utterly unable to fly them without a computer controlling the instability. Essentially, we mitigate the extra difficulty of flying the aircraft to gain the roll rate. Foiling yachts have the greatly added complexity that they have wings operating in two mediums at once, and those media are not both impinging on their respective foils from the same direction even in steady state straight line travel. The water foil sees the water from basically straight ahead and the wing sees the air from a variety of angles depending on what you are doing. So we have established that anhedral is not helpful for turning the boat, so is it still a meaningful penalty for stability? I think it is. 10cm (I’ll use your number since I’d only be guessing 🤷‍♂️) shift inboard of the centre of lift due to leaning to leeward might not seem like a lot, but it is dynamically unstable. By this I mean that the more you lean the more the righting moment decreases which tends to produce more lean… Added to this is the uniquely complex couple that happens when a foiling monohull leans, ie as you lean the bow lifts which increases angle of attack of the foils which creates more lift which raises the bow etc etc. Very small perturbations greatly increase the difficulty of maintaining a stable platform under these dynamically unstable conditions. An aircraft has it very easy. As long as the thrust balances the drag, and the lift balances the weight and all vectors pass through the the centre of mass or balance each other out, you fly in a straight line. That centre of mass is almost always well inside the fuselage. In one of these boats, the forces that need balancing are created from widely separated sources well away from the centre of mass inside the hull. Centre of thrust from somewhere up the wingsail or alternatively from somewhere underwater on the foil depending on what you choose to use as your frame of reference. (The energy differential between the water and the air that drives sailing makes either reference valid) Drag again is both below and well off to the side. Lift is off to the leeward side, plus acting at an effective bank angle at all times. This is far more like a twin engined airliner after a single engine failure. We lean the aircraft towards the good engine and use the rudder to bring all the forces together through the centre of mass. My point is that whilst you mention the 6m arm of the foil, I’m not sure that the 6m distance is the valid one. Remember that the vector of the lift produced does not go straight up. It is perpendicular to the wing so passes much closer to the centre of mass than 6m. The total lift from the foil including the vertical member is angled in towards the hull providing a force rotating the boat to windward. The total weight of the yacht acts through the centre of mass and acts straight down (Unless turning). This centre of mass is almost certainly somewhere behind the front foils attempting to pitch the nose up. The foil on the rudder pulls down trying to rotate the bow up and the rudder provides a force to leeward turning the bow to windward. The drag pulls backwards from under the water trying to rotate the bow down and also off to the side attempting to turn the boat to leeward. The wingsail pulls forward from somewhere up the sail trying to rotate the bow down. All these forces have to act through the centre of mass and/or be balanced if the boat is to continue without deviating. This is a very long winded way of saying that anhedral is bad from a stability point of view, even if the effect is seemingly small. I strongly suspect that the anhedral curve unbends under pressure to be straight or slightly curved up. Flexibility also adds damping in turbulence which can be very helpful. The 787 is specifically designed with soft wings that flex up to 20ft at the tips in flight to smooth out turbulence. This also potentially helps if you want to have the tip cut the surface without aeration which has multiple benefits including enhanced dynamic stability.

@@MozzySails Not sure I understand what you are saying about the boats rolling the wings to initiate a turn. As I see it the opposite happens. The windward foil is lowered and BOTH are lowered further than the normal position to make the foils as horizontal as possible and the supporting foil becomes vertical thus REDUCING the turning moment from each foil (to stop them wastefully fighting each other) at which point the vertical member acts as a fin to do provide the actual force for turning?

You can find the class rule here: drive.google.com/file/d/1OeQF7A0-u7iU99W9hBxOrZmU_E2-gwfA/view?usp=drive_web Look to section ten for the foil box dimensions

@@MozzySails thank you, working on it

nice video, thank you for sharing