Boats and foils comparison

kai_

New member
G'day Kai, welcome to the forum.

Interesting idea. Since under the rules they are required to have one flap on each of the two wings of the foil, how could the flaps be hinged inboard - ie at the foil centre line?
Not too sure about that one. Perhaps that could be why their little flap joiner is considered a part of the control system, as it could be connected to a linkage in the main wing that allows the middle of the flap to move backward and forward instead of rotating around a fixed point that lies within the wing somewhere, if that is even allowed within the rules. I just though that would be the simplest solution to being able to deflect a curved flap around a straight line.

 

Zaal

Anarchist
515
698
Italy
The direction LR (and INEOS) took is very different from the ETNZ philosophy. AM was kind of in between, but closer to ETNZ. The way I think about the anhedral foils is that imagine a foil with 90 deg anhedral. One wing half completely horizontal, the other completely vertical. With a foil like that, you could completely separate the actions of your flaps. The flap on the vertical wing half would be used to adjust lateral lift (leeway / yaw angle), whereas the flap on the horizontal wing half would be used solely for height / pitch control. If you wanted to go into a high mode, you would only deflect the flap on the vertical wing half while keeping the other flap angle the same => differential flap setting.

Of course a 90 deg anhedral foil would not fit in the class rules, and would probably be very draggy and slow as well, but I think LR's anhedral foils operate in a similar, although more complex way, since the vertical and lateral force components are not completely separated. Still, they have the ability to quickly manipulate the ratio of vertical vs horizontal foil forces with differential flap deflections.

ETNZ don't have this option. They don't have any anhedral (or very little), so even if they deflected the flaps differently, the lift from the two wing halves would point in the same direction. Plus, it seems they can't even deflect them differently, since they only have one actuator. They still have other options to control the vertical-horizontal force ratio. They can change the cant angle (relatively slow, and if you mess with the FCS too much, it may cause problems),and change heel angle - which basically changes the cant angle of the foil.

The advantage of ETNZ's approach is that since the load is symmetric between the two wing halves, they can get away with a smaller foil. On LR, differential flap settings mean one wing half is carrying more than the other, so I assume it has to have a larger surface area as well. Since the foil has to be symmetric, it means both wing halves are larger than if they were carrying equal loads. Furthermore, the asymmetric load on the wing halves probably creates a big vortex behind the foil with higher drag.  Maybe there are structural issues as well. If the sum of foil forces is not lined up with the foil arm, they may need beefier wings and thicker lower foil arm sections to withstand the twisting forces. (Counter argument: what happens when TR sticks the tip of the outside wing out of the water?)

On the other hand, LR can adapt quicker and has more control in maneuvers. If they can tack more efficiently, they may be able to compensate for their lower top speed/vmg. I expect their game plan is to win the start with a more maneuverable boat, then force ETNZ into a tacking duel so TR can never fully gain from its higher top speed. The problem with this plan is that once they fall behind, it's going to be very difficult to get back. 
Thank you, great explanation ! 

 

Zaal

Anarchist
515
698
Italy
I have a theory about how ENTZ could be deflecting their foil, even with the anhedral. Instead of putting the axis of rotation parallel to the foils, they could put the axis of rotation as a line between the two foil tips (I attempted to draw this below). This would be more difficult to package, but would allow the flap to deflect freely while also giving them the added benefit of extra foil area in the middle of the foil when the flap is deflected, and eliminating the need to rely on bend as the flap is free to move at all times. This would also still be allowed in the rules, as no part of the flap is changing its axis of rotation as the flap is deflected, and could be accomplished by attaching a hydraulic ram to their little center "control" joining piece between the two flaps. Excuse the crude drawing and bad handwriting, I hope it is clear enough.

Also, this is my first post! I'm an aerospace engineering student and have been racing since I was 11 (9 years now), and I hope to one day be able to work on AC tech and engineering.

View attachment 431907
Welcome mate ! 

 

MaxHugen

Super Anarchist
Not too sure about that one. Perhaps that could be why their little flap joiner is considered a part of the control system, as it could be connected to a linkage in the main wing that allows the middle of the flap to move backward and forward instead of rotating around a fixed point that lies within the wing somewhere, if that is even allowed within the rules. I just though that would be the simplest solution to being able to deflect a curved flap around a straight line.
I see your point. Your suggestion could still be used - and stay within the Rules - if considered from the centre line (or the bulb) to the outer end of the foil wing prior to the turned up tip...

 

mauriciogfj

Member
228
122
Brazil
I have a theory about how ENTZ could be deflecting their foil, even with the anhedral. Instead of putting the axis of rotation parallel to the foils, they could put the axis of rotation as a line between the two foil tips (I attempted to draw this below). This would be more difficult to package, but would allow the flap to deflect freely while also giving them the added benefit of extra foil area in the middle of the foil when the flap is deflected, and eliminating the need to rely on bend as the flap is free to move at all times. This would also still be allowed in the rules, as no part of the flap is changing its axis of rotation as the flap is deflected, and could be accomplished by attaching a hydraulic ram to their little center "control" joining piece between the two flaps. Excuse the crude drawing and bad handwriting, I hope it is clear enough.

Also, this is my first post! I'm an aerospace engineering student and have been racing since I was 11 (9 years now), and I hope to one day be able to work on AC tech and engineering.
Welcome to the fray @kay_

 

kai_

New member
I see your point. Your suggestion could still be used - and stay within the Rules - if considered from the centre line (or the bulb) to the outer end of the foil wing prior to the turned up tip...
I think I'd need to do some rule reading to see if this would be possible under the rules or not, but I think it would clear up some of the confusion about the interpretation where the join between Emirate's flaps counts as a control system and not as a piece of the flaps themselves.

 

The Advocate

Super Anarchist
The direction LR (and INEOS) took is very different from the ETNZ philosophy. AM was kind of in between, but closer to ETNZ. The way I think about the anhedral foils is that imagine a foil with 90 deg anhedral. One wing half completely horizontal, the other completely vertical. With a foil like that, you could completely separate the actions of your flaps. The flap on the vertical wing half would be used to adjust lateral lift (leeway / yaw angle), whereas the flap on the horizontal wing half would be used solely for height / pitch control. If you wanted to go into a high mode, you would only deflect the flap on the vertical wing half while keeping the other flap angle the same => differential flap setting.

Of course a 90 deg anhedral foil would not fit in the class rules, and would probably be very draggy and slow as well, but I think LR's anhedral foils operate in a similar, although more complex way, since the vertical and lateral force components are not completely separated. Still, they have the ability to quickly manipulate the ratio of vertical vs horizontal foil forces with differential flap deflections.

ETNZ don't have this option. They don't have any anhedral (or very little), so even if they deflected the flaps differently, the lift from the two wing halves would point in the same direction. Plus, it seems they can't even deflect them differently, since they only have one actuator. They still have other options to control the vertical-horizontal force ratio. They can change the cant angle (relatively slow, and if you mess with the FCS too much, it may cause problems),and change heel angle - which basically changes the cant angle of the foil.

The advantage of ETNZ's approach is that since the load is symmetric between the two wing halves, they can get away with a smaller foil. On LR, differential flap settings mean one wing half is carrying more than the other, so I assume it has to have a larger surface area as well. Since the foil has to be symmetric, it means both wing halves are larger than if they were carrying equal loads. Furthermore, the asymmetric load on the wing halves probably creates a big vortex behind the foil with higher drag.  Maybe there are structural issues as well. If the sum of foil forces is not lined up with the foil arm, they may need beefier wings and thicker lower foil arm sections to withstand the twisting forces. (Counter argument: what happens when TR sticks the tip of the outside wing out of the water?)

On the other hand, LR can adapt quicker and has more control in maneuvers. If they can tack more efficiently, they may be able to compensate for their lower top speed/vmg. I expect their game plan is to win the start with a more maneuverable boat, then force ETNZ into a tacking duel so TR can never fully gain from its higher top speed. The problem with this plan is that once they fall behind, it's going to be very difficult to get back. 
I think your explanation has gone a long way towards explaining why LR appear to be more stable in flight.

 

dorox

Member
138
217
I've done something pretty cool: synchronised video feeds with boat data and GPS tracks. Should keep you guys busy while the racing keeps getting delayed.

P.S. if VirutalEYE people are reading this: Lets see how long it takes for you to copy that, too!

https://ac36.herokuapp.com/map

image.png

 

RobG

Super Anarchist
2,874
749
The anhedral vs flat discussion was done to death maybe 12 months ago, I've forgotten which thread. There are pros and cons to each design, it's not a case of one is better than the other, it's how the whole package works that matters.

For anhedral, once you cant the foil, biasing the lift to the inboard foil gives more lift to windward, but reduces RM because the centre of lift moves inboard. With a flat foil, you can bias the lift to the outboard wing and get less lift to windward but more RM as the centre of lift move outboard. So it depends on what you have designed the rest of the boat to work with.

The flat foil is inherently more efficient as there is only one lift vector, not two fighting each other. It also might mean a simpler control mechanism, if that's worth anything.

ETNZ's flat foils means it thinks outright speed is the key and it can make up for the loss of VMG from the foil in other areas, such as main sail trickery. LR's anhedral means it thinks VMG is the priority and it can make up for the drag penalty elsewhere, e.g. the minimalist slippery hull shape.

 
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