How much lift force does a foil generate? I used to think the answer is simple - it must be exactly the net weight of sailor + gear. But I suspect this is too simple since it completely ignores leverage.
In the "Going Faster" videos from Sam Ross, the main thing he suggests is to put more weight into the harness by bending the knees. Playing around with this, it's easy to notice that just putting more weight in the harness is a great way to drop the nose of the board down onto the water. It seems that the "more weight into the harness" has to be coordinated with sheeting in and picking up speed. As the foil accelerates, the lift it creates increases with the square root of the speed, so putting more weight to the front is required to keep steady flight height. That's done by shifting weight from the back boot to the front foot, and from both feet to the mast foot through the harness - basically, using leverage to control the foil.

That raises the question where the fulcrum is. The logical points are the tuttle box, or perhaps somewhere near the middle of the foil mast. With my SB GT-R+ (800/95+) setup, the center of the front wing is about 12 inches from the center of the fuse-mast connection. The distance to the mast foot is about 3.5 times larger (~42 inches). Simple lever physics would say that if I could put all my weight in the harness, the upward force of the foil would have to be more than 3x my weight. Everything else being equal, that would enable about 80% higher speed than if my body weight is fully centered over the foil.

Maybe leverage can also explain the trend for race fuses to move the front wing more and more forward. That effectively increases the leverage of the foil, so it needs to generate less lift to fly, enabling higher angles or the use of smaller wings. If so, will we see front wings move even further forward?

Lift will equal weight otherwise you will breach.

What you are doing is adjusting the center of gravity, not the total weight.

An aircraft flies most efficiently with the center of gravity on the thickest part of the wing or even behind it. This is because less leverage is required from the tail, leverage=drag.

However the further back you go to less stable things get, which is why race fuselages are long to get some stability back.

This is an interesting one. If I can get some time soon to draw a diagram I will.

Not all of the leverage nor entire weight can go through the mast foot. Because the joint can't take a moment load, only the direct forces, the leverage from your weight gets counterbalanced by the forward pull of the sail.

So, how much weight you can load the harness with is related go how high and how much forward pull the sail has.

Tall sails with high center of effort get more leverage to keep the sail falling backwards, so you can get more weight in the harness. The rest will have to go where your feet are.

I'll probably sit down and do the math too. I bet this is also a reason why you want longer luff foil sails for racing, and shorter mast base to foil distance on freeride.

Another reason for longer fuse is you don't run race foils level, but tilted over, which adds lift sideways and reduces upward lift. That requires more rear foot pressure to make it equal body weight vs riding the board flat. Kind of like fighter pilots rolling sideways and pulling back hard to keep the plane from losing altitude.

I grabbed this from a reference to Jim Drake's paper. Note that the "Drive" here, or forward pull of the sail is only about 32.5 pounds. This is the force that would counteract the moment from the sailor's weight vs. the upward force from the mast base. So, somewhere likely around 14% of the sailor's weight in this diagram.

I want to draw a better breakdown of the forces between the mast and mast base but that doesn't leave a lot of capacity for leverage, and a big multiplication of the sailor's weight thru the mast base. Only a small fraction of the sailor's weight will go there. Also see how he has the lift of the planing board and the weight offset by about 13 inches. That shift in the lift is what we are talking about for foils but it needs a better diagram.

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Grantmac said..
Lift will equal weight otherwise you will breach.

Sounds logical. But it's a fact that the lift a foil generates increases with the square of the speed. As aeroegnr points out, some of this extra lift is "spilled" by dipping the windward rail and angling the foil, but that is an optional step. In the Sam Ross videos, he increases speed from 20 to 25 knots by bending the knees and loading the harness while keeping the board flat. He's also on an 1100 foil, which should have a stall speed of 12 knots or lower, when it generates about 1/4th of the lift it creates at 24 knots.

Changes in angle of attack play some role in controlling the higher lift. But to control a 4-fold increase in lift, the AOA would have to go down to 1/4th, since the lift vs. AOA curve is basically linear (up to somewhere between 6 and 10 degrees). I'd think that even the ~ 50% increase when going from 20 to 25 knots would require a visible AOA change (the nose dipping) if that was the primary mechanism to control lift.

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aeroegnr said..
Not all of the leverage nor entire weight can go through the mast foot. Because the joint can't take a moment load, only the direct forces, the leverage from your weight gets counterbalanced by the forward pull of the sail.

So, how much weight you can load the harness with is related go how high and how much forward pull the sail has.

Good point. I was wondering about that, since in slalom sailing, a fully loaded harness often goes with most of the remaining weight on the back foot. But foilers keep the sail more upright and have a higher COE in the sail, which translates to transferring more weight on the mast foot. Another example where leverage comes into play .

I think one of the things that obscures the effect of leverage in the weight location vs. lift game is that the board is moving through the water. Imagine a board that is stationary with a fixed fulcrum, which is located behind the front wing. Push up on the front wing, and put weight on the board to counteract this lift so the board is balance horizontally. If you now push up harder on the foil (increase the lift), then you'll have to move the weight forward. Perhaps a couple of Wikipedia images help:








Image by Jjw - Own work, CC BY-SA 3.0, commons.wikimedia.org/w/index.php?curid=12872799

We obviously don't have a fixed fulcrum when foiling, but we do have considerable resistance against the foil moving straight up or down in the water. If this resistance works like a "dynamic fulcrum", then it would be possible that the lift of the foil can exceed the weight in stable flight.

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boardsurfr said...

We obviously don't have a fixed fulcrum when foiling, but we do have considerable resistance against the foil moving straight up or down in the water. If this resistance works like a "dynamic fulcrum", then it is possible that the lift of the foil can exceed the weight in stable flight.

Definitely. The front wing puts out more lift than the overall weight of sailor/board/sail. BUT, a lot of that surplus is counteracted by the tailwing.

Complicating it is the heeling force (upward pull of the sail that helps support some weight). In his diagram it's only 16.5lbs. When you throw in a tilted board that is seeing some windage, that will really start to lever and crank on the board nose and can help counteract excess foil lift. The Patrik designer talked about that a bit in his post.

Lift does go up with the speed squared of the foil. But, the angle of attack changes, so you need less angle of the front wing for the same lift. I have a book here on related stability/engineering issues but I haven't gone back to refresh my memory. Static balance is what we are talking about, but design and trim aspects start getting into stability things that aren't as straightforward.

I am starting to wonder if the heavy front foot feeling at speed is more of a limit of the sail's forward pull, or a center of effort of the sail too low. But, I haven't convinced myself that it is correct. The boom height/mast foot pressure thing where it helps to drop the boom is related but I haven't been able to visualize it in my head without drawing it.

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aeroegnr said..
I grabbed this from a reference to Jim Drake's paper. Note that the "Drive" here, or forward pull of the sail is only about 32.5 pounds. This is the force that would counteract the moment from the sailor's weight vs. the upward force from the mast base. So, somewhere likely around 14% of the sailor's weight in this diagram.

I want to draw a better breakdown of the forces between the mast and mast base but that doesn't leave a lot of capacity for leverage, and a big multiplication of the sailor's weight thru the mast base. Only a small fraction of the sailor's weight will go there. Also see how he has the lift of the planing board and the weight offset by about 13 inches. That shift in the lift is what we are talking about for foils but it needs a better diagram.

I love Jim Drake's theory papers, but keep in mind he started writing those in times of the original windsurfer. I think Rick Hanke's "How fast can we go" papers are a little more up to date. In the revised version from April 2017, he puts the theoretical maximum "thrust" (which is "drive" in Jim Drake's figure) at the sailor's weight. After taking into account the need to counteract the "side force" of the sail, he ends up with a thrust that's about 40% of the body weight. In Jim Drake's diagram, that would be somewhere around 80 pounds, not 32. This is at speeds above 50 knots; at lower speeds, the thrust is higher, around 80% of the body weight.

One thing that Hanke has missed is the effect of body tension. Anyone who's ever done the "toe pressure exercise" in an ABK camp knows that you can increase power with muscle tension. If you have not done this exercise, think of strong man contests where guys pull 20-ton truck with a rope. Those trucks would not move at all if they'd just use body weight.

When windsurfing, I'd guess that I can put more than 50 % of my weight in he harness. A lot of that would be counteracted by the "heel" in Drakes diagram, though, and some by the "side force" in Hanke's diagrams. In foiling, I know that my numbers are usually quite a bit lower, but then, I am a lot slower than racers would be on the same gear.

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aeroegnr said..
Definitely. The front wing puts out more lift than the overall weight of sailor/board/sail. BUT, a lot of that surplus is counteracted by the tailwing.

Really? The tailwing list does indeed point down. But since it's on the other end of the mast, that actually pushes the board up. As wind and board speeds increase, shims usually change to flatten out the tail wing, decreasing its lift. While at the same time, the front wing lift should go up due to the higher speed.

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boardsurfr said..

aeroegnr said..
Definitely. The front wing puts out more lift than the overall weight of sailor/board/sail. BUT, a lot of that surplus is counteracted by the tailwing.

Really? The tailwing list does indeed point down. But since it's on the other end of the mast, that actually pushes the board up. As wind and board speeds increase, shims usually change to flatten out the tail wing, decreasing its lift. While at the same time, the front wing lift should go up due to the higher speed.

The front wing has to angle down to get less angle of attack for the same speed.

The tail wing will keep making negative lift but with shallower angles I think it makes it less stable. Also, the front wing throws a downwash behind it which causes an interaction with the tail wing. Then you have to get to stability theory to really understand what's going on.

If the total upward lift would exceed the weight, then Newton's laws require that this would cause an upwards acceleration, leading to a breech. So that cannot be happening.

I think in the end it does boil down to angle of attack for the front wing. It's easy to see with beginners, who tend to go very slow and have the nose of the board pointed very high. They need the higher AOA to keep foiling at the low speed. It's harder to see for faster foilers. Here's Sam Ross at 18 knots:

A few seconds later at 25 knots:
Is the nose pointing down more? I'd say it's a definitive "maybe". The board is tilted a little bit to windward, but it is such a small angle that it does not change the upward component of the lift by very much - less than 5%. The speed difference corresponds to a 90% increase in lift from the foil if nothing changes. If we assume that the front wing had an AOA of 4 degrees at 18 knots, and a linear lift vs AOA curve, then he's have to reduce the list to 2.1 degrees. Over the ~ 2 meter board length, that would mean a 7 cm drop of the nose relative to the tail - a little less if we consider the leeward tilt. That would be hard to see in the pictures.

So basically, the shifting of weight into the harness, and in general the shifting of weight forward, is to keep total foil lift constant by reducing the angle of attack. In addition, the dip of the windward rail reduces the upward component of the lift, and adds a windward component. For angles like in the picture above, the "board tilt effect" is quite small, but racers often have boards tilted much more, so this effect becomes more important.

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boardsurfr said..
If the total upward lift would exceed the weight, then Newton's laws require that this would cause an upwards acceleration, leading to a breech. So that cannot be happening.

I think in the end it does boil down to angle of attack for the front wing. It's easy to see with beginners, who tend to go very slow and have the nose of the board pointed very high. They need the higher AOA to keep foiling at the low speed. It's harder to see for faster foilers. Here's Sam Ross at 18 knots:

A few seconds later at 25 knots:
Is the nose pointing down more? I'd say it's a definitive "maybe". The board is tilted a little bit to windward, but it is such a small angle that it does not change the upward component of the lift by very much - less than 5%. The speed difference corresponds to a 90% increase in lift from the foil if nothing changes. If we assume that the front wing had an AOA of 4 degrees at 18 knots, and a linear lift vs AOA curve, then he's have to reduce the list to 2.1 degrees. Over the ~ 2 meter board length, that would mean a 7 cm drop of the nose relative to the tail - a little less if we consider the leeward tilt. That would be hard to see in the pictures.

So basically, the shifting of weight into the harness, and in general the shifting of weight forward, is to keep total foil lift constant by reducing the angle of attack. In addition, the dip of the windward rail reduces the upward component of the lift, and adds a windward component. For angles like in the picture above, the "board tilt effect" is quite small, but racers often have boards tilted much more, so this effect becomes more important.

Yeah, that small of change is hard to see but yes with the length of the board you may be seeing it. Hard to tell with the camera work and change of perspective. It's a lot more obvious at slower speeds when the board slows down so much that the nose pitches up at slow speeds right before stalling and falling back onto the water. Symmetric, straight airfoils with no sweep stall around 12-14deg or so, but with low aspect swept wings you can get much more angle before stalling.

He's on a 1300 superflyer which I'm guessing is pretty straight mid-aspect? It's more obvious with slingshot foils near stall because they can pitch more.

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boardsurfr said..

When windsurfing, I'd guess that I can put more than 50 % of my weight in he harness. A lot of that would be counteracted by the "heel" in Drakes diagram, though, and some by the "side force" in Hanke's diagrams. In foiling, I know that my numbers are usually quite a bit lower, but then, I am a lot slower than racers would be on the same gear.

I really need to break down the forces through the mast base vs. feet to help understand this better myself. In his diagram, the vertical portion of the heel is 16.5 pounds. In the corresponding lateral view, it's way more at 63.7 pounds times the cosine of 63.7, or it should balance the lateral force at 61.6 pounds. I bet most of that goes through the sailor.

I think on the water, we will feel a large force, like that 63.7 total pounds moving through our bodies, and not really know the direction. It's mostly taking us sideways, not lifting up our bodies.

But, again, I need to redraw the diagram with the components going through the mast base itself so we can see how much is really going through that vs. our feet.

The simplest way to think of it is to break it down.
Imagine having a front wing only.
This is the pivot point and to maintain a level flight the lift / force up is equal to and directly above the combined down forces ( rider / board and rig weight, and mast foot pressure.)
Any change in speed will need a change in AoA.

Now add a fuselage and rear wing.
Pivoting on the front wing any down force on the rear wing needs to be balanced by moving the rider / rig weight further forward.
Faster you go the more you reduce the front wing AoA, which unfortunately increases the rear wing AOA and the down force, which means you have to get your weight even further forward.

Summary the load on the front wing is equal to the combine rider rig/ board mast weight plus the rear wing down force (which increases with speed)

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aeroegnr said..

boardsurfr said...

We obviously don't have a fixed fulcrum when foiling, but we do have considerable resistance against the foil moving straight up or down in the water. If this resistance works like a "dynamic fulcrum", then it is possible that the lift of the foil can exceed the weight in stable flight.

Definitely. The front wing puts out more lift than the overall weight of sailor/board/sail. BUT, a lot of that surplus is counteracted by the tailwing.

Complicating it is the heeling force (upward pull of the sail that helps support some weight). In his diagram it's only 16.5lbs. When you throw in a tilted board that is seeing some windage, that will really start to lever and crank on the board nose and can help counteract excess foil lift. The Patrik designer talked about that a bit in his post.

Lift does go up with the speed squared of the foil. But, the angle of attack changes, so you need less angle of the front wing for the same lift. I have a book here on related stability/engineering issues but I haven't gone back to refresh my memory. Static balance is what we are talking about, but design and trim aspects start getting into stability things that aren't as straightforward.

I am starting to wonder if the heavy front foot feeling at speed is more of a limit of the sail's forward pull, or a center of effort of the sail too low. But, I haven't convinced myself that it is correct. The boom height/mast foot pressure thing where it helps to drop the boom is related but I haven't been able to visualize it in my head without drawing it.

Please draw it! The free body diagram is the way and the light! Your the guy for the job areo!

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DarrylG said..
The simplest way to think of it is to break it down.
Imagine having a front wing only.
This is the pivot point and to maintain a level flight the lift / force up is equal to and directly above the combined down forces ( rider / board and rig weight, and mast foot pressure.)
Any change in speed will need a change in AoA.

Now add a fuselage and rear wing.
Pivoting on the front wing any down force on the rear wing needs to be balanced by moving the rider / rig weight further forward.
Faster you go the more you reduce the front wing AoA, which unfortunately increases the rear wing AOA and the down force, which means you have to get your weight even further forward.

Summary the load on the front wing is equal to the combine rider rig/ board mast weight plus the rear wing down force (which increases with speed)

Thanks DarlylG, that is interesting. This might explain why I hate a shimmed stab. Any one try stabless sailing yet? Phillipe @ Horue has it in his videos from forever ago.
It is the sum of the moments not the sum of the weights that balances around the hard to define (in this case) center of rotation.

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utcminusfour said..

aeroegnr said..

boardsurfr said...

We obviously don't have a fixed fulcrum when foiling, but we do have considerable resistance against the foil moving straight up or down in the water. If this resistance works like a "dynamic fulcrum", then it is possible that the lift of the foil can exceed the weight in stable flight.

Definitely. The front wing puts out more lift than the overall weight of sailor/board/sail. BUT, a lot of that surplus is counteracted by the tailwing.

Complicating it is the heeling force (upward pull of the sail that helps support some weight). In his diagram it's only 16.5lbs. When you throw in a tilted board that is seeing some windage, that will really start to lever and crank on the board nose and can help counteract excess foil lift. The Patrik designer talked about that a bit in his post.

Lift does go up with the speed squared of the foil. But, the angle of attack changes, so you need less angle of the front wing for the same lift. I have a book here on related stability/engineering issues but I haven't gone back to refresh my memory. Static balance is what we are talking about, but design and trim aspects start getting into stability things that aren't as straightforward.

I am starting to wonder if the heavy front foot feeling at speed is more of a limit of the sail's forward pull, or a center of effort of the sail too low. But, I haven't convinced myself that it is correct. The boom height/mast foot pressure thing where it helps to drop the boom is related but I haven't been able to visualize it in my head without drawing it.

Please draw it! The free body diagram is the way and the light! Your the guy for the job areo!

I broke down drake's diagram into board and sail forces and got about 75lbs vertical through the mast foot. I'm not convinced that it is right yet. I'll share a diagram and then start changing it for foiling.

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DarrylG said..
....

Now add a fuselage and rear wing.
Pivoting on the front wing any down force on the rear wing needs to be balanced by moving the rider / rig weight further forward.
Faster you go the more you reduce the front wing AoA, which unfortunately increases the rear wing AOA and the down force, which means you have to get your weight even further forward.

Summary the load on the front wing is equal to the combine rider rig/ board mast weight plus the rear wing down force (which increases with speed)

....

I think you're missing the effect of drag. Apart from adding stability, the stabiliser's other job is to counteract the turning moment created by the drag on the foil and the forward thrust from the sail above it. If you have the correct stabiliser size and angle then the increased downforce at higher speeds will balance out the tendency of the nose to be pulled down by foil drag & sail thrust.

A foil setup that becomes much more front footed or rear footed at speed is caused by not having the stabiliser setup correctly. Most fuselages have a baked in 1 - 3 degree angle for the stabiliser. That may need adjusting with shims depending on your weight, stabiliser size and front wing selection. Lighter riders on the front same wing use a lower angle of attack as they don't need as much lift. That means less drag on the front wing and less stabiliser downforce required.

With a correctly tuned stabiliser you'll notice the foil becomes more rear footed if you pick up even a tiny piece of seaweed. That's caused by the extra drag not being balanced by the stabiliser downforce.

I played around with Drake's figures a bit. I think there's still a lot to do before I can get front/back foot loading on a foil. i just wanted think about if what I did was sensible.

I only looked at forces/moments in one plane. Here's what I did.

Copied Drake's diagram:


I broke out the board separately and added mast weight/mast drive, boom weight, and boom drive. I assumed the mast foot interface point is inline with the drag (which is wrong, but, good enough for a first run), and assumed 20in distance from mast foot to the center of the person's weight:





Then I took the sail portion with the same opposite forces:





Because I didn't draw a free body for the person himself and I didn't put a moment at his feet, the boom drive, or forward drive from the boom, has to be zero. So, only the boom weight and mast weights are unknowns, given the other forces (like 233lbs total weight, 216.5 lift on the board) are identical to the Drake diagrams.

Doing all of that, I get the mast weight is 82.3lbs. The boom weight is 101.8lbs, and the foot weight is 131lbs. 47% of the total weight goes through the boom, the rest of the majority goes through the sailor's feet, with some relief from upward pull of heel.

I need to break out the equations and diagrams in the other dimensions because the boom drive is going to be off to the side, and has to be balanced with the sail pull and differential on the sailor's legs. I also think that a lot of the felt "front foot" or heavy front foot bias is from lateral forces, which aren't in this diagram. So, this doesn't tell the whole story, especially since I haven't even taken into account foil particulars yet...but I wanted a good starting point with a known source.