The Wing Tack

I've had a few flatwater wing sessions recently that allowed me to work on the wing tack. I've started this the Andy Brandt-suggested way: straight from a jibe into a tack, so it's actually a 360. Here's are GPS tracks for one 360 from a couple of sessions ago: 

I was happy with that - it was dry, one of 5 dry tacks in this session. But I was pretty far from foiling through it, with my speed dropping down to 2.2 knots. For comparison, here's a tack from Nina:

She foiled cleanly through this one, keeping a minimum speed of 7 knots.

The difference? She started her tack with a speed of 15 knots. I started my jibe with a similar speed, but by the time I had turned 180 degrees and was riding toe side, I had lost 5 knots. We both lost about 8 knots speed in the tack - but since I entered the upwind turn with 5 knots less than she did, she kept enough speed (7 knots) to stay up on the foil, why my speed dropped below the stall speed of the foil.

Theoretically, I'd could pick up a lot of speed riding toeside before doing the tack, but that's rather hard; I'm always a few knots slower toeside. The alternative is to step to toeside instead of jibing into it - that's a lot faster and should be possible with less speed loss. But every time I try, my feet get stuck next to each other.

I've been looking for tutorials on how to go switch, and this is an interesting one I found: 

It's actually quite similar to what Andy Brandt teaches - 4 little steps instead of 2 big ones. Andy teaches to take the first step with the front foot going back, but that's largely because most of his students are windsurfers, who have stepped this way every time they jibed on a windsurfer. It took me a long time to break that habit! But the similarity is the breaking down of the movement into smaller steps. One thing I liked in the tutorial video was that they said that it's ok to wing a while with the feet next to each other, before taking the next step.

By change, I saw a video from a top racer today. It's Sean Herbert, who has won multiple national titles in New Zealand, and just placed third in the world in formula wing racing. Here's a series of how he switched his feet:

 

 

 

Four small steps - right, left,right, left. The wing is powered for the first step, and then progressively depowered for the next three steps. 

My tries to switch the feet were a bit different. I'd start bringing the rear foot forward a bit, and the second step would be another step forward so both feet at in the front. Then I'd either go down slowly (my trim is front foot heavy), or I'd decide the only foot I could move was the one I just move forward, and then move it back. Needless to say, neither got me to switch! Now I know what to try - the only question is if the temperatures will drop so quickly that I stop trying new things so I don't fall. I've seen the first frost warning of the season for tonight!

More Wing Jibe Progress

Here's a picture of my (partial) tracks from my last wing session:

In near-perfect conditions, I did 67 jibes in 70 minutes, and 60 of them were dry. The longest run was 17 foiled jibes in a row. I'm very happy about that - this is the best I ever jibed winging. On my good side, 33 of 34 jibes were dry. On the bad side, that number drops to 27 good jibes out of 33. This was my 297th wing session, so yes, I am a slow learner. But then, I am stubborn, too.

There's still plenty to improve. If you compare the jibes to my last post, you may notice that the speed drops more in most jibes. That's because I switched to a smaller stabilizer: the Axis speed 400/60, instead of the anhedral freeride 500/90. The smaller stab seems to be about half the size of the bigger one, and it makes the board much more sensitive to foot placement and weight changes. On my good side (port), the board touched down very briefly after the foot switch in most jibes; only about 5 were without any water contact, and some of these were wobbly. On the bad side, I somehow have a hard time convincing myself to switch the feet in the air, so many foot switches happened only after the board touched down. It usually pops right back up after the old front foot steps back, but any water contact means loosing a knot or two of speed. 

For the next flat water sessions, I'll go back to the larger stab to practice the foot switch in the air a bit more. That was a suggestion in a foot switch tutorial video that I found really helpful. Another helpful suggestion from the video was to practice foot switches on land. I added a little variation to that: I place yoga blocks under the board to feel right away if my steps are off. Here's an image that shows the setup:

In the next image, I moved my feet closer together, so that I can put a lot of weight on the back foot for carving without raising the nose of the board: 

If I step too far to the front, the nose dips down:

If the step back is too far back, the tail dips:

 

And if the step is too far towards the edge, the board dips to the side:

 

For the first few steps, I look down to see where my feet are; but after that, for the next 100 or 200 reps, the board reaction tells me if the step was good enough. This "yoga block stand foot switch exercise" helps to reduce one error I often made: making steps that are too large. 

Some jibe tutorials suggest to use foot straps, at least in front, since they make sure that your foot ends up in the right position. I've tried that, but found it to be quite the opposite of helpful. Without looking, my foot would often end up on top of the foot strap, or cleverly avoid it and step over it to land in front of it - a sure recipe for a touch down or nose dive. 

From watching jibe tutorials and lectures, I concluded that there are two ways to avoid the touchdown. Check out some Johnny Heineken videos for examples of the first approach - let's call it the "dynamic" foot switch. Before switching feet, extra pressure on the back foot pushes the nose of the board up. Then when the back foot steps forward, the weight shift "catches" the nose and prevents it from rising too much, and the old front foot steps back before the nose goes down too much.

The second school of thought on foot switches aims to minimize any pitch changes of the board. There are different variations on this: hopping to switch both feet at the same time, and quickly; starting with the feet close together, or even next to each other on both sides of the centerline; breaking the foot switch down into several mini steps; switching the feet without shifting the hips; and various combinations of these and more things. 

On flat water, the "feet close together" seems to work quite well for me, but I wonder if it really will work in chop, where I often need to widen my stance to control pitch when sailing in a straight line. Also, experts often switch feet "strap to strap", which makes for extra-wide rather than narrower steps. But the same experts can usually jibe at least 10 different ways without ever touching down. So there's definitely still a lot to try and learn!

 

A Ray of Hope

I have not posted for a long time, largely for a simple reason: little had changed. I'm winging most days now, with just an occasional day of windsurfing or windfoiling. But making progress on jibes has been frustratingly slow, if not absent. I've had the occasional day where I managed to get 80% or more dry jibes, with a lot of them mostly foiled through, but they were always interspersed with a lot more days where most of my jibes were ugly and often wet. The good days were always when the wind was reasonably strong, but not crazy, and I had some flat water to jibe on. But in the summer, we almost only foil at Kalmus, and the usual southwest wind creates plenty of chop. The off season escape to flat water in the Hyannisport Harbor is mostly closed off, too, thanks to lots of anchored boats and boat traffic.

But just in time for the annual ABK camp in Hyannis, things are looking up a bit. In this post, I'll share some of the things that helped me. Many wingers learn to jibe quickly and don't need this tips, but perhaps they can help others who also struggle to learn the jibe and foot switch.

Move the back foot! 

 The first change was that I copied what I had seen top level freestylers do at the GWA event in Fuerteventura: they often moved the back foot forward a lot before starting their jibes - close to the middle between where their feet were when foiling in a straight line. This made sense to me: to turn, you need to move body weight onto the back foot, since the front foot is on the center line of the board, or even on the other side. But if the foot remains in the back of the board, then this will also cause the nose to rise. To stop this, you need to move weight back on the front foot, but that stops the turn - something I often noticed happening. Moving the back foot forward makes it easier to keep carving without changing the pitch angle of the board.

In the next session where I did this, my dry jibe rate finally inched up above 50% again, with many of the dry jibes almost foiled through (with just a brief touchdown for the foot switch).

Use the stable gear!  

The next improvement started with a Youtube video about the jibe foot switch (which is where most of my crashes happen). It suggested to use a big foil and a big stabilizer to slow down the reactions of the board. That's what I had done the last time I made progress on the jibe, but at some point, I had switched back to the small stab, and also tend to use a smaller foil in the usual Kalmus chop.

Out came the big foil (Axis HPS 1050) and stabilizer (FR 500). With life guards finally back to school, we could launch at the Lewis Bay side of Kalmus when we got a northerly wind day, which meant flat water. I managed to switch my feet in the air three times, which was nice progress. The board set down every time after the foot switch - once only very briefly, barely loosing any speed, and twice a bit longer, but I was able to pop right back up every time. On the other side closer to shore, winds were lighter, so I practiced duck jibes. I made good progress in the wing handling, and foiled out once or twice, which I was rather happy with. Definitely an A+ session!

My luck continued with two more flat water sessions. The first one was in easterly winds at Kalmus. On my better jibe side, I switched feet in the air almost every time - still a bit clumsy, with some touching down, but at least starting to switch in the air became normal now. 

The second session was in perfect conditions in Duxbury Bay: nicely powered on 6.0, extremely flat water, no noticeable currents. That made it really easy to switch feet in the air, and I managed to get 3 or 4 jibes where the board never touched the water - yeah! I also worked on duck jibes on the other side, and was surprised how well they worked in the somewhat stronger wind (near 20 mph). I had a few good ones, including one where I foiled through cleanly, without any touchdown. Finally, I also did a clean switch jibe - no big deal since they are easy, but I almost never do them, and this was probably the best I ever did.

Encouraged by the jibe success and easy switch foiling on the smooth water, I then started to try some tacks. I have tried less than a handful in the past - typically, a try would end up in a nasty crash that dissuaded me from trying again for a while. This time, the crashes were harmless, and I made a little bit of progress, which was quite encouraging. Here's a section from the GPS track that shows some of the tack tries, and the jibes preceding them:

Note that you can barely see any speed loss in the jibes that I used to get toeside for the tack tries! The jibe numbers from that session looked a lot better than anything I had seen before:

 I kept between 70-80% of the entry speed in a bunch of the jibes, with the minimum speed not dropping below 10 knots. In my typical jibes where I touched down before the foot switch, those numbers tend to be lower, with typical minimum speeds of 6-8 knots, and only 50-60% of the entry speed kept.

Nina had a great session in Duxbury, too. I saw her try a couple of Frontside 360s (an upwind jump where you turn the board 180 degrees in the air, and then complete the full 360 degree rotation in the water). It's very similar to the Flaka in windsurfing, which she worked on for years with thousands of tries, only getting a marginal one a couple of times, with months in between. She made progress on the wing Flaka a lot quicker, and had landed her first on about a month ago. The tries I saw were "should have tries" ("Glucks" for those who know what I mean), where she completed the move but fell at the very end. But a bit later, she landed one cleanly! Needless to say, she was very happy about that, and started thinking about what she'd have to do to complete the full rotation in the air (which would make it the equivalent of an Air Flaka in windsurfing). Here's an image of Nina in one of here earlier Frontside 360 tries at Kalmus:

In the picture, the nose of the board has turned through the eye of the wind, but she crashed (partly) because she did not not rotate far enough. One interesting thing compared to the windsurfing Flaka tries is that her crashes look a lot less dramatic when winging, even though she has to jump higher to get the foil out of the water, instead of just a tiny little freestyle fin.
 

Better Foils

 Recently, my lovely winging wife almost cut the size of here foil in half: she switched from a 1250 square centimeter foil to one with 680 square centimeters. She goes out on the much smaller foil in the same conditions - when my wind meter shows 10 knots on the beach, she'll be on her 4.2. In the past, she'd switch down from her 1250 to a 725 square centimeter foil when the wind was strong and the chop/swell high, but that's a thing of the past: the new 680 foil is all she uses, in 10 or 25 knots, flat water or big swell. She's also having more fun, which sometimes is very obvious by much longer wave rides while flagging the wing.

It is pretty obvious that the new foil is a lot better - but why? The new 680 is a "high aspect" foil, but so was her old 725, which did not work nearly as well in lighter conditions, so that can't be it. But a closer look at the foils shows some large difference in the foil shape. I grabbed my contour gage and measures the foil profiles near the center of the foils. Here's what the old 1250 looks like:

And here's the new 680:

While the 1250 is a pretty standard, nearly symmetrical foil shape, the 680 looks very different: only the top section is curved, while the bottom section is very flat (easy to verify by just putting a ruler on the foil surfaces). If you often think about fluid dynamic force diagrams, you'll probably say it's obvious that the lower shape is a lot more efficient - but seriously, who thinks about fluid dynamic force diagrams?

Well, let's look at a pressure diagram for an asymetrical airfoil (from this tutorial): 

The blue section above the foil shows an area of low pressure that the foil shape generates. That low pressure basically sucks the foil upwards - it's the lift that the foil generates. Now if you would create the same image for a symmetrical foil instead, you'd also have an identical blue low-pressure area below the foil. A symmetrical foil does not create any lift if it goes through the water at a straight angle - it can only create lift if it is tilted so the front edge is higher than the rear. That's call a positive "angle of attack".  At an angle of attack of 0 degrees, a symmetrical foil only creates drag, while an asymmetrical foil creates lift.  Things change when we go to a positive angle of attack, but I get the suspicion that the "bulge" on the lower side is counter-productive even then.

Intuition is one thing - but can we find support for the theory that foils with a flat bottom are better? How about we check on http://airfoiltools.com/, a public database that has profile and drag and lift data for 1638 different foil shapes?

Let's start looking at a "classical" symmetrical foil, the NACA 0015:

First, we can check how the lift the foil generates depends on the angle of attack:

No lift at 0 degrees, and then a steady linear rise to about 10 degrees, before the lift curve flattens out. Of course, the drag also increases at higher angles of attack:

If we look at the ratio of lift to drag, we get the "glide" a foil has at different angles of attack:

This particular foil is most efficient at and angle of attach of about 8 degrees. Past this angle, the drag increases faster than the lift, and the foil becomes less efficient.

Now let's look at a asymmetrical foil for comparison. I picked a more or less random one that has a bottom that's almost flat:

It's not exactly the same as the 680, but close enough for this discussion. Here's the lift vs. angle of attack graph:

Unlike the symmetrical foil, this foil generates lift at a 0 degree angle of attack - almost 30% of the maximum lift it can generate. Here's the drag curve:

And the lift-to-drag ratio ("glide"):

The angle of maximum glide is close to 5 degrees, lower than for the symmetrical foil. But more importantly, the maximum lift-to-drag ratio is about 150. The symmetrical foil we looked at had only "glide factor" of about 78 - roughly half of what the asymmetrical foil had. Q.E.D.

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Of course, this analysis is simplifying things a lot. There are lots of things we are completely ignoring - drag from mast, fuselage, and stabilizer; the effect of water turbulences; the fact that the foil is a 3-D shape, not just a constant 2-D profile; differences in thickness and actual shape; and more. But we have seen some good theoretical evidence that "flat bottom foils" can be more efficient than more classical, symmetrical shapes. For wing foiling, we have seen a somewhat similar reduction in drag from board shapes that are longer and narrower. I believe that a few years from now, the "classical" foil and board shapes will be mostly replaced by more efficient shapes. Personally, I can't wait for my brand to go and copy the profile of my wife's new 680 foil.