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.

Wing Jibing Progress

 It's been 4 months since my last post about wing jibes, so it's time for a progress report. In this time, I have winged about 30 times, with more than half of these sessions in Florida last months. One thing that briefly set me back a bit was switching from Starboard foils to Axis foils. My new HPS 1050 front wing has a lot more glide than the Starboard Supercruiser and Wave foils, so for a few sessions, I was constantly overfoiling in the middle of jibes. On the less efficient Starboard foils, turning of power during the jibe required shifting more and more weight to the back - something I had apparently automated well. On the higher aspect Axis foil, the weight shift was not necessary at all, so when I did shift my weight unconsciously, the foil climbed up out of the water. It took a few sessions to break the old habits, and stop overfoiling in jibes.

Several things happened after that which helped me to finally make more rapid progress. Before I go into these, let me show you some GPS tracks from the first hour of a session I had 2 days ago:

Check out the speed graph at the bottom: it never drops down to zero, which means I never crashed. That's 29 dry jibes in a row:

In about 1/3rd of the jibes, my speed never dropped below 5 knots, meaning they included just a brief touchdown, after which I was able to pop right back up on the foil. This may not be impressive compared to good wingers, but it's a huge change for me. A month ago, I would have crashed somewhere around 90% of these jibes! 

Before the session above, I had a few of sessions in Florida with even better jibes (judged by minimum speed and percentage of speed kept during the jibe), but I still crashed every few jibes.

Here are three things that helped me get to an hour of winging without crashing:

1. Switching to a larger tail wing. Based on forum advice from someone who also had had a hard time learning to jibe, I replaced my 400 Speed stabilizer with a 500 freeride stab. The effect was dramatic - the board became much steadier than before, giving my slow self time to react when that was necessary. Finally, the board was not reacting twice as fast as I was anymore!
2. Practicing gliding. On the same forum thread, someone had posted a video from Alan Cadiz with instructions how to learn to glide while winging. Pretty basic stuff, but perfect for me.
3. Switching back to my wide, long Fanatic Stingray 140. This is the board that I had some of my best jibe success on many months ago, before I switched to the Starboard 115 wing board. The initial hope was that the extra width and volume would make it a lot easier to recover from mistakes, like slightly wrong foot placement, without crashing.  That certainly is the case - but another benefit of the long, heavy board is that it again slows things down. My "dry rate" immediately went up a lot when I switched to the Stingray. Another benefit is that the board just keeps planing in touchdowns, where the shorter 115 l board tends to loose a lot of speed. That makes popping right back up on the foil after a touchdown and foot switch much easier.

I also switched to a longer mast (82 cm instead of 75 cm), but that made just a small difference. In fact, the session above was with a 75 cm mast, albeit on very flat water.

Encouraged by the "dry wing hour", I decided to switch back to the 115 l wing board yesterday. The wind meter readings were a few knots higher, and stronger wind often had helped me in jibes. Alas, things did not go quite as planned. Here are the tracks:

No fewer than 5 of the first 6 jibe attempts ended in crashes! However, there were a few things that contributed to the crashes in addition to the board chance: the wind was more southerly and stronger, so we now had some closely spaced chop; and I had forgotten my booties, which I always use when foiling, and being barefoot was quite distracting for the first half hour.  Of 13 jibe tries, only 5 were dry, and just 2 were halfway decent. Eventually, I got the suspicion that I was perhaps a bit overpowered, and went in to switch from the 6 m wind down to the 5 m wing.

That turned out to be a much better size for the wind. It's also helped that I had completely forgotten that winging barefoot is not something I usually do, and had stopped worrying about hitting the fin or something nasty on the ground with my bare feet. Of the 15 jibe attempts in the next half hour, 9 were dry, and 4 were halfway decent. I even switched my feet in the air once or twice, rather than waiting for the board to touch down first. 

One peculiar thing about yesterday's session was that it was slow. My top speed was about 1.5 knots slower than most days, and even the mighty, never-crashing Nina was a couple of knots slower than usual. Our best guess is that the closely spaced chop just disturbed the water so much that it slowed us down significantly. But if we're lucky, we'll get a nice northwesterly tomorrow that's strong enough for a slalom session, and will go at more than twice of yesterday's speed in really flat water.