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.