This is a polar plot of 4 different foil sections… Any guesses as to the thinnest & thickest foil sections? Higher on the page is higher drag, right on the page is more lift.
Bonus points for which section is aft-loaded/concave bottom!
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Red is thick, green is thin
Purple is aft camber
Is this software accessible? Is this one of those theory says X but in practice we all feel Y
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What about speed? Is this graph a constant speed and varying AOA to get the different lift and drag numbers or constant AOA and different speeds to get there? - I’m guessing the latter -
I feel like a lot more info is needed! This is especially confusing since lift on a foil never really changes….Lift is always your weight so the graph should be drag vs speed at whatever AOA is necessary to get a given lift at each speed! I feel like we fall into lift drag coefficient as a way to understand these things because It’s EASIER. Plug in speed and AOA, get a lift and drag and compare them but it’s not what’s happening on the foil. When your riding your varying AOA to generate that constant lift for whatever speed so a good model should reflect that. Much harder math I know!
This is a constant lift polar, minimum to maximum aoa (shown as CL) and speed with Reynolds number specified to be in the expected range
So was I right? I want to know more regardless. Feels like the purple is the interesting one, good until a point and then…?
Well, the aft camber part was a bit of a trick question since they’re all aft loaded sections. The purple one does have the most aft loading though
You forgot to rate the gray one, that’s the thinnest and the green one is on the thicker end.
Gray: 10.5%
Orange: 12.2%
Green: 13.3%
Pink/purple: 13.5%
for reference, CL= 0.4 - 0.6 or so is where most riding is done, with 0.2-0.3 usually the top end and 0.8 or so the bottom end. CL = 1.0 or above is only during a slow takeoff and pumping
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This type of polar is very difficult to apply to how we surf in my opinion. We generally ride foils with more or less fixed amount of lift (equals our weight + gear + board weights). But we span across lots of speeds.
A better plot would be speed on the x axis; drag on the y-axis - for a fixed lift. angle of attack is varied at each speed to find desired lift at optimal drag. The plot would drop out at stall speed, so it would also be possible to see the lowest possible speed for a given amount of lift.
If comparing multiple foils, we’d need to pick a lift value that makes sense. But another way to visualize a single foil’s data would be to have multiple colored lines for different lift values - corresponding to rider weight.
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Well, this is a 2D analysis of foil sections so only coefficients are generated since there are no dimension; CL is the output instead of speed. The way a 2D fixed lift analysis works is that you find the reynolds number that you expect to see IRL at CL=1.0 and the model runs the calculations for a range of AOA or CL at the corresponding reynolds numbers. This graph covers a reynolds number range from about 300k to 1 million. That’s around 8-30 knots in the real world
A 3D analysis can output speed since there is a surface area applied, but 2d is always the first pass when designing a section.
@jondrums trying to pull the same BS I did but this guy is way ahead of us.
@J_L lets being this back to something we can all understand - looking at CL .9 and 1.2 comparing red and purple - complete this statement.
At _____speed(CL .9) _____ wing generates more drag than _____ but at ____ speed (CL 1.2) they are equal. Can we do that with this graph? Do we know that this the range we’re operating in when foiling(the range shown on the graph)?
@TooMuchEpoxy here’s a 3D analysis of those two sections at 100kg weight and a 800cm2 AR10 foil.
This plot is lift/drag ratio on the Y axis vs speed in knots on the X axis.
As you said, they have about the same drag at CL 1.2 and again around CL .45 corresponding to 9 knots and 14.5 knots . You can see the wide drag bucket of the purple one allowing it to glide at high L/D down to 11 knots while the red one peaks at 12 knots and a lower L/D ratio. The purple one also starts a little earlier. Red is a full knot faster at the top end though.
This is nice. I feel like I can start to put a story to all this now in terms of things
I feel on the water. I think there’s a few ways to go from here….
The most analytical approach would be to make a distribution of time spent on foil at which speed and overly that to get a feel of which foil complements which speed range.
The other would be to target specific moments in riding - digging out of the hole, high speed glide, etc and - find the speeds associated with those maneuvers and see if that helps develop a narrative for one over the other. Is there a specific moment that really is helped by that purple outlier at 10kts or is the reds superior performance in all other speeds more applicable?
Thank you for posting the updated 3D analysis - awesome! Are you using XFLR5 for this? I’m playing around with now, and yes I am familiar with XFOIL for 2D analysis, which is the core of XFLR5. I do have a plan to compare directly the CODE 1075 to the ART PRO 1201, two foils I have a lot of experience riding. I’ve extracted the foil sections from both and working on figuring out the software to get them plotting correctly. Need a bunch more hours of fiddling to get my workflow sorted out. I have a total boner for analysis like this.
Just to get an idea of the data format I think is most useful for a given foil, I extracted the curve using “engauge digitizer” and inverted the Y-axis to Cd/Cl which is a dimensionless version of drag for comparison purposes. For me, much easier to see that the purple foil has nominally less drag at low speeds but gets draggy at speed. Obviously possible to deduce from Cl/Cd as well, just have to stand on your head.
curious what % thickness is the Code section?
I haven’t graphed it out, but it’s basically like this: Winging flat water, 12-25 knots with the majority around 15 knots. Winging big waves 12 - 22 knots mostly 15 knots. Downwind in the ocean, 10 - 18 knots with the majority around 12-14. Prone/SUP 12 - 18 knots mostly around 14
I think the high glide of the purple one is useful to keep moving at that speed without having to pump, since drag is lower you can stay on a smaller wave/bump. It should also pump better at 12-14 knots since as you load the foil up you are pushing it up the L/D curve vs pushing into the area of falling efficiency.
Where things get interesting is that you can leverage the earlier takeoff of the purple one to reduce area a bit and keep the same span for a higher AR and then the speed differences become less and the glide advantage is even bigger…
My favorite winging foil uses the red one since it can be pushed fast and turns really well.
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I use Flow5 but it’s mostly the same as xflr5. have you gone into the inverse design module yet?