Thoughts for the optimum finish for foil wings and masts for low drag?
Should wing (lift) surface prep be different from mast (no lift)?

I know it's been covered many times before re. America's Cup boats, windsurf fins etc. but are foils different?

Video below (from 7.40 on) has interesting things to say re. laminar vs turbulent flow.

Turbulent boundary layer should Increase lift and lower stall speed, so need a slightly rough surface. Think turbulent boundary layer will also be least resistant at lower speeds, not sure about high speed, although likelihood of turbulent boundary layer increases with speed, so maybe it's moot.

To improve lift, maybe we should try some vortex generators on our foil wings?

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azymuth said..
Thoughts for the optimum finish for foil wings and masts for low drag?
Should wing (lift) surface prep be different from mast (no lift)?

I know it's been covered many times before re. America's Cup boats, windsurf fins etc. but are foils different?

JJ, you tell us, are you going faster now you have worn the anodising off your mast
Maybe too much TOW

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oscardog said..
Turbulent boundary layer should Increase lift and lower stall speed, so need a slightly rough surface. Think turbulent boundary layer will also be least resistant at lower speeds, not sure about high speed, although likelihood of turbulent boundary layer increases with speed, so maybe it's moot.

To improve lift, maybe we should try some vortex generators on our foil wings?

Some advocate sanding a rougher strip 10-20% back from the leading edge to do just that. Zeeko also uses an anti-ventilation coating on their wings which is scale like



Make sure your wings are clean from grease and fingerprints. Once I cleaned on of my problematic wings, its behaviour improved significantly. I had gotten tef-gel on my fingers putting in bolts.

I sand my wings with 800 or 1000, and my mast with 3000, seems to work as I passed 30 knots of speed quite a few times now. I also clean my foils regularly with alcohol / thinner and try to touch the foil as little as possible. If your foil is in good shape I feel the second thing makes way more difference than the first, oily / fatty foils feel a lot less locked and ventilate much more at higher speeds.

We used to have this very same discussion in the days of formula racing. Fins and board bottoms. Slalom racers talk about this, too.

Consensus was that a surface with a 600-1000 grit finish was faster than a mirror finish. Boundary layer separation was a thing.

"Some advocate a rougher strip further back from the leading edge", here's FHot's version.

The optimum position for the rougher area will really depend on where and why the flow is separating.

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Grantmac said..
The optimum position for the rougher area will really depend on where and why the flow is separating.

Agreed and I doubt it's that far back as pictured. Even very efficient laminar flow aircraft wings are laminar for about 40% of the chord. I've read that while the P-51 Mustang was using a laminar flow wing, in practice, it usually wasn't - just a good, low drag design. This company that makes vortex generators for small aircraft recommends the placement of theirs at less than 10% back from the leading edge: vortex-generators.com/installation-of-vortex-generators.html

As to the video, vortex generators on jets usually have more to do with boundary layers in transonic flight regimes rather than trying to massage low speed stall.

Some VGs are all the way up at 3% of cord. That keeps them from creating drag at cruise.

I definitely agree that past 40% isn't going to do much.

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Grantmac said..
Some VGs are all the way up at 3% of cord. That keeps them from creating drag at cruise.

I definitely agree that past 40% isn't going to do much.

At 3%, probably just easier to sand the whole top with 200 grit.... I'll have to look up the 3%

3% back from the leading edge. So basically far enough forward that they aren't really exposed in level flight.

A person could probably do some fun experiments using tape cut with pinking shears. I know RC Gliders have done that.

Closest I've seen is about 6% in cruising the internet - mainly regarding full size airplane wings. At 3%, it seems you get into the "bugs on the leading edge" issue. Do you have any links to 3% installations?

However, I did see the following which is very interesting - a way to trip the boundary layer: www.deturbulator.org/TheDeturbulatorTape.pdf

Also, tapes, tapes, tapes: wingsandwheels.com/tapes-seals/turbulator-dimple-tape.html

I've been aware of tape for years but this whole topic is intriguing for low end foiling. Anything that delays either stall or ventilation without a noticeable speed penalty would be interesting.

I believe its Microaero who do the leading edge installation. But that is mostly for STOL aircraft. No idea how that translates to laminar flow or higher Reynolds numbers.

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Grantmac said..
I believe its Microaero who do the leading edge installation. But that is mostly for STOL aircraft. No idea how that translates to laminar flow or higher Reynolds numbers.

Thanks - found it. "Micro Vortex Generators are placed in a spanwise line two to fifteen percent aft of the leading edge of the wing. They control airflow over the upper surface of the wing and the tail surfaces by creating vortices that energize the boundary layer." I appreciate your sharing that. Off I go down the rabbit hole...

Remi Vila on sanding

800 grit perpendicular to the flow on wing and mast and along the flow on the fuse if I'm undertanding him correctly
He also says to square the trailing edge of the mast/wing to stop foil whistle
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