Design for windsurf hydrofoil wing

Hello guys, I'm looking for help

Next year we are going to have the southamerican windsurf championship in our country and the main event is foiling. We have racing equipment, but we want to introduce more people into the sport, so the idea is to make a wing that turns the racing equipment into a very slow and easy one.

I want to build a cheap, easy building front wing for windsurf foil. Something around 1800/2000 cm2. The challenge is made it on plywood and glass, so, should be flat, but has to be super stable, and allow to fly at very low speeds and use very small sails.

I was thinking in an elliptical wing design, straight, with a wingspan around 900 mm, thickness around 20 mm and a flat-bottomed profile like the S7055(10%). The idea is to put a plywood board into a CNC and then glass it.

I'm an engineer, but not this kind, so, I tried downloading XFLR5, tried to learn as much as I could to realize I know nothing on this and is way beyond my possibilities.

the racing equipment I want to adapt is this one:
2019 Race https://starboardfoils.com/pages/2019-race

and hopefully make it work like this one:
2019 SuperCruiser https://starboardfoils.com/pages/2019-supercruiser-2

Any help is appreciated, opinions, suggestions, anything

Best regards.

 

I would worry about the connection between fuselage and wing. An 1800/2000 cm2 wing with 90cm span is very large, and the torque that will go into the fuselage is substantial (for instance when you would waterstart with only half the wing in the water, or land a jump sideways, or some big surf dude pumping that wing). I am not familiar with the Starboard products, but looking at the pictures it seems the second model uses a much stronger type of connection than the race model, and there is probably a reason for that. The race fuselage seems narrow, and seems to mount using M6 screws. You might find those screws bent or pulled right out of the wing.

Making the wing with plywood and glass will be heavy, but certainly strong enough.

If you have access to a CNC, you can use any profile, you don't need to limit yourself to one with a flat bottom. Just use some of the same plywood to CNC a negative bottom holder. Then cut the bottom side of the wing. Turn it around and put it into the holder, connecting it with screws or something, make sure it is in place. Then cut the upper side of the wing.

Hopefully someone with more knowledge can add better insights, but I don't know about S7055 as a hydrofoil section. A flat underside means the underside will have very little laminar flow at useful angles. S7055 seems to prefer high lift, at alpha=0 it already has CL over 0.3, and it works best around CL=0.7. A 2000cm2 wing will lift 100kg at a slow 5 m/s at CL=0.4, so those higher coefficients are needed only during take-off. But hey, there are some successful commercial brands that also use flat-underside sections (Naish), so maybe it is not too bad in practice. If you use it, I'd advise connecting it at 0 degrees to the fuselage.
If you want an easy wing, I'd also advise using some sweep. Making it flat is perfectly fine.

Although there is lots of information about airfoil sections available, I found it very hard to find useful information about hydrofoil sections, and specifically useful sections to start from. Most fundamental research is ancient. The recent interest in foiling sports has not yet translated into publication and research of practical sections for specific design goals that I know of. The best information I found is here in this site and a few other sites like http://www.tspeer.com/. The publicly available options seem to be H105, NACA 63-210 or E817 (do not use that one). The hydrofoil sections that I use are custom designed using XFoil and are optimized for lower lift coefficients that occur once you are foiling. I tried, but was unable to design efficient hydrofoil sections under 12% thickness, so my sections are around that thickness. I optimize for CL between 0 and 0.4, and Re 500k - 2Mln, maybe for the slow wing you want, you may look at even lower Re numbers.

 

Thanks for the answer, I really appreciate it

The fuselage is 3 cm wide from aeronautic aluminum (Starboard 115), and in windsurf we mostly start only by uphouling, so I think we will be fine

we read about the E387 profile, we decide to go with it, but from what I read from you, maybe we should make it thicker (max thickness 9.1%)

we are going to follow your advice and go with angle of atack zero

Here is a picture of the first drawings, please tell me what you think, I belive we are going with the one in the middle. Our target is 1850 cm2

Sorry if my english isn't perfect, is not my native language and is kinda rusty, I can only say I'm super happy for your answer and I'll be very grateful if you guide us

Thank you very much

 

That middle plan, or the right one, will probably work fine.

That E387 section again has very high lift. Remember that after the start you will be mostly using low lift coefficients. At 7 m/s, the 1850 cm2 wing will be working at CL=0.22, which is a negative angle with this section. With 25cm span, you will have a Reynolds number around 1.4M at that speed, which is not that low. The nose of the board will be pointing slightly downward, and you will probable have to shim it to make it feel right. Also I suspect that a concave underside might be hard to control if it gets near the surface.

I don't know everything, and have not done windsurf foiling, but for kitefoil I use sections that have CL=0 at alpha=0, and mount them at around +2 degrees, or CL=0.2 to the horizontal.

 

We'll take your advice, we are changing to H105. For this profile we should go with alpha=0 right?

I'm super ignorant on this topic, but I'm studing, so I hope to during this week I'll understand what said with the CL thing (sorry)

The design is basically (from center to the tip) a rectangular wing from middle to 20 cm at 25 cm long, then the next 25 cm reducing in delta from 25 cm to 10, completing a wingspan of 90 cm

I hope by fith of july I'll allready be able to test it on XFLR5, then I'll will ask you then what numbers should I chase

thank you very much

 

Hi Maarteen88

I've made my research, I found some graphics on the H105, it match perfectly on the parameters you gave me, so I belive it would be an H105 mounted at apha=0, to get near CL=0,3

What do you think?

 

You are getting there. But you should not use a graphic of H105 as its source, you should use the .dat file that Tom Speer posted, and use that directly in XFLR5 and your CAD program. Airfoils are very sensitive, you will get some inaccuracies during the build, but there is no reason for inaccuracies during design and analysis.

What I do is open the .dat file in XFLR5, normalize it and refine it to something like 81 coordinates. Then I set a trailing edge gap of something like 0.4 % (which will translate to 1 mm for your 25cm chord), so the trailing edge will not be dangerous / sharp. Then I manually edit the coordinates, I specify the end to be exactly 1.0,0.0, I change the one closest to 0.0 to be exactly that, and the last one again to be 1.0, 0.0. That way you are sure to have a closed section that will intersect the center line at the LE and TE exactly. I then save that file as a .dat file.

Then I start my CAD program (I use Fusion 360). I first draw the outline. Then, starting from the root, I create several sections, by creating a sketch at that point, intersecting with the outline to get the section centerline, and then using a macro to insert the .dat file as a airfoil in the sketch. There are several macros in the Fusion marketplace that will help with that.

In my wings, I also draw anhedral and twist, which takes more steps in the design and is harder to get right. But you don´t have to do that to get a good enough wing.

When I have the wing sections drawn in their correct positions in space, I use the loft command to get the 3D wing, using the outline as guides. I use a second loft to get the wing tip. It takes practice to get this all working, and I have had Fusion crash and hang many times on sections that have too many points, or that are not well formed.

If you are using another software for CAD, I think most of these programs have comparable features, and have some way of lofting profiles to get an organic 3D shape.

Good luck!