Foil section? For foil board

Revintage,
You should try XFLR5 available on Sourceforges
For rookies like me XFOIL is a bit intimidating as it requires DOS commands
XFLR5 is a Window environment XFOIL, so much more user-friendly
You download the Selig UIUC database in the same folder, and things become very easy.
The price is some XFOIL functions are not available, like the Boundary Layer's H parameters.
Or I was unable to find them.
You have also airfoil-tools.com or something like that, which is running XFOIL too and nothing to download or unzip.
The learning curve is not that difficult for most of the XFOIL analysis, a bit more challenging when it comes to full-inverse method.
The time put at learning this software will be a great investment for your foil designer's life.
Best of luck
Erwan

 

Thanks for the tip Erwan, I have actually downloaded and installed XFLR5 and also spent a few hours digging into it. Had no problem to import my N12FLAT. Need more time though. About H, I think I have found it in the program. It was for that reason I initially wanted to try it. Will have to give it more time.
About N12FLAT I am already building a 100cm prototype foil for load testing.

 

I'm especially happy to have found this thread, especially for the useful replies. Thanks to one and all.

I'm considering a build that I hope would be able to fly a typically small light rowing dinghy as fast as is reasonable through a deadhead infested Georgia Strait.

I'm considering bringing two modes of propulsion: windsurfer style sails, and kites. A short, narrow rowing dinghy (8 ft LOA?) would require a VERY low take off speed (because of hull speed issues), but that seems to come with more drag. I would love to figure out how to calculate what the typical highest speed would be for a given foil.

Another option I'm considering is longer hulls in catamaran or trimaran form (16 ft LOA) (again with windsurfer sails and/or kites).

Basically I'm looking at a low wind option and a high wind option.

I'm imagining building a few test foils from wood (CNC milled perhaps?), without fibreglass or carbon to try to get the hydrodynamics sorted, and then predict how a fancier build would work by just removing a sandbag or two from a tow test. Is that the right approach?

Should the test foils have square ends? Or should I round them off?

Any recommended foil sections, and/or areas / aspect ratios for those two modes of operating?

 

@Christian Nally I suggest you read "hydrofoils: design build fly" by Ray Vellinga. He goes over pretty much all of the design process for a small hydrofoil without delving into too much math, and copies are cheap. I don't think a windsurf sail is a good idea for using on anything but a windsurfer. They are hard enough to use on a board that is designed around them, I can't imagine trying to foil a old style dinghy. I don't know enough about kites to comment on them.

making preliminary foils out of wood is not a good plan. The material, and therefore strength, is inextricably linked to what the best design is. A wood foil needs to be thicker in order to be strong enough, and will be higher drag at high speeds. If you want wood, fine, but design around it from the start and realize that if you swap materials you will need to design over again.

Planform is best elliptical or tapered, unless construction demands the foil to be rectangular. Vellinga goes into detail about foil section and lifting area and aspect ratio.

 

Thank you @ziper1221 much appreciated

 

Given the very low drag of a foiling Moth, and the much higher drag of a boat wide enough to row and only 8ft long, it seems hard to see how it would foil well.

The Moth also has the benefit of a very high righting moment and large but low drag sail, both of which it needs. When you pop up on the foil the apparent wind can increase dramatically.

I know an engineer with an excellent record as a sailor, designer and builder of high performance craft who gave up building his own foils because he couldn't make the junction of the mast and main foil strong enough in carbon at home, even vacuum bagging. If the junction is too bulky the drag is too high. It's hard to see a wooden foil working where carbon won't.

 

I've always heard that a well-faired junction will be much lower drag than a direct T-junction.

 

I remade Tom's diagram as an exercise (and because his seems a bit cluttered). I also made one with some different 12% thickness symmetrical sections.

Edit: looks like the forum downscales images. Here they are as PDF also.

 

I must have missed some postings to this thread as I hadn't noticed the mention of connection difficulties between the foil and the vertical element.Back in the 1990's I made my first T foil rudder and used a simple fillet with microfibres and epoxy as the filleting medium and it worked for a while.After a breakage that may have been caused by contact with a UFO I needed to re-attach the foil.The solution I chose was to use a 3 ply laminate of carbon to make a connection web.From memory the laminate was 200gsm 0/90,45/45,090 and each arm was embedded completely through the necessary elements.I have since made others with the carbon web inserted before covering the whole assembly with glass or carbon and never had a failure.The span of the foils has been 250mm-290mm and while the boat isn't s fast as a fully foiling Moth it does plane as well as most other racing dinghies.I did upload a file of the carbon webs a long time ago but may still have the ability to include it here.

 

I know this post is quite old.
How did it go with the foil design and perfomance ?
I'm designing a racing windfoil and looking into what foil section I should use.

Looking at the currently best windfoils (Aeon from Patrik), I believe it is best to start with something physically similar, to minimize the risks of failure in the design.
The Aeon 650 (cm2) has a ratio of 10,1 and a span of 810mm. cord is approx 100 mm at the widest and approx 45 mm, 50 mm form the tip.
it is said to have "Double concave shape in frontwings for increased lift" Not sure what this means, I haven't been able to find any hydrofoil sections with double concave ?

Speed range for this foil is from 20 to 30-something knots, this should be the most efficient range.
There could be several parameters to look at, not only the lift/drag ratio, but also torsion and flex stiffness according to foil-section geometry.
Can any of you come up with some advice ?

Br.
Rasmus

 

Hi Rasmus,
So far what I have observed on windsurf foils regarding the double concave wing section is the following:
1-First concave on the upper side at around 60% of the chord from the leading edge is the shape for a Stratford recovery typical of Liebeck's wing sections as presented in A.O Smith "Hight Lift" workpaper. It aims to create hight lift by extending the "Rooftop" therefore it is possible to minimize leading edge velocities for the same lift.
2-You can find probably another concave shape on the bottom of the wing section around the last 20% very close to the trailing edge, It is called "Aft Loading"
AS I am not a CFD specialist, take time to check what is above mentionned
Cheers
Erwan

 

Hi Erwan
Basically it is still the lift/drag ratio that determines the efficiency, so adding a concave on the top of the profile might increase lift, but what about drag ?
Could it be other parameters, like control and "sailability" that makes the double concave fast ?
Does the double concave make the foil less sensible for angle of attack or less nervous ?
Control has always been a key point for windsurfing. You can't go fast without control.
With wind-foiling, control is even more crucial to be able to go fast! (or big cojones )

Do you know any std. foil sections with double concave ?

Br. Rasmus

 

Would that foil section be similar to the type intended for the Miles M52 aircraft and later,the Bell X-1? Both were designed to reach supersonic speeds and the wing section was a critical aspect of the concept.A quick search hasn't produced any detailed information but I suspect that a more prolonged effort might achieve a result.

 

Supersonic/ super cavitating sections (if that's what you mean ?) offent have a completely opposite geometri, thin leading edge and thick trailing edge. Not suitable for low speeds (below 50 knots) at all .

 

This from the Selig site The Incomplete Guide to Airfoil Usage https://m-selig.ae.illinois.edu/ads/aircraft.html is as much as I have so far found Miles M.52 wing root Biconvex 7.5% wing tip Biconvex 4.9%. there are some images here: History Shall Be Kind https://apostlesofmercy.tumblr.com/post/142976015410/the-miles-m52-was-the-worlds-first-supersonic
This being one:


This drawing is the only source a few more minutes of searching has produced and most definitely does not have a thick trailing edge.