Basic Foil Design (I14 T-Foil)

I’m at the very beginning of designing a foil with the long term goal to design and simulate a T-foil for an International 14. My goal is not to improve on the currently existing foils but to better understand fluid dynamics and why we use our foils like we do.

I would be glad if someone could give me some hints regarding the general approach or point out errors in my early calculations.

So far, I took an aerodynamics class, played around with XFOIL and did some very basic math.

I am aware that the foil configuration of an i14 is somewhat unique and that it will take advanced tools and knowledge to fully simulate a foil behind a hull. So, for now I will ignore the (probably significant) effects on the hull wake and just look at a simple fully submerged wing.

Current designs do not use the maximum allowed area of 0.14m² so I will start with a square foil with span = 1m and chord = 0.1m. The foil shall operate best at 10kn or 5.14m/s as we spend most of the time sailing upwind. For lack of knowing better I will assume a lift force of 700-800N is optimal.

The Reynolds number will be around 500k upwind and peak around 1M.



Assuming two-dimensional flow those tables give me lift and drag force for the related coefficients (I guess I can ignore the temperature here).



Any errors so far?

How should I proceed when choosing an airfoil? I understand that I want best Cl/Cd for my Re Number and I know approximately what Cl is required. But what about other properties like stall characteristics or behaviour at large alpha?

Can I even trust these numbers or is a real three-dimensional foil on a skiff rudder exposed to such irregular flow, that this won’t help me a lot?

And is XFLR5 a tool worth looking at for this scenario?

If someone could answer some of these questions or recommend some further lecture I’ll be very happy.

 

XFLR5 could be a place to start. Or you might start with a vortex lattice code like AVL. I've not used XFLR5, but I think it is a lifting line plus 2d section approach. The vortex lattice would be better at the 3D interaction between the rudder and foil.

Unfortunately, Neither XFLR5 nor AVL can handle the flow from the hull, and the hull-foil interaction is very important for the I-14. That's why the foil is not located at the bottom of the rudder.

A major effect of section camber is to shift the zero lift angle of attack. But if you measure angle of attack from the zero lift reference line, all sections produce about the same lift. So you can start with a symmetrical section or even just a flat plate. Then you can add thickness and camber later after you have a better understanding of what is going on.

The next step up from vortex lattice is a panel code. Aerologic sells CMARC, a version of PMARC that has been ported to the C language, along with a gridding tool called Loftsman and the Pmarc post-processor. A panel code will allow you to calculate the effects of thickness and the interference between the rudder and foil. You can get the surface pressures to see if there is any potential for cavitation. The free surface will be a problem, though. CMARC can't represent the stern wave from the hull, but I believe it does have the infinite Froude number approximation, which has a flat surface but nonzero vertical velocity at the surface. The same goes for AVL and the free surface.

After that, if you want to handle the hull, foil, and free surface, you're getting into some really heavy computation. Probably some sort of Reynolds Averaged Navier Stokes (RANS) code. You'll also need a much more sophisticated gridding tool and post processor to visualize the results.

You might contact Paul Bieker at Bieker Boats and ask him how he designs I-14 foils.

 

Thanks for the input!

Yes, the hull-foil interaction is what I want to understand one day. With all the different gantry designs and foil/cassette combinations out there I think the position of the foil varies quite a lot between boats and most people don't seem to care a lot. And then there are quite big differences in crew weight, too.

Also, I kind of feel when the foil is in an inefficient angle but knowing how the aoa/drag behaves cannot hurt.

I remember Paul Bieker showing a simulation at 5:00 in this video. Contacting him might be a good idea.


Can someone recommend a commonly used foil section so that I have a reference an can get a feel for the characteristics?

There was another answer in this thread but I cannot see it nor can I find a private message function in this forum. So if this wasn't deleted purposely I would be glad if you could share the information with me.

 

I am curious about the reference to the Alpha angle.Is this a reference to the hull at "normal" trim conditions or the actual incidence of the foil relative to the course of the boat and the horizon?I haven't set foot in a 14 for a very long time but my understanding is that they use the foil to optimise trim for the leg of the course they are sailing and don't adjust the angle very much before the next course change,not that it would be the most frequent adjustment for a busy sailor in any case.Is it customary to hope that a negative incidence of the foil might delay or prevent nose diving?In which case a cambered section might be sub-optimal.I am curious also about the means of indication of the foil incidence,I presume that with the tilting gantries there might be a mark on the adjustment system to indicate neutral incidence and the human eyeball estimates the standard deviations from that condition.As far as the vertical position of the foil on the main rudder blade is concerned,would I be right in thinking that the reduced pressure on the upper surface might have some effect on suppressing the stern wave,a notion that only came to me after the van Oosanen institute released the papers relating to the Australia 2 12 metre which showed some effect from the reduced pressure on the windward side of the keel adding to the depth of the trough amidships.

 

Except when sailing downwind in strong wind and waves we do not really trim the bow up so the hull stays level around the course.

We have some marks on the gantry to indicate foil angle and I added a ball stopper on the adjustment system to limit movement. When in a hurry I can just uncleat the control line for maximum safety (bow up) without having to look at bow or gantry and then trim it back on later without sacrificing too much speed.

In general, we put the foil into max lift setting and then move the crew weight as far back as possible. Once we cannot move further, we start easing the foil control. Upwind, where the speed does not change that much, we usually don’t touch it.
Downwind we adjust lift more often but with more experience we learned that it’s faster to stay in the foot loops und stuff the bow from time to time in a big wave. 14s don’t pitchpole unless something goes really wrong.

I’m not sure whether the foil creates downforce during regular straight-line sailing. But during a bear-away downforce can be necessary.