This is a concept I have been working with for 3-4 years. A different way to induce counter-rotating vortexes, similar to the humpback whale’s tubercles. It has the advantage of not catching the leash. These are back quads (fronts coming soon).
The leading edge ondulates so the foil’s angle of attack varies by 5 degrees. The front pair of quads have a lower frequency but still 5 degrees of amplitude. Without going into the science, the effect is similar to tubercled fins, smoother stall in exchange of a lower maximum lift.
I make no claim of lightspeed-hyper-space-vortex effect, I am just exploring possibilities, looking at nature and computer charts with equal awe.
In my opinion we do not have enough guts on most waves and skilled surfers to have a design that actually enhances what just we have. I think the simple test is to ride them in smaller gutless surf then in the same session change the fins for others with the SAME area, size; cutaway, with an 80/20 foil conventional shape and see. Very possible nothing noticeable.
I can see them shedding a leash easier than a tubercled fin, which basically cant, but I could still see it getting stuck in some situations.
I’ll guess the undulations make it act like a bigger fin. Wonder if it makes the passing water too ‘busy’?
I thought MrMik’s tubercled fins would be too busy too, but the only ones which felt draggy were those with more numerous smaller tubercles. The others only feel draggy when dragging a lot of kelp or grass, At least on a singlefin longboard.
I like the raked ‘spitfire’ ish template too.
“It’s all been done before”, or:
'so and so started doing this 50 years ago"
…does not seem to apply to fins such as yours, but I am sure some will feel the need to denigrate or dismiss the design, instinctually.
Wild, wild stuff. I hope you’re not making them by hand.
In isolation, are each of the undulating chords single or double foiled? If they are single foiled, does the foil swap with each undulation, or are they all on one side of the fin?
Thank you Mr Thrailkill, coming from you it is an honor!
I am afraid that I’m not an accomplished enough surfer to give an interesting surf report. The ones in the pictures haven’t been surfed yet, I will finish the front pair of the quad set next week-end hopefully. But from an earlier experiment I can say that I was surprised that there was no noticeable increase in drag, when paddling or trimming. The stall softening effect seems to occur more gradually and predictably than with tubercles.
Are they better ? A lot more testing by good surfers would be needed before answering this question. Also “better” is a tricky term, some surfers are happy with conventional fins and their technique is build around “normal” fins behavior. For surfers who like to experiment, I think this solution is as interesting to explore as tubercles or other passive flow control measures.
The channels created by the leading edge undulation create a horizontal component which needs to be taken into account though. So I orient the foil in relation to the water flow by compensating for the tail rocker. In the above fins it’s 3 degrees to fit my board in progress. I think that the more complex the design is, the more it needs to be tweaked to the board and the surfer.
Greg, about toe-in, until now I have been aligning the center of the fin with the box. So if the AoA varies by 5 degrees the lowest AoA will be 2.5 deg under the toe-in angle and the highest Aoa 2.5 deg above. If that makes sense. There are obviously many possibilities to explore.
Reverb, I totally agree. Although I would think that the near to post-stall behavior should be noticeable for a decent surfer, as with the turbercles.
Absolutely, they feel bigger than they are, but most of my fins do, possibly because of the thicker foils, in the 10% chord range. (for thrusters and quads, singles can be much thicker)
The water will be too busy if the flow is not aligned with the “channels”. I compensate for the tail rocker. I guess at least 2 models would be needed for good results on most boards, one for low and one for high tail rockers. 3 different orientations would be covering all boards properly, may be.
Thanks ! I believe that this is the first human application of this concept, accross all types of foiled objects. But I won’t cry if it’s not. In the meantime I give all credits to the scallop for the inspiration, and an honorary mention to the humback whale, again, for her ventral pleats.
Yes some people don’t like change, but some are understandably wary of all the revolutionary claims, which pop up every year but are just gimmicks. That is why I insist that anything I do is pure exploration for the sheer stoke of it.
No I make them by brains, I have a robot for the rest :). Actually I like to joke that my cnc is a Terminator who has gotten tired of terminating and decided to become a carpenter. I must say he’s pretty good, if you tell him what to do.
They are cambered foils, although almost flat on the inside on these, and the foil stays oriented on the same side, like a normal fin. It is like wings that have a twist at the tip, except that here it’s a wave all along the LE.
Thanks again for the encouragements ! It means a lot.
I would like to emphasize the word “seems” in the sentence you quoted, until better surfers get to test the idea.
By the way Mr Thrailkill, a few months back I have made some visualizations of the flow in your twingle set-up (with the foiled sides in and a 2 inches from center gap), which shows that the paired fins allow the flow to stay attached to the outside fin (in a turn), way beyond the stall angle compared to a single fin. I haven’t posted them here yet because I need to run the same simulations with a single fin to make the point more convincing.
I know you are practically oriented and you already know that it works but I thought you might appreciate some visualization of your idea.
This deserves it’s own thread but I’ll just post one image here. This is at 3 m/s, 15 degrees AoA, halfway down a pair of 7" fins. You can see one fin is stalled an experiences re-circulation, while the other one still has the flow fully attached.
Then a screen shot of the fin models used for the simulation.
Never noticed it until I looked at your graphic image. The space between the inside convex surfaces of the twingle’s two fins is a true Venturi channel. That should generate some interesting flow dynamics: increasing pressure > decreasing pressure > increasing pressure.
Stoneburner, yes this system is like a bi-plane wing with a venturi effect because of the inverted foil.
Monkstar, I have a few more images, I will start new a thread to post them. May be a video or 2. But I must say again that the analyses that I can do with my dumb laptop aren’t sufficient for serious scientific studies. They’re more like food for thought, while still allowing to visualize some of the more obvious phenomena like tip vortex, stalled turbulences etc.
First, the top wing of a biplane would not use an inverted foil.
Second, for maximum biplane benefit, I believe there is a specific foil separation distance. I could be wrong.
By placing an inverted foil too close, the resulting Venturi effect might be detrimental. As I recall, Mr. T uses a chord thickness of 3/4". The twin foils figure I posted would represent 3/4" chord thicknesses with a 1" separation.
Chord thickness, chord length, maximum camber, foil separation and internal vs. external camber placement could be critical variables.
For clarity and understanding, the fins in the TWINGLE setup are on two inch centers. Each single foiled fin is 3/8th inches thick. In effect each single foiled fin has the same camber as a double foiled fin that is 3/4th inches thick. The above model showing one fin in a stall, while the other fin is fully engaged, is EXACTLY the performance I was seeking with the twin single setup. The sensation the rider experiences, is that the ‘‘gas peddle’’ is down, throughout your turns and direction changes.
Unless WAO doesn’t mind, this discussion belongs in another thread. We are hijacking WAO’s undulating-surface fin thread. This will be my last post about twingles in this thread.
I do not dispute that your twingle works. I am in agreement with Hans that your twingle does in fact work.
However, Hans felt inwardly directed camber (asymmetric foils) was not the optimum foil type for the twingle.
The disrupted flow in WAO’s diagram is on the flat surface of the upper fin in the diagram.
I think areas of high and low pressure should be identified for inverted single foils vs. double foiled fins in a twingle setup. Flow diagrams need these to accurately represent the fluid dynamics involved.
WAO those fins are works of art. You put a ton of thought, science, engineering, and technology into your creations. Just curious, how much time went into the design of these fins? How much machining time on the terminator CNC? What kind of bits do you use for roughing and finishing? How much post CNC sanding/clean up do you do?
I don’t know if it is the pictures or the fins, but did you build in any cant into the fins/finbase? It could also be the cant from the boxes also. Another thing I noticed is, ignoring the undulations, the fins are straight over the length, except the last couple cm. Is the small curve towards centerline an effort to make a winglet of sorts?
I don’t mind, I derailed my own thread by posting these images. I’ll start the Twingle Visualization thread now but I can’t post all the images yet. I need to resize them for the web, put them in order and think of smart things to say about them, tomorrow morning hopefully.
Thanks Phillip. Now that I have the technique to draw them I guess I can draw a new one in 1 hour if I have the planshape ready. But it took a few models before I could control the angle variation all along the leading edge.
About cutting time, for these I cut 6 fins at the same time in the same plank. 2 fronts and 2 back quads and 2 center fins to have a thruster options. The roughing pass took about 25 minutes and the finishing pass about 90 minutes. Same for the other side. I use the biggest ball-nose that will fit the design, 8 mm in this case because of the smaller bends at the tips. For more conventional fins I use 1/2" ball nose, so it goes faster and gives the best finish. I use the same ball nose bit for roughing and finishing so I don’t need to re-set the Z origin, this way I’m sure to have the maximum accuracy for the foil.
I make the finest finishing pass that I’m willing to wait for, typically 0.4 mm between pass or 5% bit diameter. With a big diameter ball nose it’s virtually sanded already. I prefer to have the minimum sanding to do to avoid altering the foil shape. This is for complex surfaces that would be tricky to sand. For a “nornal” fin, 1 mm passes will need sanding but it will be easy and won’t take too much time.
The bent at the tip is because I end the sine curve by a “down”, but it is along the same mean line as the rest. It is more logical to end the curve by a lower AoA and yes at the same time it does a little winglet.
Well it’s nice to know that your process isn’t far off from mine, considering the results you’re getting. I’m pretty limited by my cnc in terms of size and control hardware.
If it’s not a trade secret of yours, how do you deal with the area where the base of the fin meets the tabs? With a fin that’s thicker than the tabs and using a ball nose I end up with a “fillet” right where the fin meets tab. I either have to clean it up with a chisel or design my fin around this so it fits in the box properly.
Also rereading the thread so far, you mentioned designing the sine wave airfoils to best work in harmony with the flow of the tail rocker. I think I understand what you are trying to accomplish. Couldn’t you assume that the flow would be parallel to the rocker and perpendicular to the fin boxes? Any compensation, with respect to the rocker, would be accounted for when the boxes were installed flush and level with the board bottom/rocker. I could be over simplifying or misunderstanding your intention, because you are clearly on a whole new level of fin design.
Lastly did you ever get a proper surf on the chambered board you built? If I remember you were basically done with the build. Keep the stoke alive in these bleak times!
