Here’s my version. No cant though, just straight up and down.
That looks awesome RDM, what is the ride feedback like?
BTW maybe you should call it the No Cant Do haha.
Huck, no can’t do ! Hahaha
Rohan, that’s a great looking fin, how’s it go ?
Got o pair of these from Greg Webber
haven’t figured them out yet probably need a pintail
Has a lot more “squirt” than a standard fin. Pushed hard it responds with a bit of a “boost” feeling.
Rohan, if you look at the physical design of your fin that’s exactly what you’d expect from it. You end up just looking at a board or fin and know what’s happening. Interestingly fins are the only flat surface on a surfboard, I know there’s an oblique flat line on concave hulls but a fin is the only flat 3 dimensional surface.
Maybe because we’ve always laid up fin panels on a flat surface.
I have proper foil only on the inside of each of the fins (as per the Thrailkill recommended setup). If you sketch a horizontal section through the fin/s and then sketch in the water flow incoming at an angle you will see why you don’t gain much by having a full foil on the outside as well.
The theory is that full foil on the outsides has a negligible effect on the point at which flow separation will occur during a turn anyway.
All the hard work is being done on the inside by the “captured” flow.
Or should I say that it is only the “front half” of the foil (up to the wide point) that does the work in preventing a stall during a turn. The “rear half” of the foil is only ever there to terminate the rear edge properly (in a sharpish edge) in order to minimise drag.
And that the effect of on earlier separation point on the outside of each of the fins for a full foil versus very a very limited foil (which would normally make an easily perceived difference) is overwhelmed by the effect of the “captured” flow.
That’s Interesting Rohan !
I see it from another angle…
With these dual fin setups both fins are virtually in the same space, take 2 fins with the foil on the outside, both fins would give identical lift in the opposite direction away from one another cancelling any lift created. Also during a turn the flow hits one fin on a flat side and the hits the other fin on a curved side. It’s a win / lose arrangement.
But the flow between the fins is unimpeded by the in facing flat sides.
Now take 2 fins set with the foil on the inside.
Once again any lift derived from each fins foil is equal and opposite to each other.
And during a turn the flow hits one fin on a flat side and the other fin on a curved side, another win / lose arrangement.
And the flow that’s directed over both of the the inside convex foils will be more turbulent especially as they’re close together and when at greater AOAs.
Only in the “straight ahead” will your foils produce opposing lift (equal and opposite) as per your diagrams.
As soon as you angle them into the water flow you will get lift in a common direction.
Using 2 double foiled fins (pic C) brings its own inherent problems.
A double foil has always been a compromise shape that was neither perfect going left or right but it had to be symmetrical. If a double foil was the most efficient shape it would be used on side fins.
And during a turn in either direction the flow hits both double foiled fins on convex sides which is a lose / lose as the most efficient inside surface is flat.
Which brings us to an option of using a foil arrangement of 2 slightly double foiled fins, a subtlety rounded leading edge and a gently tapered trailing edge. Less foil has a greater top speed, less form drag, provides the best dual surfaces for turns in both direction. And an almost parallel flow between the fins.
This thread is worth rereading.
http://www.swaylocks.com/forums/toe-inside-foil
Excellent link, thanks Rohan ! Pity a lot of those guys don’t post anymore, I enjoyed those all in discussions.
I found a study on “Aerodynamics of biplane and tandem wings at low Reynolds numbers” online which might shed some more light on the subject. Seeing as surfboard fins are operating in the same Reynolds Number range as this study, I think it is particularly relevant.
The layman’s terms summary was this. Experiments were performed to investigate the aerodynamic characteristics of biplane configurations at a low Reynolds number of 100,000. The wing models were rectangular flat plates and it was concluded that lift, drag, aerodynamic efficiency and power efficiency ratios show that for small incidence angles, performance compared with the single wing is degraded. However, for single-wing post-stall angles of attack, lift performance improves and stall is delayed significantly. For a fixed angle of attack, there are optimal gaps between the wings for which total lift becomes maximum. Measurements also show that performance improvement relies heavily on the strength of the inter wing flow.
These results are sort of what I would have expected given my own experience with the fin I’ve shown in this thread. The fin gap to chord length of fin ratio and its resultant effect on the inter wing flow is the variable to experiment with in order to optimise the performance.
Pitts Specials and their aerobatic capabilities are the “proof of the pudding” of these results too I guess.
That’s the beauty of surfboard design, everyone’s got a way of looking at it.
You’re correct. it is quite relevant. Points given, for your insight. As a licensed pilot, since 1966, the Pitts Special was an early favorite of mine. If I win the Lotto, I’ll get one !
I have found another study dealing with low Reynold Numbers in regards to biplanes and wing spacing in particular.
It was observed that increasing the gap between the two lifting surfaces of a biplane will result in an increase in the total lift coefficient up to a point. A greater rate of increase in lift coefficient as a function of increasing gap is observed until the gap reaches approximately 1 chord length distance. Above one chord length gap, the rate of change of lift coefficient decreases with increasing gap. Further increases in the gap result in minimal interaction between the wings and leads to the lifting surfaces acting individually. Thus, any gap greater than a 1 chord length distance between the individual wings would likely not merit consideration.
Bill Thrailkill might be able to confirm or deny if he found this to be the case in the development of his Twingle design.