# Force direction of foiled fins?

[=Black][=1][ 3]I’ve just got my Futures Vector 3/2 fins with the center hatchet from Fiberglass Supply and can’t wait to try them out. My new board should be done in about 2 more weeks. [/]

I have always had a question about the vector foils, though. I haven’t seen it when reading through searches, so I thought I’d ask:

When air passes over a curved airplane wing, due to the curvature and the longer distance required it causes H and L pressure spots, etc and creates lift, allowing the plane to fly. That’s fine and all. Now on a surfboard fin with a concave inside foil, lift is supposedly created. But what direction does this force act? An airplane wing is parallel to the plane, but a surfboard fin is nearly perpendicular to the board – so wouldn’t the lifting force created act to the left or right and parallel to the board’s surface? Wouldn’t this force be acting in a direction that is only useful during turns?[/][/]

Lift is ALLWAYS measured perpendicular to the flow of the fluid. Drag is ALLWAYS measured parrallel to the fluid flow. Check http://www.dreesecode.com/other/aflprimer.pdf for a really good basic outline on foil theory.

All assymetric foils create lift and hence induced drag at zero angle of attack. My take on toe-in is that for the most part it alters the angle of attack to the zero lift zero induced drag point.

(I’m not sure I’m very good at explaining this)

So although lift allways acts perpendicular to flow toe-in means that mean lift and induced drag are zero, and you only get the lift off you side fins when you need it.

A point to note is that different foil sections have different zero lift angles, i.e you might want to change toe-in for different fins, I’d hope that the fin designer would take this into account, but the differences are probably pretty small.

I hope this makes sense?

Quote:
A point to note is that different foil sections have different zero lift angles, i.e you might want to change toe-in for different fins, I'd hope that the fin designer would take this into account, but the differences are probably pretty small.

How much does the zero lift angle change from a double foiled fin to a single foiled fin?

What angles are fins usually toed in at?

The info on the asymetrical foil I have states 2 degrees. I’m guessing this is the angle to produce zero lift.

My text books with all the data are in hiding at the moment so from memeory 2 to 4 degrees. A symetrical fin will have zero lift at zero AOA.

Are these data derived from very high aspect ratio foils or low aspect ratio foils?

Are they altered if one end of the foil is attached to a surfboard?

And, following logic is the best angle for a symmetrically foiled fin 2-4 degrees less toed-in than the best angle for an single foiled fin? (Side fins are usually toed in from 1.6 to 3.2 degrees, or 1/8th inch to 1/4 inch on a 4.5 inch hypotenuse)?

What about using symmetrical foils as the rail fins. The lift produced would be a factor of the angle of attack of the fin. Looking at aerobatic planes, they are the most similar to surfboards in the fact that they work both at positive and negative Angles of Attack (inverted flight) and also routinely vary their AOA very drastically and stall and recover back into lift producing AOA. (think of a snap off the top of a wave).

Just a thought. Here’s the foil I had in mind:

Like blakestah said though, the toe-in would probably need to be adjusted accordingly.

The forces and energy acting on the side fins are asymetrical, hence the need for an asymetrical foil.

A single foiled fin generates better lift:drag ratios at positive AOAs, and its drag is not so bad at negative AOAs if the front edge is radiused. Its a trade-off - the rail fin does most of its work at positive AOAs, so the single foil works.

The concave rail fin develops drag much quicker at negative AOAs. This paper on leopard sharks shows their pectoral fins are concave when generating lift at positive AOAs, and flat on the low pressure side when descending. If only we could change camber and rake of our fins at will - so far we can only change AOA…

http://jeb.biologists.org/cgi/content/abstract/203/15/2261

A single foiled fin generates better lift:drag ratios at positive AOAs, and its drag is not so bad at negative AOAs if the front edge is radiused. Its a trade-off - the rail fin does most of its work at positive AOAs, so the single foil works.

BINGO!

…vector-schmector

although, redirecting the flow on the hp side towards the back of the board instead of towards the rail might be a bit better for thrust and forward drive

onward

I recently picked up an old Infinity from the late 70s which is a precursor to the Secret Weapon. It has a fin setup similar to the current Cluster setup Infinity uses, but the rail fins are symmetric, double foiled with a small amount of toe-in. This has been bugging me, since if the fins really are completely symmetric, the positive angle of attack from the toe-in should be producing a little lift toward the stringer from each rail fin. Sanity check, anyone?

I was thinking about re-foiling the rail fins a little to try and get them to a 0 lift situation for their angle of attack. The rail fin boxes are standard Bahne boxes, so I could always buy a few more fins to try this with. After carefully measuring the foil, I could do a little research on NACA foils to determine what new foil to shoot for. Feedback?

You need to be very carefull comparing wing sections, a lot comes down to Reynolds number. Surfboards operate at very very low Rn’s. As far as the foil is concerned even a relatively slow stunt plane is several orders of magnitude faster, the closest thing in the sky to a surfboard fin is a model airplane and these guys have done loads of work and research surfers could really tap into.

Now this might be contensious but I’m not convinced that fins actually operate at very high angles of attack with respect to the direction fluid flow. Think about it, at the moment of reversing the board direction (snap or 360 or whatever), there probably isn’t that much flow over the foils, in fact a 360 involves popping the fins free.

I really like the idea of partially rotating foils (Blakestah), I’d love to try this on outside fins only such that the lazy or outside fin feathers itself to reduce lift and therefor induced drag.

"You need to be very carefull comparing wing sections, a lot comes down to Reynolds number.

Now this might be contensious but I’m not convinced that fins actually operate at very high angles of attack with respect to the direction fluid flow"

WHO NEEDS TO BE CAREFULL?

Keep researching, but this time try a different tree…

PS - have you actually calculated Re #s?

MeeCrafty,

You got me with your provocative thoughts. Nice brief work. Mark

Here goes:

“have you actually calculated Re #s?”

Anyone can do this. I believe many on the forum here have done the math. You don’t have to be a Naval Architect or a fluid physicist with PhD. Now there are websites that do the math for you. Just plug in the variables and click.

MY brother had me do the math many years ago with a thing called a calculator, and our numbers agree with Simonc. Specifically, my brother and I found model sailplanes and surfboard fins have similar Reynolds numbers. A very solid point to consider throughout is Simonc’s point that, if I may take the liberty, we should be sure we are comparing oranges to oranges. Not just any oranges either, but, first, oranges from the same tree, grove, state etc.

Regarding AOA, how many times does anyone make a hard past 90 degree turn? Good questioin. Kelly? Yes. Plenty of times in a single session, in a three session day, with a higher wave count than anyone out. But when he does, he usually does it on rail, so it’s not all just fins. Side slip can be compensated for with tail plan, tail rocker, tail bottom contour, rail, rail outline template, and chine design. To name a few. Then there is his technique!!! The analysis of which would require a lot more space than available here. Therein lies the real genius. It’s his technique that is still outside the box. Can’t package that, yet.

“WHO NEEDS TO BE CAREFULL?”

Being careful in the bigger picture means applying scientific method to your reasoning. That

is so that no one jumps to conclusions and begins to grandstand. If you want to find out what one thing is doing look at that one thing and compare it to similar one things then only change one thing at a time. But even then some things may not be compatible with others. See, it’s a rather complex formula that some folks here are searching for. In fact it’s so complex that only a few shapers have the slightest grasp of it in all its’ glory and that just may well be the secret to their success. Their designs are mostly from intuition and experience. Some shapers have been very systematic in their approach. They compare different oranges from the same tree.

Scientific method can start with ID or OD. Intuitive data, observable data: Inductive or Deductive reasoning. YOU can form an idea (hypothesis) that you want to test from an idea you may have had (eureka flash whatever it’s Inductive)or observe something in nature deduced from the particulars by observation. IN some sense it’s all observation based, but the subconscious mind may be working overtime causing a eureka. Both are valid approaches, but knowing which one you are testing determines which step one should follow next and how you should approach your data.

“Keep researching, but this time try a different tree…”

This is actually a very good metaphor. I’ve used it throughout. Which tree are you picking your fruit from? All ideas currently being tested and or discussed here are just fine, and why shouldn’t they be? There is a lot of accumulated experience available right here and now, therefore, this forum is a valid source for stimulating ideas. But it’s that next idea no one has had yet that keeps me interested. Not the current retro trend, recycling at it’s commercial finest.

That doesn’t mean I don’t love my retro fish, combining proven ideas from the 70’s 80’s and 90’s in a way that was not possible, until all the ideas were on the table. And, might I add in a way that they don’t conflict. (70’s tail, 80’s deck, 90’s outline and rocker, or something like that.) But then my favorite fish predates the current retro trend, too. (March 2001.)

As for me, I don’t do a lot of testing. At least not any more. Several reasons. One, I can’t take changing on the street in winter anymore. Thirtyeight degree water, thirty degree air cost me the next day. Then as everyone now knows I have a bad shoulder that keeps popping. Another is my ideas are outside the box, and I don’t have many new ones till I get my test results back from my riders on current ideas. However, some other fin companies have chosen to market designs based on some of my earlier ideas. A little of that is okay, because those were just beginner ideas. As for my latest it’s too soon to get excited about, and I only just named it a few weeks ago, but it’s been in testing since last Fall. What’s funny is it has taken almost 9 months.

Anywho, point is ideas and testing take time. So, even though I’m not shaping every minute, that doesn’t stop me from thinking. For whatever it’s worth, ideas don’t just grow on trees, but they grow about as fast. That is why we don’t see a whole lot of new ones.

I wanted to see the theoretical fluid flow past a thin plate inclined at 90 degrees. This situation corresponds to the maximum co-efficient of lift for a thin airfoil in a perfect fluid.

Theoretically the lift co-efficient for a thin plate is: Cl = 2Pisin(Angle of Attack)

Therefore Cl(90 degrees) = 2*Pi

Note: For a thin plate in an ideal fluid the drag is zero and the lift is up.

We all want to find /build the perfect board,BUTTT

the best surfer can ride a DOOR !!!

don’t think so, got to keep trying evene though my name ain’t KELLY.

Shouldn’t the flow round the plate be symetrical?

I think we’re all missing a trick here. Since participating in this forum I’ve taken more interest in fins beyond “why are all surfboards fins such dreadful foils”, and think increasingly we need to be looking at the lift force distribution, angle of attack and minimising drag.

Imagine a bottom turn when you’re pushing as hard as you can. The actual amount of lift is fixed, Newton’s laws mean that the amount of force will be the same as the force put in by the rider. Now because coefficient of lift varies at a particular angle for different foils one fin might have a turning radius greater or less than another, so that’s one variable.

I can’t help thinking that a very important set of variables that really effects the way a board feels is the lift distribution. Think of single fin with an area of X, the force will be distributed in three dimensions in a relatively small space. The depth of the fin will affect efficiency but also the moment required by the rider, a deep fin requires more effort than a shallow fin. The surfer is trying to turn against the fin, with the centre of effort being on the underside of the board somewhere in the tail region. Now with a thruster you have three fins that have a force distribution much closer to the bottom of the board which makes it easier to initiate turns due to the reduced moment, and spread more along the length of the board which should give a smoother ride with more grip than say a twin fin.

In very simple circumstances a single fin producing X amount of lift should produce less drag than any combination of multiple fins. But there are so many more factors involved that multiple fin configurations actually can work better (I know all the debates here!).

To really improve things we need to understand how water flows across the tail of a surfboard in wide variety of circumstances, we need to know the direction and speed. Once we know this we can compare different fin setups more objectively that at present. We should then be able to compare pressure distribution (feel), drag (speed), Cl at AOA (tightness of turns), and stall angle (spin out, snap and slide).

Quote:

Shouldn’t the flow round the plate be symetrical?

I agree…and the lift coefficient should be zero since–by definition–lift is the force perpendicular to the free stream velocity. Moreover it would also be zero for a “thin plate” (technical jargon normally meaning in the limit as the thickness approaches zero) even if the flow field were as shown in the diagram since the only net force generated (for inviscid flow, as assumed in the calculation) is parallel to the free stream velocity.

mtb

** Edit: Upon closer inspection, I see that a circulation has been imposed on the flow, which, in combination with lower bound of the figure apparently being an impermeable boundary, could account for the flow pattern. However, the pattern is still left-right symmetric, which means that the drag of the thin plate would be zero–a situation (arising from assuming an inviscid fluid) that is far from real life (with possibly one exception), where eddy shedding would be occurring with the accompanying production of drag. I also retract my statement that the lift coefficient is zero (which would be the case for a real–i.e. viscid–fluid) since I guess that it is possible that in the limit as the thickness of the plate approaches zero, the flow speeds at the upper and lower ends of the plate could approach infinity in such a way as to yield a finite lift coefficient.

Diagrams should be designed and used to help to clear up matters not add to the confusion. Therefore, I’m not putting a lot of stock in that diagram. It leaves to many things up in the air. Hardly worth attempting to draw any conclusions based on it. I can’t tell what direction the flow is moving in or if the plate is or was moving in the flow or if the plate is being dragged through the flow sideways or straight. Just that there are questions about the symmetry of the flow around the plate is a clear indication of the confusing nature of this diagram. I asked the same question.

I don’t know where it comes from or who made it and there are very few words to help describe what it is attempting to show. It could be from some early software, I don’t know, but it is clearly an oversimplification. There is so much missing we know should be there, for example where are the vortices?

I am avoiding making to much of to little. If the poster (from memory, vincus?) has more information perhaps they could share a little enlightenment with us. I know I could sure use it.