Hi guys , anyone thought of putting horizontal side fins on the flat inside surfaces of side fins? Be they on a thruster or quad.
Depending on the angle of attack you could have a real jumpy board for popping airs or a longboard with a bit of tail hold for nose rides. Being attached to the inside surface, theyd be in a prime place to add control when you bury a rail. And theyre kinda out of the way of stray fingers
Whattayathink, has it been done before ? Any suggestions?
SF.
Experiment with it. It wouldn’t be very difficult to do. You could use an existing board that you are familiar with and attach the side fins. If you mount the side fins with some sort of a screw/bolt you could make them adjustable and thus figure out what angle works best.
waterski folks have done something similar for a while…
JSS
There’s been a variety of ways people have tried to add horizontal control surfaces to rail fins.
These were made in 86
Turbo fins are commercially available with a tube in the middle of the fin, Probox will supply them (and posted pics in a recent thread).
Over sand and stone bottom breaks only; If you’re in the kelp forests forget wings. They hang so easily that you’ll find yourself anchored when you’re paddling in or doing the kelp trick when you least expect it.
I have toyed with wings some and there is potential there, but you have to build it and ride it if you want to know how it works.
So many possibilities ~ so little time.
No Worries, Rich
As applied to surfboard fins that idea is at least 40 years old. They were first commercially available to assist in nose-riding. Two small adjustable fins bolted through opposing sides of a large single skeg.
This is an intriguing idea. Will have to try it myself.
One question.
Why to the inside and not the outside?
Greater water flow in there than on the outside?
The FCS CRV’s are also curved to the inside and I wondered when buying them, why not curved to the outside.
Put 'em on the rail. See the L-fin thread(s).
Think of a fin as a shovel.
Which side holds the most dirt?
Curve the fin tip to the outside
and you’ll dump your load.
When doing the heavy lifting
why not make your job easier?
If not you’ll wind up doing a lot of swimming.
Nothing wrong with that unless you planned on
spending your afternoon surfing.
In which case…if you must have a curved fin
go with the tip curved in. It’ll improve your odds of remaining upright.
Search the internet for the term wing fence.
Yes Surfifty,thats what I was thinking about adding the side-bites at ~90 degrees. Start with a familiar board and fins so any new effect would be simple to evaluate/ adjust.
Maxmercy, thanks for the pic, thats pretty much the idea, maybe epoxy glued on to the fin ? and if the base of the side bite is flared out ( I forget the correct term…?) then there would be a goodly amount of area to adhere to.
The idea is to add an effect for boosts and/or accentuating lift/ hold esp. in the tail. I think that pic shows that the theory is valid and effective.
Blakestah, the blue fin looks more like a blend of thoughts rather than combining 2 seperate ideas, I was thinking of keeping the fins that people like and then adding a seperate effect.
Thanks for the kelp tip Halcyon, I’m thinking of a ‘Spitfire’ shape-- base 1.5",height 2"
If youve got any more tips on the wings you’ve tried, I’d be grateful.
Riddim, I think the in-side is the way to go for a few reasons.1. the effect will be right under the centre of the surfer, where the weight and the rotational axis is. 2 the water that hits the in-side of a side fin is less turbulant than the out-side so any effect should be cleaner.
I always thought water travelled along the chord of a fin rather than from base to tip so I too wonder why the CRVs are curved.
MikeDaniel, Ive read and contributed to the L-fin thread but I was thinking more of a centralised effect directly under the rider.
MarkSpindler, I wasnt altering the lateral hold but thinking of an aircraft pitch type effect. Theres not a lot of work on altering pitch and I hoped that a few of these would give small wave boards a real “lift”.
I’ll make a few of these side-bites, but Ive just had 6 screws put into my scapula last week, so I’ll have a few weeks to think about it first. Any suggestions are most welcome.
SF
Hey SF,
Water skiers use the ‘winglets’ as decelerators mainly. They lift downwards, putting more of the ski in the water, so you can slow down fast enough after crossing the wake to make the bouy without much overshoot. The fin has a winglet on the opposite side as well, to balance the effect for turning to both sides.
Acceleration is mainly handled by the 300-odd horses yanking you out of the turn courtesy of the boat, so no worries. Since you hit almost 50-60mph crossing the wake (you get pulled by the boat at 32-36mph) , you need some serious braking power pronto to turn around the bouy and have any hopes of making the next one. Slalom skis also have a lot of rocker, so the short radius turn can be made at the relatively low speed.
Sounds like you want a supplementary lifting surface to add to the board itself. Only thing I can see as being detrimental is riding such a design in chop, where the angle of attack on the winglets could be going from lifting upwards to downwards and back in a flash, making for an interesting ride. If the angle of attack could be controlled well by rider stance and shifts in weight, it could be a very effective tool.
Otherwise, winglets can be drag savers, cutting down on induced drag, the drag from generating the tip vortex, which is what wing fences try help out. Here’s an example of wing fences, on a MiG-15:
But this is where research and computer modeling is almost necessary, as winglets improperly done can cause more problems than they solve, as they add their own small tip vortices, and interference drag from being ‘attachments’ to the main fin. If those two new sources of drag are significantly smaller than the main tip vortex they quell, it’s a good design.
In the case of the MiG-15, I believe they were added to help stall characteristics after test flights discovered the stalling tendencies of swept wings are not too pilot-friendly, but I’m not completely sure…
JSS
Hey SF,
If you’re up for some reading, here’s a good site on winglets, pros and cons:
http://www.mh-aerotools.de/airfoils/winglets.htm
JSS
I think you should go for it.
Three other thoughts. Thought one, if you’ve never tried one, once your scapula gets better (ouch! is there a good story there?) you should try a shredda…crap, that’s not it, I meant a star-fin. Solo (if he has any in stock) sells star-fins at a nice price. On the right board, you can really feel the wings give you some interesting feelings. Helps fight the tendency to get blown off of waves when the wind is howling off shore, helps keep a board engaged when slogging through the soup, and can give a feeling of lifting the board out of the wave hydrofoil style if you have speed and trim way back (had that happen once…freaky).
Thought two, by having it on the inside of the fin, and by putting it midway up the fin, or even higher, you might avoid a lot of the kelp problems. You could also increase the chord length of the winglets as the fin chord length will be wider, this could allow you to make them less pronounced.
Thought three, I know almost nothing about hydrodynamics, but I do remember some of the crazy fins Larry posted on that historical fins post, including some with “fences”. Might be able to adopt something like that.
Maxmercy, Ive added into your post to add to some of your ideas…
Quote:
They lift downwards, putting more of the ski in the water, so you can slow down fast enough after crossing the wake to make the bouy without much overshoot. Hi Max, thats what I was hoping the effect would be, but also available in reverse, so as you pumped a board along the wave face it would generate extra vertical grip or lift. The fin has a winglet on the opposite side as well, to balance the effect for turning to both sides. I figured if I installed 2 of these side bites then the effect would be doubled on the flat, like take-offs but the effect would still be apparent when leaning over on a bottom turn.
Slalom skis also have a lot of rocker, so the short radius turn can be made at the relatively low speed. Speaking of rocker I thought of setting them parallel to the rocker where the fin is, but do you think a bit (1 or 2 degs) of +ve AOA would hurt or help?
Sounds like you want a supplementary lifting surface to add to the board itself. Yes, the additional lifting surface could hopefully pick up a board on a sluggish wave, and I think most of us encounter that problem far too often.Only thing I can see as being detrimental is riding such a design in chop, where the angle of attack on the winglets could be going from lifting upwards to downwards and back in a flash, making for an interesting ride. Max, are you thinking that the s/bites are flexing? The plan was that they were well fixed to the inside of the side fins so that they didnt flex. Kinda like horizontal speed dialers.(?)If the angle of attack could be controlled well by rider stance and shifts in weight, it could be a very effective tool.
Otherwise, winglets can be drag savers, cutting down on induced drag, the drag from generating the tip vortex, which is what wing fences try help out. Here’s an example of wing fences, on a MiG-15:
JSS
Max, Thanks for all the info and the link, Ive got 3 months on my hands to study/build and hand out a few test models.
Im not sure how close to set them to the base,heres a few options.
Ive mucked around with lifting hydrofoils before so this is a kinda mini submerged version. All suggestions most welcome.
Thanks, SF.
“Speaking of rocker I thought of setting them parallel to the rocker where the fin is, but do you think a bit (1 or 2 degs) of +ve AOA would hurt or help?”
That depends on the overall trim attitude of the board when surfing. What I mean by this is an imaginary line from the nose to tail, and the angle that line makes with the water’s surface. Since rocker is often a continous curve, it would be difficult to say what AOA the winglets would normally assume. The best solution for this is adjustable AOA on the winglets, as this could make or break the design, just as board rocker can make or break the performance of a star fin. The AOA setting is also highly dependent on the foil section. With a symmetrical section, AOA setting would be different than an asymmetrical section.
“Only thing I can see as being detrimental is riding such a design in chop, where the angle of attack on the winglets could be going from lifting upwards to downwards and back in a flash, making for an interesting ride. Max, are you thinking that the s/bites are flexing? The plan was that they were well fixed to the inside of the side fins so that they didnt flex. Kinda like horizontal speed dialers.(?)”
I’m not thinking flex, but instead of the porpoising of the board in choppy conditions, making the winglets run through excursions in angle of attack that could cause more harm than good drag-wise.
As far as foil depth, I would want them deeper rather than shallower, so they see less of the deflections in flow from the board itself, but I really don’t know. It would be great if MTB chimed in, he has lots of experience with hydrofoils.
Another thing to consider is the foil section, area and aspect ratio of the winglets. Too small of either of the last two, and the lift you are making could cost you more than it’s worth in drag, making for a net loss overall…
Please let us know what you find out, this is pretty interesting.
JSS
For what it’s worth, here’s the fence fins from that post I mentioned:
Good point on minimizing drag…these could be drag machines, but relevent to your current endeavor, perhaps.
–BCo
Maxmercy, I see what you mean about the AOA being the same as the angle of the rocker at the fin position, I was suggesting that as a starting point but yes I see how it could make or break the design.
As for the foil section, I was thinking of a fairly neutral or mild Clark Y section which has a bit of +ve lift even at 0 degrees AOA ? Worth a try…
The penny has also dropped about the board riding in chop where the side bites might be skipping in and out of the water, NOW I get it !!
With the question of foil depth, I remember MTB saying that foils lose efficiency when they are at a depth LESS than their chord, so it shouldnt be too hard to work out a few suitable shapes and depths.
And the question of area is made easier to work out as the FCS website surffcs.com (?) has all their fins and their surface areas listed for guidance. Because there would be a side bite on each side fin, then the area of each could be quite small given that their combined area would quickly add up to that of a G7 10137mm,G5 9525mm or PG7 10137mm and a low speeds these bites wouldnt change much but as the speed picked up and the flow became stronger I can see them having an effect as important as an additional fin.
11ft, great pix of the fence fins, Im not sure if they were trying to do the same thing or if the fences were to halt cavitation? I saw something on the Hydroptere and sailboards where they put thin fences in to stop the air being sucked down towards the tip of the fin. But they might have had a milder effect.
I’ll do a few drawings and pass them by you guys, thanks for your input, SF
SF,
To make things easier at first, a simpler foil like the Aquila would be simpler to manufacture, with its flat bottom:
I bet this looks familiar to some people, as a lot of side fins have flat insides. Its performance is similar to the Clark Y at low speeds (lots of lift), but it will generate more drag at higher speeds than the Clark Y. But, it is much easier to make. This type of foil used to dominate in model sailplanes of the 60’s and early 70’s, especially in kits, because of the easy build capability and great slow flight characteristics. That was before folks like Prof Selig went to work researching low Re # stuff. Now foils like this are only found on beginner type aircraft, again due to its slow flight goodness & ease of manufacture.
A great (BUT COMPLEX) solution would be adjustability, but if you are going to make the first few winglets fixed, know that for an asymmetrical foil like the Aquila or Clark Y, its zero lift AOA is slightly negative, around 3 degrees negative compared to the chord line (imaginary line from the very leading edge to very trailing edge of the section), so I’d be hesitant to install it at a positive AOA to begin with, but that would depend on the rocker on the board, so it’s a tough question…
JSS
Are your estimates of lift and drag based on low aspect ratio foils on raked templates, or are they the generic models based on infinite aspect ratios and no rake (which generally hold for aspect ratios >5)?
Great point, my estimates of differences between foils are made using infinite aspect ratio 2D theory/computation (I use profili software). The codes do not take any 3D factors into account (like planform). I think comparisons between foils only hold up when all other variables (like planform and rake) are held constant.
Definitely something to think about, though. Would there be a significant difference between foils at lower aspect ratios and higher rakes even if those and all other variables were held constant during the comparison?
JSS