Hey, Did anyone else see the article about Continental Airlines fitting winglets on their 737’s to improve glide and reduce resistance for improved fuel economy?? 8’tall 4’ wide. They curve up off the end of the wingtips to vertical. They appear to be modifying and/or controlling tip votices. Interesting…How does the tip vortices affect drag on our surfboard fins???
this has come up a number of times before in discussions here; search for “winglets” or “finlets” and you should find those threads.
Right, and a lot of the 747s have 'em. The simplified explanation is that you have less pressure on the top of the wing than the bottom and air flows around the tip towards that low pressure, creating a kind of whirlwind ( the tip vortices or vortexes) and a lot of drag.
http://www.swaylocks.com/cgi-bin/discussion/archive.cgi/read/35663 has a discussion on it, among others. There’s also some similarity with the winged keels that a few racing sailboats use.
Now, the question is, is a surfboard fin ( center fin which is neutrally foiled or side fins that are not but at the same time are toed-in so the overall pressure is at least intended to be neutral *) going to have those pressure differences and thus tip drag? Lets assume straight-line travel on the part of the board and that the centerline of the board is in parallel with that direction …which isn’t necessarily so either.
*overall pressure might be intended to be the same but the centers of pressure/lift would likely differ somewhat, making the possibility of tip drag.
I’ll discount the winged keels on racing sailboats which are ( I think ) more intended to reduce leeway: how a sail propelled boat tends to be blown sideways to some degree.
Okay, given that - the center fin won’t benefit from winglets or tee-like projections from the end. Water pressure is the same on both sides of the fin and the flows should be pretty much symmetrical. Side fins, now, that may be a different story. But then you get another thing happening, assuming you installed winglet-like foils on the ends of those side fins: Are they going to be parallel to the direction of waterflow or not? The angle of attack of the board itself is going to be changing with respect to waterflow…
Thought the change looked very interesting in the photo I saw. A surfboard/thruster is somewhat of a complex arrangement when you break all the aspects down and analize the cause and effect of every design component. At the efficient top end every little change is usually a compromise at the expence of another performance component or design. Constant speed, direction, angle of attack and pressure changes coupled with the desire of all riders to be able to bust it loose in a controlled slide and bite it back in again for the huge sucking barrel wall(without sliding) the current design seems to fill all those requirements pretty well. I’ve seen some video of some of the pros out playing on single fin retros and what we (us older guys) thought used to work pretty good looks squirrely and tedious at best. You can even see some problems with the yellow “Bolt” when Lopez was pullin’ in at Pipe. I love the free release a single fin gives for that weightless down the line …seems I can feel the drag of the thruster cluster when I unweight…mmmmmhard to teach an old dog new tricks when I’ve been used to surfing a particular way for so long. The fin discussion in the other ongoing thread was interesting since I would like to get a little more glide out of the thruster set up without compromising the toe in too much. Going to experiment with the cutaway base on the side fins and see how that feels. All interesting…saw the FCS video Kokua sent (Thanks Mike) and thought the FCS test tank was pretty cool. Although the test fin set up could measure and expose flow characteristics/cavitation on design fins it still was a static set up as far as I could tell. I suppose there is no way to cater to all the dynamic flows involved…there has to be a compromise and leaning towards one characteristic at the expense of another. Just ramblin’ and thinkin’
Of course there is a benefit for surfboards.
The fins don’t do anything unless there is a pressure differential across the two sides.
Anytime there is a pressure differential there will be tip eddies.
Putting something on the end of the fin to prevent flow from crossing sides is an inherent reason for the Downing bulb on the end of the fin, and for the winged keel. The winged keel also generates lift, which complicates seeing the impact only of preventing tip eddies.
Note also that progressive rake and lowering the chord length near the tip are also adaptations to minimize tip eddies. With a winglet there, you ought to be able to have a thicker tip, and less progressive rake.
Aircraft winglets have intrigued me for a while as far as how they could correlate to surfboard fin design. They recapture some of the energy from the wingtip vortices in effect making the wing more effecient.
The current thread on fin design had brought up the issue of tip vortices and this would be one way to minimize them. Granted, what size and shape to use is a whole other issue in itself.
I’ve been riding the FCS g-crv fins for a couple years now and really like how drivey and loose they are. The curve on these fins in effect creates a winglet on the tip of the fin. These fins really opened my eyes to the fact that there are other designs out there that will work well without being the typical flat two-dimensional fin shapes we are so used to.
I’m very curious to make a set of fins that incorporate camber into them to increase their lift efficiency, and I’m thinking that a small winglet would be something else to try down the road. It wouldn’t need to be very large at all due to the density of water, more of a small flip at the tip of the fin.
Here’s an airfoil that would seem to work quite well translated to a fin design:
It has a high lift to drag coeffecient which seems to be ideal for fins.
FYI ~ The barrier island website cross sectional view of a fin foil is very close the the way I foil my rail fins. It is my sense that having one side of the foil close to the mirror image of the other is enhances flow and helps performance as a result.
The double foiled leading edge has a very positive effect as well. I like to ease the trailing edge on the high pressure side a little. It’s a little extra touch that I think smoothing things out.
Will you describe in a little better detail what how these winglets you are refering to are placed and configure?
Boeing 737 uses mvgs too. Grab a window wing view seat next time and try and count them. There are more than 30 per wing.
Here’s a quick primer on some other “old” (grin) technologies. It doesnt have the Sharkskin Speedo link though. Anyone interested can Google that.
I was doing some digging tonight to try and better understand what makes the winglets work. They reduce induced drag and by carefully calculating the toe and cant to create a small forward thrust component they recapture some of the thrust lost due to wingtip vortices. Interestly enough the winglets serve much the same purpose as the two side fins in a thruster setup. Their lift component is used as a forward thrust in much the same way fins are used for drive. Here’s a link I found with a few winglet pictures:
After looking at all the pictures, I couldn’t help but notice the resemblance of all the winglets to the fin template that blakestah posting in the fin design thread. Very similar leading and trailing edge rakes and the aspect ratio even looks similar. Maybe he’s on to something useful there.
Here’s another link with some oversimplified definitions but what I found most interesting is the image of stall progression on varying wing planshapes.
Basically what I found was that to properly integrate a winglet is very tricky at best and you are only gaining 2-7% in lift-drag ratio which would probably be negligible on a surfboard. Most of the benefits in aircraft are in extended range and fuel savings. Probably not of much use in fin design.
edit: spell check
is it possible…
that you coudl actually have different foils on your fins to where lets say it would let it looser backside, but still drivey?
to where your side fins are 2 totally different shapes and/or foils?
i have seen pics of the board that was meant for all left point breaks and i got me thinking…could u dial in your fins to your own personal needs that much?
on a solid bottom turn,is it your rail fin closest to the lip that is generating the drive off the bottom?
sorry if im off? just thinking on fins
Probably not of much use in fin design.
Try one of these. You’ll be surprised.
OK, in this case the winglets are probably adding their own lift and the lack of rake creates a whole different feel alone, but they are fast and fun. Well worth $45.
How flexy are those Starfins?
I ask b/c I hand-made some Lexan fins (cut template from 3/8" sheet, foil by sanding), and they were quite unsatisfactory due to the flex. And I checked the numbers, Lexan is several times more flexible than fiberglass (roughly a factor of 5 compared to a laminated sheet).
I took my initial template from airplane templates, mainly to be able to independently modify leading edge rake and base chord length. So any similarity is probably from my copying. I’d be willing to be the airplane wing designers chose the templates for the same reason. But to do a nicely controlled set, I make the templates on the computer with known geometric relations, print em, trace them on fiberglass plate, and go from there.
So, progressive rake is thrown out, as well as any potential benefits from that. If anyone is a strong proponent of progressive rake in a fin, I’d like to hear it. At least, I’d probably test a set.
But now I’m playing more with the trailing cutaway - to try to move the long chord of the foil about 2 inches away from the surfboard, while maintaining rake to avoid eddies on the cutaways. The last one before this was the same fin with roughly half the rear cutaway size.
It’s all an effort to try to get the best drive characteristics out of a fixed area. And, to inhale lots of sanding dust!
Not much flex in them at all. The newer Starfins are molded plastic; Lexan was used in the 80’s (90’s ??).
The stiffness, depth (leverage) and upright design of these things can weaken a normally glassed finbox. Just my unscientific observations.
Future fins have side fins that are foiled on both sides, go to futurefins.com, click on the “vector” link. They don’t show much, but try to find one at a shop to look at they are cool looking and they have a cross section view that is similar to Lawless’ last picture.