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rtz
Joined: 31 Oct 2010
Posts: 296
Location: Oklahoma City
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 Posted: Sun Mar 24, 2019 3:04 pm Post subject: Lift from fin? |
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| I often read about fins providing lift. On an airplane; the vertical tail doesn't provide lift; so how does the vertical fin make lift? |
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DanWeiss
Joined: 24 Jun 2008
Posts: 2296
Location: Connecticut, USA
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| Posted: Sun Mar 24, 2019 4:18 pm Post subject: |
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Angle of attack vs actual direction of travel. That gets the windsurfer upwind.
Large fins like the soft, FW fins clearly flex longitudinally. This flex works like a foil, adding vertical lift to the equation. Racers often prefer softer fins in very light conditions because the vertical lift component helps keep the waterline back, something critical on high aspect planing surfaces. These are those with a wide but short planing surface. If the wetted surface moves forward, the board's drag increases very quickly compared to a low aspect design, like the slalom boards of the late 1980's that slowed down more gradually but were less stable in optimum trim.
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U2U2U2
Joined: 06 Jul 2001
Posts: 5467
Location: Shipsterns Bluff, Tasmania. Colorado
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| Posted: Sun Mar 24, 2019 6:10 pm Post subject: |
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| DanWeiss wrote: | Angle of attack vs actual direction of travel. That gets the windsurfer upwind.
Large fins like the soft, FW fins clearly flex longitudinally. This flex works like a foil, adding vertical lift to the equation. Racers often prefer softer fins in very light conditions because the vertical lift component helps keep the waterline back, something critical on high aspect planing surfaces. These are those with a wide but short planing surface. If the wetted surface moves forward, the board's drag increases very quickly compared to a low aspect design, like the slalom boards of the late 1980's that slowed down more gradually but were less stable in optimum trim. |
I like this explanation. I prefer drive to lift.
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swchandler
Joined: 08 Nov 1993
Posts: 10588
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| Posted: Sun Mar 24, 2019 8:23 pm Post subject: |
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As I like to look at it, it's a balance between the sail and the fin. Of course, the board and its planning surface come into play, but I still think that there needs to be a balance that leads to power and direction. Ideally, the fin's lift establishes the needed power balance with the sail.
I have to say that I look at it abstractly, because the science easily gets lost in the details. |
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westender
Joined: 02 Aug 2007
Posts: 1288
Location: Portland / Gorge
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rigitrite
Joined: 19 Sep 2007
Posts: 520
Location: Kansas City
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| Posted: Mon Mar 25, 2019 6:28 am Post subject: Re: Lift from fin? |
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| rtz wrote: | | I often read about fins providing lift. On an airplane; the vertical tail doesn't provide lift; so how does the vertical fin make lift? |
(See's can of worms open and spilled all over the ground): "Goddamit! Who did this?!?!"
The BEST answer is: "It doesn't matter, and whatever the answer is, it won't effect how you sail"
So, symetrical foils can create lift: some helicopter blades are made this way, as are the dive planes on submarines. However, they only create true lift within a VERY narrow angle of attack, like a couple of degrees, before they stall.
I think that on a windsurfer that the fin does not create lift, the SAIL is what gets you upwind, and the fin is governed by viscous forces (prevents side-slip). If you sail a freestyle fin, you can really see how much side-slip you get vs. a slalom fin at speed, but when you're slogging, they both are about equally ineffective. An experiment [/i]could[i] be devised to figure this out, but it would be expensive and difficult to set up.
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bericw
Joined: 07 Mar 2006
Posts: 90
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| Posted: Mon Mar 25, 2019 8:57 am Post subject: |
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You have perhaps already read,
https://en.m.wikipedia.org/wiki/Tailplane
As previously noted in related discussions here, the fluid dynamics definition of lift force is simply the force normal to the fin surface, whereas drag force is the force parallel to the fin surface. There are pressure and viscous force terms in both lift and drag. Orientation affects the magnitude in a given fluid. One may consider any text similar to Engineering Fluid Mechanics, John A. Roberson, Clayton T. Crowe, any edition, for definitions of and expressions for lift and drag. Or, more immediately,
https://en.m.wikipedia.org/wiki/Lift_(force)
Specific to windsurfing fins, please forgive me for again providing this cool reference:
https://www.google.com/url?sa=t&source=web&rct=j&url=http://eprints.bournemouth.ac.uk/12235/1/Simon_Fagg.pdf&ved=2ahUKEwi2me-Sx-zbAhXH5IMKHaJpCuIQFjAAegQIBRAB&usg=AOvVaw2UnL7FZRJQ-f3oqYHsIfWO
which provides the thesis of Simon Barry Fagg, Bournemouth University, 1997, which ihas numerous references related to lift on "fins". Statements are made that describe that the primary role of a modern planing hull windsurfing fin is to provide a lifting force (the force normal, i.e., perpendicular, to a surface, reagardless of that surface's orientation to gravity) to counteract the sideways force of the sail as well as provide directional control and stability. The magnitude of the lift force on a fin on a planing windsurf board is referenced to vary between 300 and 600 N, while the drag force on the fin, 1/6 of the total drag on the system, varies in magnitude from 20 to 50 N.
Have a great day everyone. |
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bericw
Joined: 07 Mar 2006
Posts: 90
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| Posted: Mon Mar 25, 2019 9:15 am Post subject: |
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Incidently and off-topic, rigitrite, I know you're going for it on the water! Please let me know if you are ever in the area and want to sail the eastern gorge. Yeah buddy!
Finally got another day on the water a couple weeks ago after a drought from the last days of west wind out here which were around Christmas. Summer is coming! Hope you're getting some! |
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DanWeiss
Joined: 24 Jun 2008
Posts: 2296
Location: Connecticut, USA
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| Posted: Mon Mar 25, 2019 9:33 am Post subject: Re: Lift from fin? |
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| rigitrite wrote: | SNIP
I think that on a windsurfer that the fin does not create lift, the SAIL is what gets you upwind, and the fin is governed by viscous forces (prevents side-slip). If you sail a freestyle fin, you can really see how much side-slip you get vs. a slalom fin at speed, but when you're slogging, they both are about equally ineffective. An experiment [/i]could[i] be devised to figure this out, but it would be expensive and difficult to set up. |
I whole heartedly disagree as I venture would every shaper, fin designer and naval architect.
Some fins are designed to stall incrementally, like freestyle fins, whereas race fins are designed to hold and hold and hold 'till nearly impossible odds. The former lets go without fanfare, the latter blows out and can break ankles.
All foils (that's what fins are, of course) require a certain amount of flow per minute to produce sufficient lift to counterbalance the forces placed upon the fin by the board. Without sufficient flow, fins wash out and force us to change the forces imparted to the fin, including holding its angle of attack at a very high angle until the rig/we generate more power to increase the flow across the foil.
The sail does provide lift going upwind, but really no more than the fin generates through the water flowing past it if for no other reason than the difference in density between air and water. The low density of air requires a comparatively monstrous difference in size of respective foils. A FW race fin flowing through water at 25 mph will generate more lift per sq. meter than a 747 wing develops when flying through the air at 400 mph -or so I'm told.
In any case, if you don't believe fins contribute at least as much lift to the total windsurfing experience as the sail, I suggest the following experiment.
Sail a FW board and appropriately sized sail (11.0+) with a 70cm race fin. Assume conditions of flat water with a 10 to 15 knot breeze. The gear will rip upwind and downwind at angles that will astonish folks new to FW equipment. Change the fin to a 66cm. Report your observations.
Then do the opposite: select an 8.0 race sail and use with the 70cm fin. You will find it more difficult to get onto a plane and in your straps, but once there you will accelerate and find your pointing angle about the same as the larger sail. You will also maintain your angle in lulls, just not quite as long in the lowest limit of wind speed.
I suggest you will find the small fin forces the board to plane later, and will take you longer to jam on the harness and extend your arms. You will be able to sail upwind, but not at the same angle -especially when sailing in lulls. In contrast, the small sail/70cm fin combination will sail at the same angle as it did with the larger sail. Conclusion: the experiment confirms that the fin is directly responsible for much of the overall lift and resultant tacking angle ie., the angle between pointing on port and pointing on starboard. A markedly smaller fin will increase that taking angle when compared to the 70cm standard fin but a markedly smaller sail affects the upwind performance do a much lower degree (if at all) than the smaller fin.
As for any difference between resisting sideslip and lift, there is none. These terms are synonymous. The fin resists sideslip because it generates lift. The lift occurs even while schlogging as long the fin moves through the water with minimal speed. As you note, some fins generate more lift at lower speeds, like the low aspect freestyle fins. A race fin of the same surface area requires more speed to generate the same lift yet has a far superior L ratio when moving at a higher speed, resulting in a much higher top speed and ease in further acceleration when sailing powered up. The freestyle fin in contrast will generate significantly more drag but not much more lift as speeds exceed 20 mph. It will start to ventilate before a race fin but any resultant spinout will be much easier to catch on the freestyle fin because flow attaches on it at a much higher angle of attack than on a race fin -typically.
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isobars
Joined: 12 Dec 1999
Posts: 20935
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| Posted: Mon Mar 25, 2019 10:16 am Post subject: |
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You guys seem to have missed one vital factor: As discussed often here and in windsurfing magazines over the years, lift, in this context, refers to lateral/upwind, not vertical, force.
From Fagg:
1.1.3 The Role of the Windsurf Fin
The primary role of the windsurf fin is to produce a lifting force to counteract the sideways force of the sail, as well as providing directional control and
Without the fin the complete assembly would be pushed downwind under the action of the sail. With the modern planing hull designs the wetted length is reduced to a minimum (as compared with a displacement type hull), thereby elevating the role of the fin in generating the required sideways force.
Google provides many sources explaining how planes can fly upside down. Pick your favorite. |
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