low-mid speed paddle board hydrofoil fences, vortilons, or nothing

Discussion in 'Hydrodynamics and Aerodynamics' started by container, May 22, 2019.

  1. container
    Joined: May 2019
    Posts: 2
    Likes: 1, Points: 3
    Location: new zealand

    container New Member

    high aspect hydrofoils for surfing has come up in another forum but I figured here might be a better place to source some answers. basically the topic was on the handling performance of high aspect surf foils and their tendency for the wing on the inside of a turn to stall sharply due span wise flow and the poor low speed characteristics of high aspect foils.

    all my questions are based on my experience with my home made foil- 1100mm span, 240mm chord at centre, 50mm of anhedral at the tips and roughly 35mm thick at the centre. take off speed is around 3-4kts in flat water while stand up paddling and it will max out around 18-20kts at a guess (however fast you foil down 6ft wind chop with 40kts of breeze behind you)
    I recently attached marlin lure 'woolies' along one of the tips to study water flow angles while being towed behind a boat at about 6-7kts.
    I found the water flow to be bending inboard slightly (~10 degrees) during straight and level flight (you can see the woolies indicating flow angles of level flight in the 4th picture) parallel to the centre line when the tip is on the outside of a turn, and a complete stall of the outboard half of the wing when on the inside of a turn- maybe the whole inboard wing? i only had woolies on the outer section.

    with this info i put a small fence roughly 100mm inboard from each tip, parallel with the foils centre line. with no numerical info and not enough time with the boat to do another flow test i can only go off feel and intuition. I think that this has made a huge difference to the foil, it feels alot faster and slipperier in flat water and more stable and 'gripped' when at high speed although harder to turn at speed on a wave.
    would more fences- either along the entire top surface of the wing, just around the leading edge and maybe 30% of the chord, or projecting from the underside leading edge a la 'vortilon' do any good?
    purely for maintaining water flow during a tight turn and without hampering the top end speed of these high aspect foils.
    this question is primarily aimed at high aspect wings (they are getting near 10:1 now) but should be roughly accurate for my wing also
    a broad question i know but any information would be much appreciated! i cant seem to answer my own questions with creating more questions 48382679_741290812891884_5408250246784352256_n.jpg 52679048_1379429172199988_2965861083245445120_n.jpg 53202210_356515948409404_2328321218773516288_n.jpg 52843657_358975081624906_1810097877951709184_n.jpg

    note all of my foils are hand shaped and built by eye, no cnc or cad programs. only what i think is right. (leave my fat leading edge alone it works extremely well for lowering the stall speed of the foil)
     
    philSweet likes this.
  2. container
    Joined: May 2019
    Posts: 2
    Likes: 1, Points: 3
    Location: new zealand

    container New Member

    more pics
     

    Attached Files:

  3. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Hopefully, one of our hydro gurus will help you, but until then, it would help if you told us what level of formal hydro or aero background you have. Are you proficient in multivariable calculus, vector field calculus, and vehicle dynamics in fluids? Do you read the technical papers and theses on the subject?

    For starters, the aspect ratio isn't the real issue here. It's just the span. If you shortened the cord on your foil, the aspect ratio would increase some, but then you would have to operate at a higher lift coefficient to compensate. For small variations, these two will cancel one another. So concentrate on comparing span as a primary measure, with aspect ratio being very much a secondary consideration.

    Yes, the topside flow has a spanwise inward component to it if you are generating lift. The bottomside has an similar outward flow. Outboard of the foil, these flows continue as a big swirl from the bottom to the top that is the tip vortex. In the wake behind the foil, there is a shear layer where the top boundary is streaming inboard and the bottom boundary is streaming outboard. There is also a strong downwash behind the wing, so wake velocities viewed from behind show downward and inward above the wake shear layer and downward and outward below the wake shear layer. The downward components are identical and the shear layer is being carried downward intact.

    [​IMG]


    You don't want to try to intercept spanwise flow on the foil at the local level. That creates jumps in the lift distribution and adds to drag. What you want to do is influence the development of this flow over the entire foil. This is what wingtips do, they act like an imaginary extension to the span, and the ideal lift distribution along the span of a foil with tips is almost the same as that of a flat wing with a bit more span. I'd move those fences further outboard, for starters. And I'd splay the top and bottom sides to match the spanwise flow. The streamlines curve spanwise along the foil surfaces. On the topside with the sweepback on your foil, it starts out outboard then curves more and more inboard. Your woolies seem to capture some of that feature.

    To handle stall when turning radii are small compared to the span, you want to keep as much speed on as possible. The major problem here is loss of net speed. The actual lift in a turn isn't compromised very much, except the center of lift moves outboard since the outboard side is traveling faster. With the anhedral you have, that will tend to pull the board to the outside of the turn, and if I'm thinking correctly, increase the angle of attack on the inboard tip. I'd take the droop out of the tip section and/or washout the attack angle at the tips. So reflex the tips, say the outboard 20%, back to zero anhedral, and twist the cord line to a lower AoA over that same span.

    And yes, thin the darn tips out some. The big leading edge radius is hurting you there. Nice job, by the way.
     
    Doug Halsey likes this.
  4. tspeer
    Joined: Feb 2002
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    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    One thing that's not helping is the pointed tips. As the angle of attack increases, a highly tapered tip will become comparatively more loaded than the area inboard. You might try increasing the tip chord and seeing if that helps.
     
  5. markdrela
    Joined: Jun 2004
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    Location: MIT Aero & Astro

    markdrela Senior Member

    Another thing that's not helping is the sweep, which makes the tip loading increase with AoA even faster. The combination of excessive taper and sweep, without any washout twist to compensate, is generally a recipe for tip stall.

    I see no hydrodynamic or structural reason to sweep the wingtips that much -- that just makes the wing worse in every way.
     
  6. Doug Halsey
    Joined: Feb 2007
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    Location: California, USA

    Doug Halsey Senior Member

    There was some NASA-sponsored research about 30 years ago that claimed improved efficiency for the so-called "crescent elliptic wing." Later studies concluded that the result was "probably an artifact of the surface-pressure integration."
    CrescentWing.jpg
     
    Doug Lord likes this.
  7. Barry
    Joined: Mar 2002
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    Barry Senior Member


  8. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Where I park my car at work has the exact same vantage point of the Asheville Airport runway. And it is adjacent to the French Broad River, so we get ground mists most mornings. I get to see these effects almost every day. There's one particular daily commuter pilot that likes long, low approaches. You can just about read the tire labels when he goes by over the approach system which splits the property where I work. You can even see these effects in the stirred-up grass pollen when that one lands. The deer wait for him to land before coming out to graze.
     
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