Foils...Long cord length V's wide wingspan ?

Discussion in 'Hydrodynamics and Aerodynamics' started by Dave_S, Oct 26, 2020.

  1. Dave_S
    Joined: Sep 2018
    Posts: 18
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    Location: Brisbane Australia

    Dave_S Junior Member

    What are the advantages and disadvantages to each ?

    Considering a T foil, narrow is stronger, less hangups but everyone seems to go for wide and skinny, I'm guessing for good reason ? Comparing the same profiles and same surface area.
  2. Maarten88
    Joined: Apr 2020
    Posts: 7
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    Location: Netherlands

    Maarten88 Junior Member

    Depends on what you want to accomplish - what is the foil for? Straight line speed, low speed take-off, maneuverability, stability? If you want straight line speed, go for wide and skinny (high aspect). Anything else: it's complicated.

    I designed and made several foils for wing-surfing, and tried to go higher-aspect recently. I have now designed three foils that have about the same surface (1400-1500 cm2) and use the same profiles, but they have different outlines, sweep and anhedral. The low-aspect version has AR of 3.8, the high-aspect has 7.1 and the medium-aspect has 4.7. It's still complicated as there are more variables at play than Aspect Ratio alone and I don't have the time and money to be scientific about this and only change one variable at a time.

    The low aspect foil is very well behaved: it lifts predictably at low speeds, turns really easily in the waves. But it is not very fast, oftentimes waves move faster than the foil and riding them does not work.

    Then I designed a high-aspect foil with less sweep for efficiency and more anhedral to improve maneuverability. It is over 40% more efficient (less drag) at the same lift (in my computer). On the water it has much lower pitch stability and needs a larger stabilizer and longer fuselage (costing performance). Breaching the surface with the wingtips is easily controllable. Steering behavior in the waves is poor - testers disliked this foil.

    The medium aspect wing is being tested now. It fits in the middle between the two earlier designs: more efficient but it also has good-enough turning behavior in the waves. When the wingtip breaches the surface this wing is harder to control: I may need to modify the wingtip design to lower pressures there to fix that.
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  3. Dave_S
    Joined: Sep 2018
    Posts: 18
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    Location: Brisbane Australia

    Dave_S Junior Member

    It's for controlling pitch on a 15m 7.5t catamaran. The objective is to increase speed in all directions and improve tacking angles through pitch control and not looking for lift just countering the pitching.

    I think drag is my biggest issue, it seems to me I'm at the lower end of performance for benifiting from foils.

  4. CocoonCruisers
    Joined: Dec 2015
    Posts: 84
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    Location: Marseille & BuenosAires

    CocoonCruisers Junior Member

    I'd say Lift/Drag is your main concern for that application. Thinness and maxing out the Aspect Ratio will help with that.

    Main tradeoff: thinness and foil length vs structure, rigidity and building difficulty :
    - At some point things will become so tiny that you will have a hard time keeping the wing smooth and matching the ideal section precisely. Boundary layers are picky about such things.
    - At some point it will get expensive.
    - At some point you can't assume rigidity anymore, and if you have to tackle the flexing behavior of the foil so that it won't flutter, you're in for a whole other level of complexity.

    There may also be a point of diminishing returns when your foil gets so efficient that the strut becomes the main source of drag. (You'll need quite a long strut to keep the foils reliably submerged)
    Last edited: Nov 9, 2020
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