Bell shaped lift distribution

Discussion in 'Hydrodynamics and Aerodynamics' started by container, May 19, 2020.

  1. container
    Joined: May 2019
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    container Junior Member

    Has anyone seen a foil using bell shaped lift distribution? I have been looking at Al Bowers' prandtl flying wing, for a downwind foiling paddleboard the concept of wing tips producing thrust sounds very interesting.
     
  2. CocoonCruisers
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    CocoonCruisers Junior Member

  3. container
    Joined: May 2019
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    container Junior Member

    Thanks I had found that just after posting this! although the talk there seems to be more on how to apply the theory to sails instead of hydrofoils. The equations all fly right over my head but I do agree with the basic principles- linear downwash distribution, not linear twist and load distribution.

    As there is an immense amount of talent and knowledge here I was just curious if anyone had seen it successfully applied to a foil - T shape, not L or J
    I should add I have already designed the foil (based on numbers defining twist/AoI given to me by Al Bowers himself) and started construction, I am only after opinions that arent mine!

    I will post some pics as soon as i can figure out how
     
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  4. Eric ruttan
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    Eric ruttan Senior Member

    It seems, if you can do an L, you would never choose a T, right? You loose a high drag corner going to a L
    And if you can make a curve work, you would choose that Overany hard corner, right?
     
  5. container
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    container Junior Member

    Maybe so but this is for a stand up paddleboard so unless i want 2 masts/struts then the foil needs 2 tips
     
  6. patzefran
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    patzefran patzefran

    Foiling stand up paddleboard !!!!!!! oops.....................!
    You need first to improve the paddle tip !
     
  7. Eric ruttan
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    Eric ruttan Senior Member

    Wow. So, lets assume your paddle board is perfect for the foiling you want to do.
    Why do you need '2 tips'?
     
    Last edited: May 20, 2020
  8. container
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    container Junior Member

    sounds like scepticism! getting a paddleboard to foil is actually quite easy if you're young and fit. to back up my outrageous and unproven claims here is me doing a flat water start on a hand made foil. the board is my creation too.



    and this is a typical downwind run. as the bell load wing will have more span it wont be as maneuverable but it is intended to tick off the miles on long straight runs ~ 20-30nm



    and to be fair, you dont even need a paddleboard. a piece of plywood big enough to stand on and something to jump off is perfectly adequate. just dont fall off once you start!

     
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  9. container
    Joined: May 2019
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    container Junior Member

    As you can see in the vids it actually has 4 wing tips! Maybe a foil version of Al Bowers flying wing could do away with the horizontal stabilizer completely?

    <photos added to post using "Upload a File" near reply area>
     

    Attached Files:

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  10. Eric ruttan
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    Eric ruttan Senior Member

    I can not say no, but i think you kinda want the tail to get your pitch osolating movement that produces thrust.

    But I think you only need a half span wing and tail.

    Typically one wants symetrical wings, but you are balancing the roll and the pitch using the board.

    So, assuming you have the stiffness, one for tip and one aft tip should be less drag. The upright comes up offset to one side, but other than looking odd, perhaps, it should be better.
     
    Last edited: May 20, 2020
  11. container
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    container Junior Member

    that wouldn't work. you are trying to balance a board on a stick while hurtling down, around and over waves. you need symmetry.

    before we get carried away reinventing the wheel, I was purely asking if anyone had seen Prandtl/Bowers theory used on a hydrofoil. ideally pictures of streamlines on tank test models or the wakes of such foils in use
     
    Last edited: May 20, 2020
  12. patzefran
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    patzefran patzefran


    Congratulations !
    Two thousands years ago, this would have been looked as a miracle !
     
  13. Doug Halsey
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    Doug Halsey Senior Member

    I don't want to dampen your enthusiasm, but I don't think there is a planform that results in a bell-shaped spanload distribution under all conditions. Instead, any given planform must be twisted to obtain that distribution under whatever conditions it is encountering.

    That might be achievable in a sail if you had total control of its twist, but wouldn't work with a rigid foil.

    Aircraft attempting endurance records (as featured in Bowers' papers) can adjust their speed to allow constant lift coefficient as their weight varies (due to burning fuel), thereby staying near their optimum loading, but a foil will need to use different lift coefficient to support a constant weight at different speeds.
     
    Last edited: May 21, 2020
  14. container
    Joined: May 2019
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    container Junior Member

    Thanks doug that does make sense. the spread sheet Al gave me was based on a 1 metre span foil doing 32 km/h carrying 90kg I think. the total twist is 11 degrees from the centre to the tip ( +9, -2) I stretched the span out to 1.4 metres (1 metre is a bit small to be giving away area to downforce) and reduced the chord to keep the surface area around 1500-1700 cm/2. Ar is 14 if my numbers are correct. span squared divided by area? it is only an approximation off the drawings so dont burn me at the stake for not taking into account surface curvature etc.

    speeds are usually between 8-15kts on a downwinder, although Al's guess was on the high side it is still realistic. I am also able to greatly vary the total lift of the foil by changing tail wings or tail wing angle (less downforce on the tail, less total lift) and can actively adjust effective load on the foil by moving my weight forward or back on the board. this works well as by taking a small step forward you can essentially double the load on the foil, keeping it in the water when you drop down a wave and your speed nearly doubles.

    I think this design could very well work for tail less foils as the tips are performing the same function as the tail wing (at our chosen speed of course, this idea is only for downwind racing. who can do 20-30-40nm the quickest. simple.. right?), and if theyre swept back to a position behind the mast then that downforce will be aft of the CoE, and out in clean water.
    I know nothing is ever black and white, especially when it comes to wing design and fluid dynamics. but I also know its not impossible and thats good enough odds for me.

    is there a way of uploading pictures directly to this post? other than adding them to site media and all that jazz
     

  15. container
    Joined: May 2019
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    container Junior Member

    what, the fact youre talking to someone on the other side of the world via a piece of plastic in your pocket?
     
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