Windsurfing Sail and mast design

Discussion in 'Hydrodynamics and Aerodynamics' started by matt_bob, Apr 13, 2015.

  1. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    Also the E396 (available on airfoiltools )very thin like a windsurf rig, but too much camber and low range of AoA with Cl between 0.8 and 1.5
    While you need at most Cl<=0.5 for 20 m/s apparent wind velocity, according to a proxy of your righting moment.
    E396 seems to have a flap pressure distribution, not bad for low drag, so if you de-camber it, increase LE Radius and cook around with inverse method, you should achieve what you need.
    For very low drag wing section look at MH22 and MH30.
    At first glance your problem will not to find horsepower, but minimize drag and make handling easy for the crew.But wing section drag will be only around 50N assuming a Cd=0.03, to be compared with other drags to establish priority.
    As you probably do not need to tack or jibe, the best result regarding drag could be to wrap the crew in an asymetric & inflatable proxy of a wing section.
    Or a loose one which can allow free movements, and like some kites, get its shape with air-intakes, as speed buids up. Also crew stabilizes his position on the board with increasing speed.

    Regarding the rig, appropriate/automatic leech twist in the gusts is probably the main parameter to get right first in order to let the crew concentrate on balance & speed.

    Cheers
    EK
     
  2. matt_bob
    Joined: Apr 2015
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    Location: Pretoria, South Africa

    matt_bob Junior Member

    Hey.

    I managed to create cambers with a very low drag coefficient and lift coefficients spanning a range of 0.8 to 1.8 which is perfectly what I needed. However, at the moment I am using the total lift force as the force in the moment calculation, which is not technically correct as the windsurfer only has to balance the side force of the sail to maintain the not only the angle of attack but also stay upright.

    I am struggling to calculate the angle which will allow me to decompose the lift force into the side and driving forces. I have the true wind velocity, the apparent wind velocity, the true wind angle, the apparent wind angle and the maximum draft is located at 33% aft on all of the batten. I have been trying to find the best way to do this but am coming up empty, any ideas?

    Regards
    Matt

    UPDATE: I managed to find formulas for the side and driving force in Principles of Yacht Design. What is a good ratio to aim for? I currently have a driving force of just under a third the side force... is that good?
     
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    The maximum drive/side force ratio you can get. :)
     
  4. matt_bob
    Joined: Apr 2015
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    Location: Pretoria, South Africa

    matt_bob Junior Member

    So I am designing the cams that are going to induce the camber into the battens by means of an axial displacement. I am struggling to find how much force is needed so that the maximum buckling displacement is the correct displacement to make the correct camber.. any thoughts?

    Picture attached for clarity
     

    Attached Files:


  5. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    In my own words ,I'd formulate your camber problem this way, but I m rookie with structural engineering, so check it with experienced people.

    You have a beam (batten set with camber) subject to compression load which create bending.

    The strain energy from this compression load must be large enough, for the aero loads not to be able to change it.

    Then you'll refine by setting inertia or EI where it's needed to get the right curve

    Cheers

    Erwan
     
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