Tornado concave naca rudder section

Discussion in 'Multihulls' started by k2mav, Jun 1, 2008.

  1. k2mav
    Joined: Jun 2008
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    k2mav Junior Member

    Which are the benefits of concave sections in current Marstrom Tornado rudders? Are they naca0009 or 0012 sections?

    Being searching info here and other technical sites without success.
    Cheers,
    Martin
     
  2. TTS
    Joined: Jul 2007
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    TTS Senior Member

    http://groups.yahoo.com/group/TornadoCat/
    Try posting this question on the tornado Yahoo group. Above is the link and you will find that many of the members are sailing tornados at or close to the Olympic level and can answer this question. Goodluck.
     
  3. k2mav
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    k2mav Junior Member

    Tom

    Maybe the 2nd part of my question could be answered there, but the 1st for sure can be found here better than any other place. I'm member of that forum too.

    To rephrase "Which are the benefits of having a concave section in rudders in a Tornado type multihulls??"

    Thanks for your help
     
  4. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    They can't be NACA 4-digit sections if their thickness distributions are concave.

    Concave thickness distributions result from designing a section that has a rapid pressure increase that blends into a more gentle pressure increase toward the trailing edge. Typically the pressure distribution ahead of the rapid increase is nearly a constant pressure at the design angle of attack or lift coefficient. This is known as a "rooftop" section.

    The design philosophy behind the rooftop section goes like this:
    - At high speeds, transition from laminar to turbulent flow occurs when disturbances in the boundary layer get amplified until they kink up into the complicated structures that form the turbulent boundary layer. These disturbances get amplified rapidly if the velocity is decreasing (pressure is increasing), and can even be dampened somewhat if the velocity is increasing.
    - However, at the trailing edge, the velocity outside the boundary layer has to get back down to a level that is close to, and a little slower than, the free-stream velocity.
    - The higher the maximum velocity on the surface, the longer the portion of the chord has to be that is devoted to slowing the flow down to get to the trailing edge without separating.
    - So a good compromise is to accelerate rapidly to the maximum velocity near the leading edge, then hold that velocity as far back as possible, then decelerate the flow as hard as the boundary layer will allow to finish up at the trailing edge.

    It turns out the minimum distance to decelerate the flow from a given velocity to lower velocity is accomplished when the boundary layer has a constant margin from separation along the whole "pressure recovery" region. When the pressure recovery starts, the boundary layer is comparatively fresh and energetic. It can tolerate a rapid pressure increase without separation. However, as it goes along, the boundary layer thickens and the velocity profile in the boundary layer becomes more prone to separation, and it can't tolerate as high a rate of increase in the pressure. So the deceleration of the flow has to flatten out.

    When you calculate the shape that will result in this kind of pressure distribution, using a program like XFOIL, what you get is a shape with a hollow in the aft part of the section.

    Now the rooftop design philosophy works for high-speed sections - say, Reynolds number greater than 2 million - but it's not a good way to go for low Reynolds number sections - say, under 500,000. The rapid pressure increase at the start of the pressure recovery will lead to a long laminar separation bubble that is prone to bursting - causing a dramatic stall - and adds drag. If you want a good all-round section at only a modest increase in minimum drag, it's better to have a more rounded shape to the pressure distribution.

    The design philosophy here is to create a short laminar separation bubble that is positioned well aft on the section at low lift coefficients, providing lots of laminar flow for low drag, but moves smoothly forward as lift increases, trading laminar flow for a longer, more robust pressure recovery. Such a section "shifts gears" automatically and will work well over a wide range of speeds. This is crucial for a sailboat, because the section has to work in light winds and while tacking as well as when blasting to windward in a breeze.

    Today, it's not necessary to be restricted to choosing sections from a catalog. Instead, catalog sections should be used as a starting point, and an inverse design code like XFOIL used to design a custom section that meets your specific design requirements.
     
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  5. k2mav
    Joined: Jun 2008
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    k2mav Junior Member

    Thanks Tom! That was the technical answer I was looking for.
    For naca 0009 or 0012 I was talking about that initial thickness ratio / catalog section used in the T. Cause basically to me the Marstrom rudders are an "extended/modified" standard section as you point.
     

  6. tspeer
    Joined: Feb 2002
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    tspeer Senior Member

    You're welcome.

    Because of the symmetrical shape of boards and rudders, it's hard to operate in the "laminar bucket" because it has to be centered on zero lift. So all you have is the width of the bucket. And a multihull operates with the board loaded all the time, since they don't sail DDW if they can help it.

    The NACA 4-digit sections are actually pretty good when you look at medium lift coefficients instead of minimum drag. The jump in drag outside the bucket can double the profile drag, giving a "laminar flow" section more drag at the operating point than a more conservative section.

    I should add to the discussion of design philosophies above, the shaping of the leading edge. A pure rooftop design, in which the velocity distribution rises almost vertically right to the rooftop level, will develop a high, sharp leading edge suction peak when operated above the design angle of attack. This is bad news with regard to laminar separation, and can lead to a nasty leading edge stall.

    Besides rounding the pressure distribution to blend from the rooftop to the pressure distribution, it's good to design the leading edge for a higher angle of attack than the design angle used for the rooftop itself. By looking at a higher angle of attack and blunting the leading edge pressure peak somewhat, you'll form a more rounded leading edge pressure distribution at the design angle of attack.

    Notice there's no mention of leading edge radius in all of this. The circular leading edge radius was something NACA used because the formula for their pressure distribution didn't encompass the entire leading edge. A circular leading edge is actually a bad idea because almost by definition there's an abrupt change in curvature between the radius of the leading edge and the rest of the section shape. The inverse methods, like XFOIL's MDES mode, explicitly shape the entire leading edge and no geometric modification is required or desired.
     
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