Gunaca aerofoils

Discussion in 'Boat Design' started by sorenfdk, Mar 9, 2010.

  1. sorenfdk
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    sorenfdk Yacht Designer

    Does anyone have any information about or experience with the Gunaca 23-series of aerofoils? If so, I'd like to hear about it.

    And if possible I would also like to have some coordinates, please.
     
  2. daiquiri
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    daiquiri Engineering and Design

    I don't have the required data (sorry), but I do recall reading something about Gunaca foils. They should be a variation of NACA 6-series, with pretty similar aerodynamic characteristics.
     
  3. tspeer
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    tspeer Senior Member

    Search for GU or Glasgow University airfoils.

    Wind tunnel test data for the GU 25-5(11)8 section may be found in Kelling, F.H., "Experimental Investigation of a High-Lift Low-Drag Aerofoil," Report No. 6802, Department of Aeronautics and Fluid Mechanics, University of Glasgow, Sept 1968.

    Coordinates, inviscid aerodynamic characteristics, and the Algol code for generating and analyzing the airfoils can be found in Nonweiler, T., "A New Series of Low Drag Aerofoils," Report No. 6801, Department of Aeronautics and Fluid Mechanics, University of Glasgow, March1968.

    Also see ESDU 99003, "Generation of GUNACA 23-series of low-drag aerofoils."

    The GU airfoils were one of the first efforts to use Lighthill's method of inverse airfoil design to generate sections whose pressure distributions had flat roof-tops and concave pressure recoveries. In practice, they proved to be sensitive to contamination of the laminar roof-top segment, leading to separation of the pressure recovery. They were used as the canard section of some early Varieze aircraft, leading to unsatisfactory handling qualities in rain.

    If you have the coordinates, you can use XFOIL to analyze the performance at your design Reynolds numbers, etc.

    There have been a lot of lessons learned in airfoil design since the GU series. If you are thinking of using the GU 23, I would use it as a starting point in XFOIL (or Profili) to design a custom section tailored to your requirements. You can round off the sudden break between the rooftop and pressure recovery to create a transition ramp that will foster a short laminar separation bubble instead of experiencing massive laminar separation at the break. And you can flatten the pressure recovery so you get a more gentle stall. These changes will make the section less sensitive to early transition (such as caused by contamination of the rooftop).

    There are also other high-lift sections you might want to consider. Eppler has a number, as well as Wortmann.
     
  4. sorenfdk
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    sorenfdk Yacht Designer

    The reason I asked is that I found a copy of ESDU 99003 and wanted to analyze a GU23 aerofoil in Profili (just like Tom proposes).
    It's just that without the coordinates, this is difficult...

    I already have the coordinates for the GU25 aerofoil, so I'll use those instead.
     
  5. daiquiri
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    daiquiri Engineering and Design

    A bit off-topic but... Mr Speer, you seem to be the right person whom I can ask for the following info. :)
    Do you happen to have some data regarding the influence of surface contamination on aerodynamic characteristics of airfoils designed with Stratford pressure recovery ramp? Does it still work as designed in case of real-life, rough wing or keel surfaces, or do contaminations induce a premature separation of the flow, as I would expect? And, finally, are there some recommendations you could share about a design of a reasonably "robust" recovery ramp?
     
  6. tspeer
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    tspeer Senior Member

    I don't have any data at hand regarding surface roughness and the Stratford distribution. But the effect isn't likely to be good! However, it is possible to make the Stratford distribution more robust.

    The Stratford distribution has the form:

    Cpbar*sqrt(x*(dCpbar/dx))/(Re*10^-6)^0.1 = S

    where
    Cpbar =1-(Ue/U0)^2
    Ue = local velocity at the edge of the boundary layer
    U0 = freestream velocity
    Re = chord Reynolds number
    x = non-dimensional distance along surface, with 0 = start of the pressure recovery

    Stratford's equation was intended to predict separation, with S ~= 0.35. However, if you use the equation backwards and calculate Cp vs x such that S=0.35 everywhere, then you get a pressure distribution that is on the verge of separation everywhere. This is the shortest, steepest way to get from a faster local velocity to a slower one.

    However, S can also be taken as the margin of separation. If you choose a value of S that less than 0.35 you will get a pressure distribution that is not as steep and will tolerate some contamination before it separates. (Or you can design for a Reynolds number that is less than your actual design Reynolds number range, and that will do the same thing.) You can tailor how much margin you have by the value you pick for S. Unfortunately, I can't tell you how much roughness will cause separation at a given value of S.

    As for what is a good transition ramp, that will depend on the Reynolds number and your lift range. The lower the Reynolds number, the longer the ramp has to be. Instead of having the abrupt increase in pressure generated at the beginning of the Stratford distribution, round off the pressure distribution between the rooftop and the pressure recovery. The transition point should move smoothly forward as the angle of attack increases. At the low end of the lift range transition should occur at the end of the ramp region just before starting the pressure recovery proper. This will avoid having a long separation bubble caused by the abrupt start of the recovery causing laminar separation.

    When laminar separation occurs, the pressure is approximately constant until the flow transitions to turbulent, at which point the pressure increases in basically a Stratford distribution (with zero margin). If that increase in pressure intercepts the inviscid pressure distribution, the flow reattaches and forms a laminar separation bubble. If it doesn't intercept the inviscid pressure distribution, then the result is massive laminar separation and stall.

    If the inviscid pressure distribution is already a Stratford distribution, when the sudden increase in pressure triggers laminar separation, it's like a ski-jumper sailing almost parallel to the slope and landing way down pressure recovery region, forming a long separation bubble. By using a convex pressure distribution for the transition area, the modest pressure increase triggers laminar separation and the convex pressure distribution is like a mound in front of the jumper that makes it impossible to jump very far without coming back in contact with the surface before the big drop-off occurs.

    The point at which the pressure gradient is adverse enough to cause laminar separation will roll forward on the convex transition area as an increase in lift makes the pressure gradients over the entire surface become more adverse. With a flat rooftop and no transition ramp, the boundary layer stays laminar until it drops off the cliff at the beginning of the recovery region. That remains the case until the gradient on the backside of the leading edge pressure peak gets to be enough to trigger laminar separation, then the transition point suddenly jumps from the start of the recovery region to the leading edge suction peak. If the peak is too short and sharp, the flow won't reattach and you get sudden leading edge stall. That's why sudden movement of the transition point is an indicator of trouble.

    Bottom line is one does a lot of whittling and shaping of the leading edge, transition ramp and pressure recovery when designing a section. If separation is occurring at the trailing edge at too low an angle of attack, you need to flatten the pressure gradient there, which means making the recovery steeper ahead of that area. If there is a long separation bubble or sudden changes in transition, the transition ramp needs to be rounded more. If transition is occurring too soon, then the ramp can be shortened and the curvature of the pressure distribution increased a bit. It helps to analyze the leading edge at a higher angle of attack and then chop off the top of the pressure peak. That will make for a more rounded leading edge and broaden the pressure peak so it doesn't promote leading edge stall.

    A Stratford pressure distribution may make a good starting point, but the pressure distribution should be tailored from there to get the characteristics you want.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Great explanation, thank you very much. :)
     

  8. Ilan Voyager
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    Ilan Voyager Senior Member

    Thanks TSpeer. Very interesting explanation.
     
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