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Best lift-to-drag ratio

  1. Feb 7, 2016 #1
    Hi,

    I am trying to find a NACA-aerofoil for a given wing geometry (AR = 8, Taper: 0.35, Cruise CL = 0.05 at 0.6 degrees angle). Do any of you know if it is possible to find an aerofoil that will give this wing the best L/D-ratio at this cruise AoA (0.6 deg)?

    I have done some analysis on several aerofoils on my wing using the XFLR5-software, but the best L/D-drag is usually achieved at an angle of attack of 3-4 degrees.

    Or maybe the only solution would be to change the wing geometry? The cruise lift coefficient of 0.05 is very small, but due to some geometrical problems this can maximum be set to 0.1, giving an AR of 5. But even at this CL, I struggle to find the aerofoil giving the best L/D-ratio, or at least very close to it.

    Hope someone can help me out with this.
     
  2. jcsd
  3. Feb 7, 2016 #2

    CWatters

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    Not an expert but low drag and low lift point to a thin symmetrical section. Perhaps look at NACA 0006 or NACA M1?
     
  4. Feb 7, 2016 #3
    Thank you for your answer. That may be an option, the only problem is that this is a low speed wing, and Im afraid that the thin aerofoil will cause some poor low speed characteristics. So this might be an impossible task if I don't change the geometry.
     
  5. Feb 7, 2016 #4

    CWatters

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    Make it thicker and live with the extra drag?
     
  6. Feb 7, 2016 #5
    Yeah, I guess that is the only solution. Then I won't be able to reach the best L/D-ratio at cruising, by using NACA 64 012 for example
     
  7. Feb 8, 2016 #6
    First of all, XFLR5 is based off of the XFOIL code by Drela. The L/D numbers you are getting are very much idealized. You might see an L/D over 200, when in actuality the best of the best wings will get you an L/D of 60. These are wings on gliders with winglets and an aspect ratio of 20+. When you have a wing with a very high aspect ratio, holding wing area constant you risk decreasing the stall angle because you lower the Reynolds number of the wing. With an aspect ratio of 5, you will probably get an L/D of less than 10. 8 is probably more realistic.

    Also, why do you want an angle of attack of .6? That is probably well within your construction and flight precision limits. And if you want an airfoil with lift coefficient so low, why not just use a symmetric airfoil and to the wing apply a trim angle and twist. XFLR5 lets you play with the twist, so pick something basic like a NACA0012 with trim of 1 degree and see how you do.
     
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