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Can anyone explain/justify why, using the 'Theory of Wing Sections' by ABBOTT the lift/drag ratios seem so high. For example, I'm doing an analysis of the NACA0012 airfoil and using their experimental data on pg462/463 at 8 degrees CL is roughly 0.9 and CD is roughly 0.001 giving a lift/drag ratio of 90. This seems incredibly high since as far as I'm aware lift/drag ratios go up to 70 to 75 practically from answers I've seen on here. My problem is that my CFD analysis also gives these high lift/drag ratios but I just didn't think it was possible.

Thanks in advance to anyone who can help!

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    $\begingroup$ I would replace "possible" with "practical" $\endgroup$
    – Abdullah
    Mar 3, 2021 at 19:54
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    $\begingroup$ Could you be looking at lift/*parasite* drag only ratios? Induced drag depends on span only, so probably isn't included in 2-D section analysis. $\endgroup$
    – Jan Hudec
    Mar 3, 2021 at 19:58
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    $\begingroup$ @JanHudec yeah I suppose it could be that not all the drag is included since it's a 2D airfoil, are airfoil L/D g ratios generally larger then? $\endgroup$
    – Barnaby
    Mar 3, 2021 at 20:20
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    $\begingroup$ @Barnaby, the lift is approximated well by the 2-D analysis, so if any form of drag is not calculated, the L/D will obviously be higher than the final 3-D wing. $\endgroup$
    – Jan Hudec
    Mar 3, 2021 at 20:28

1 Answer 1

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The difference is that one figure looks at 2D flow around an isolated airfoil, where L/D ratios of up to 200 can be reached, while the other figure is for the 3D flow around a whole airplane, warts and all. The additional sources of drag which are absent in 2D flow around airfoils are:

  • Induced drag. This will double the total drag right at the polar point of best L/D.
  • Fuselage and tail drag. Of course isolated airfoils only have their own surface where friction drag occurs, but all the non-lifting parts on whole airplanes will bring D up without helping L.
  • Lift loss at the tips. L/D for airfoils uses the pressure distribution and maybe a drag rake at the windtunnel center section, so no tip effects will tarnish the result, but the outer wings of airplanes will have reduced lift relative to the local surface and drag because of tip effects and to improve stall behavior.
  • Trim drag: The tail can very well add a downforce to balance the lengthwise lift distribution with the location of the center of gravity. Now extra lift has to be produced by the wing, and both, downforce at the tail and extra lift on the wing, will produce more drag.
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