XFLR5 and inverted airfoils

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Aerowerx, Sep 22, 2012.

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  1. Sep 22, 2012 #1

    Aerowerx

    Aerowerx

    Aerowerx

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    I have seen that some designs use an inverted airfoil for the horizontal tail. The Zenith STOL designs come to mind.

    Has anyone modeled this idea in XFLR5? I have tried it, but the outline looks like a normal airfoil.

    Is this just the displayed outline and the calculations use the inverted foil, or can't XFLR5 handle the inversion?
     
  2. Sep 22, 2012 #2

    Hot Wings

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    XFLR5 V6 will show an inverted file just fine.

    Take the ordinates of the foil you want to invert and input the data into Excel or the OpenOffice equivalent. Make a new column (D2) using =Sum(-1*b2) to copy the right hand column and multiply it by negative one. This inverts the file under column D2. Copy the left column with file name in A1 to column (C1). Copy both column C+D. Paste into notepad. Save as "airfoil file XYZ.DAT" using the " " around the whole name. This should save the file as in inverted file under the name airfoil file XYZ with a DAT extension.

    Remember to rename the ordinate file name in cell C1 to avoid confusion down the road.

    Note: a simple data sort does end up with a "normal" looking airfoil.
     
    Last edited: Sep 22, 2012
  3. Sep 22, 2012 #3

    Aerowerx

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    I have some data that I inverted.

    My question was not how to do it, but if XFLR5 handles it correctly even if it does not look inverted.

    Not sure what version I have. Will have to check.
     
  4. Sep 22, 2012 #4

    wsimpso1

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    Why should it care which way is up?
     
  5. Sep 22, 2012 #5

    Aerowerx

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    If you follow the link in my orginal post you will see that Zenith uses a nonsymmetric inverted airfoil for the H-stab. This gives them more H-tail authority at the low speeds they need for STOL performance.

    I would also think that it would allow you to use a smaller H-stab and/or shorter tail for a non-STOL design.
     
  6. Sep 22, 2012 #6

    highspeed

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    What he's getting at is that the airfoil doesn't care which way is up. All it cares about is the angle of attack and velocity. The data will be the same whether the foil is upright or upside down.
     
  7. Sep 22, 2012 #7

    clanon

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    The Pilot will sure knows!:gig:
     
  8. Sep 22, 2012 #8

    wsimpso1

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    Highspeed gets it. Computational software does not know anything about up and down. It just computes the speed and pressure of the air on the foil and sums it up...
     
  9. Sep 23, 2012 #9

    Aerowerx

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    Except that the lift vector will be negative instead of positive.

    Did you look at the application that I referenced (Zenith STOL)? With a conventional airfoil for the main wing (negative pitching moment) you want the 'Lift' on the horizontal tail to be negative. That is, it 'pushes' the tail down.

    With an inverted (non symmetrical) airfoil for the horizontal tail this 'pushing down' is more than if you had a symmetrical airfoil (Assuming the same AoA for both).

    My original post was about what XFLR5 did when you used an 'upside down' airfoil. Does it 'sum it up' in the correct direction even if the outline drawing on the screen indicates a 'normal' airfoil?
     
  10. Sep 23, 2012 #10

    Hot Wings

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    If the data was truly inverted XFLR would have shown the foil as inverted. Trust me here - I'm a bit dyslexic and know there are 2 mirror images of everything. :ponder:

    The other guys are right about the results being the same, just inverting the AOA - until you go to put the thing on a whole plane in XFLR. Then the HZ stab will need the inverted ordinates or it will think it has a normal airfoil.
     
  11. Sep 23, 2012 #11

    fly2kads

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    It depends on what you want to do with XFLR5. If all you are doing is 2D airfoil analysis, then it doesn't matter which way is up, as others have pointed out. Invert the signs of your results to suit your needs. If you are using the 3D analysis on the whole aircraft, then the direction of "up" does matter.

    I have never had any issues with XFLR5 drawing an airfoil correctly. I would double-check the coordinate file to ensure that the camber is pointed the way you want it. For an inverted airfoil, the Y values should be greater in the negative direction. Sorry if that sounds obvious, but it should be double-checked if you're not seeing what you expect.
     
  12. Sep 23, 2012 #12

    Norman

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    Xfoil, the 2D analysis engine behind XFLR5, has three analysis types.

    Type 1 is a fixed Reynolds number analysis like wind tunnel data

    Type 2 is a fixed lift analysis where the Reynolds and Mach numbers vary as the square root of the coefficient of lift. This type will not show negative lift because it get's into divide by negative number errors.

    Type 3 is a rubber chord analysis which I haven't tried but I suppose it's good for honing in on the critical Re of an airfoil.

    XFLR5 needs a database of 2D airfoil characteristics before it'll do any 3D analysis so if you have an inverted airfoil on your plane do the calculations on it with type 1 repeated over the entire range of Re that your plane will operate at.

    That last sentence is just a guess:ermm:
     
  13. Sep 23, 2012 #13

    Aerowerx

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    :emb::emb:<<<doubly embarassed. Should have verified that the data was correct in the inverted .dat file. It wasn't.
    You have to do the 2D analysis with the inverted airfoil coordinates so the database will be correct in the 3D analysis. As far as the range of Re, that applys to any airfoil---normal or inverted.

    As far as the results I am getting...You can't just shorten the tail, it seems, as it does affect the stability damping. Not sure if an inverted airfoil for the H-stab is really a benefit. Zenith must think so, though. Anyone have experience with the Zenith STOL designs?
     

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