Flap neutral setting for wing-fuselage junction?

Discussion in 'Soaring' started by Exian, Jul 18, 2019.

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  1. Jul 18, 2019 #1

    Exian

    Exian

    Exian

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    Hello!

    For my project (15 m racing class glider with electric propulsion) I will use the AH93K131 airfoil.

    Like all recent flapped glider airfoils, it was specifically designed so that :
    - The bottom surface is continuous with flap slightly up for optimal transition performance. Airfoil coordinates are given with the flap in this position (considered 0°).
    - The top surface is continuous with flap slightly down for optimal thermal performance. Flap position is 12° down compared to the above reference position.

    My performance estimations are the following for my project :
    - With flap at 0°, best L/D is 39 at 126 km/h
    - With flap at +12°, lowest sink rate is 0,54 m/s at 80 km/h.
    But overall best L/D is obtained with +6° flaps and is 45 at 97 km/h (consistent with other gliders of the same class).

    I must now design (and build on the almost completed fuselage) the shape of the wing root fairing but I can’t decide what is the best “neutral” flap position that will be “frozen” in this wing root! Is it 0°? Is it +6°, or something between?

    On older flapped airfoils, 0° flap setting gave already the best L/D and you used the airfoil as it was, but it not the case with the new ones....

    What are the other big sailplane manufacturers doing? (pictures on internet are never zoomed properly at the interesting spots!).

    Opinions of soaring specialists and others are welcomed! AH93K131-15.gif
     

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  2. Jul 18, 2019 #2

    BoKu

    BoKu

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    I think LS6 has it at the -5 (full negative) setting.

    I chose the 0 setting.

    Edit add: If I was doing it again, I'd probably go with -5.
     
    Last edited: Jul 18, 2019
  3. Jul 18, 2019 #3

    Hot Wings

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    I'm one of the "others".

    IMHO, and it's just an opinion, I'd probably try to figure out how much time is spent with the flaps at cruise and how much time you spend with it down. Which ever time is the greatest determines the fuselage mate.
     
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  4. Jul 19, 2019 #4

    pictsidhe

    pictsidhe

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    I'm also an other. I concur with Hotwings, set it wherever it will spend the most time.

    It's good to see someone with a sensible electric aircraft plan, add electric propulsion to a very low drag airframe...
     
  5. Jul 19, 2019 #5

    Hot Wings

    Hot Wings

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    Upon further thought my simplistic opinion is further humbled. It may in fact be better to choose the condition where less time is spent.

    For example if 60% of the time is spent in thermals but but the thermal mated fuselage profile reduces performance by 70% in cruise it might be better overall to take the lesser percentage performance hit for a longer time and design for cruise - provided time to goal is the desired result rather than simple time aloft.

    Just another example of when a seemingly simple problem may not be so simple and why experience is so valuable.;)
     
  6. Jul 19, 2019 #6

    pictsidhe

    pictsidhe

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    With the design and use uncertainty, I could be tempted to make removeable fairings. If I found they didn't work, swap them...
     
  7. Jul 19, 2019 #7

    pictsidhe

    pictsidhe

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    For other others, here's an Eppler paper on these flapped sailplane airfoils.
     
  8. Jul 19, 2019 #8

    Hot Wings

    Hot Wings

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    Or make a little "rubber ducky" one that can follow the flap.

    But when Boku said he did it one way and would do the opposite next time :cool:
     
  9. Jul 21, 2019 #9

    Exian

    Exian

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    Thanks for your answers and comments.

    What I first had in mind :
    When talking about gliders, first thing that comes to mind is best L/D.
    So common sense would be to do design for this best L/D.

    But thinking about it again and considering most of your comments:
    From my gliding experience, you rarely fly at best L/D (~100km/h), you are usually going slower in thermals, and faster between them!

    What I am sure is that no one sets the wing root fairing at the "thermaling" position (12° flap down in my case). So the other configuration (0°) is right?

    So I will probably go for the 0° position.

    Having the fairing follow the flap mouvement seems difficult.
    It will not be easy, but if after the flight tests I find that 6° would have been better, I could still modify it. Because of how I built the fuselage and will build the fairing, the part of the fairing next o th flap will be a massive block of dry micro, nothing structural. I can sand it away and make a new one ...

    I attached a picture of the current state of my project. Summer vacations will be used to made the surface finish (sanding a lot of dry micro, spray putty and sanding again, gel-coat and lots of wet sanding).
     

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  10. Jul 29, 2019 #10

    Exian

    Exian

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    So I set the flap position at 0° for the wing root fairing on the fuselage.
    Too late if this was wrong, it is already built!
     

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    BoKu and Hephaestus like this.
  11. Jul 30, 2019 #11

    BoKu

    BoKu

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    It's never too late for sandpaper and shmoo!
     

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