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Thread: Anyone ever wanted to build a wooden biplane sailplane?

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    Registered User HumanPoweredDesigner's Avatar
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    Anyone ever wanted to build a wooden biplane sailplane?

    The reason for wood is it is cheaper than carbon fiber, I assume.
    I'm aware that carbon fiber is not instantly stronger than a wood design, but has to be engineered right to be rigid, just like a wood design.

    I know biplanes have more drag than monoplanes. But because of their strength, I think you can pick any airfoil you want without worrying about spar depth.

    But here is my twist: Make one of the wings much smaller corded than the other, so it just streamlines the spar cap inside it. The webbing is the struts and wires. The struts would be V's with the small wing at the point.

    There would still be the drag of the wires and struts, but far less interference between the wings since one of the wings would be so much smaller than the other. There would be the strength of a biplane wing, but the same choice of thin airfoils and span available to carbon fiber planes, at the price of wood.

    What I don't know is if the best airfoil could make up for the struts and wires, compared to a very think airfoil of 18%. I also don't know if a wooden biplane could compete with a carbon fiber spar for span. But there must be a reason why early airplanes were biplanes instead of 18% thick mono wings. I think the L/D would be close to some of the thicker airfoils, but the strength might be higher.

    Do you guys think this could be a good idea for high aspect ratio wooden wings?

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    Moderator Topaz's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry James Thoreau
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    TFF
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Biplanes were used because of strength; it is one big truss structure. I dont think you will get gains from it. I have seen some older slope hang gliders that were biplanes. Not a sail plane but I have always thought the WingDIng II was cool.

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    Registered User Dan Thomas's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Quote Originally Posted by TFF View Post
    Biplanes were used because of strength; it is one big truss structure. I dont think you will get gains from it. I have seen some older slope hang gliders that were biplanes. Not a sail plane but I have always thought the WingDIng II was cool.
    The (powered) biplanes I've been in had the glide angle of a sackful of anvils. They're really draggy; you don't want to underestimate the effect of struts and wires and interference between the wings. When I towed gliders in the '70s the easy tows were the cantilevered gliders, no wing struts or tail brace wires. The old ex-military trainers (Schweizer TG-1 and TG-2) were horrible things by contrast even though they weighed no more than some of the slicker machines.

    Dan

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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Dan, I think the biggest reason why traditional biplanes are so draggy is because they have the skin drag of two wings, but the lift of at most 1.2 wings. That is 60% the L/D of a mono-wing before we even count the wires and struts, which I think are less than the double skin drag. Then there is some interference drag where the struts meet the wings.

    I could be wrong about the extra skin drag being the dominant factor. But if I'm not, then I wonder of my small wing/big wing biplane would be a lot less draggy than a traditional one. I think a good way to find out would be to look at the L/D of a rigid skinned hang glider that has wires. That would tell us how much the wires are actually contributing.

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    Registered User HumanPoweredDesigner's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Quote Originally Posted by Topaz View Post
    Wow, thanks. Already been tried. I knew they had one wing with a shorter span than the other. But not one wing with a shorter cord. It says they have the same performance as a single surface hang glider. One disturbing thing is it weighs as much as the pilot even with that small span. I wonder if it was not designed well, or if that means a larger span would necessarily weigh a lot more.


    OK. Drag aside, could a wooden biplan have the same span and cord as a fiber carbon sail plane?

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    Registered User ultralajt's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    First of all, you must ask yourself, what your sailplane should be capable of!
    What for you will design it?
    Where your sailplane will fly mostly and for what purpose?
    Ridge soaring, thermal soaring, long distance flights, home airport or hill soaring just for fun....

    As Bug (Mike Sandlin`s tube/fabric biplane design) sailplane prooves, biplanes are draggy and they lack a glide angle for efficient interthermal glide. They stay in the air mainly due their light wing loading that are main source for low flight speed and low sink.

    So, as they have relatively low speed (and sink speed) they can soar in ridge lift and if catch a thermal they can stay in its strongest core as low speed resulted also in very narrow circling without need for a strong bank angle.

    Fun flying machines can be biplanes, but as soon we speak about efficient cross country flight, sleak aerodinamicaly clean monoplane is the only reasonable option.

    If you do some math (aerodinamic calculations of different design configurations..layouts), you will see for yourself...othervise is just guessing.

    Research some different sailplanes data and their performancdes (trough a history), to get some feelings about what is good and what is bad.

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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Quote Originally Posted by HumanPoweredDesigner View Post
    What I don't know is if the best airfoil could make up for the struts and wires, compared to a very think airfoil of 18%. I also don't know if a wooden biplane could compete with a carbon fiber spar for span. But there must be a reason why early airplanes were biplanes instead of 18% thick mono wings. I think the L/D would be close to some of the thicker airfoils, but the strength might be higher.
    The early bipllanes had to be biplanes because they used such thin undercambered wing sections. These weren't more efficient-- a more modern section like the Clark Y or even thicker NACA sections are much better-- but they didn't know any better back then. Biplanes fell out of favor when the thin sections were abandoned, and structural techniques improved.

    The fact that long span wooden sailplanes have been made proves there's no need for the aerodynamic disadvantages of a biplane.

    The only reason (and it's a good reason) to build a biplane today is if you simply like biplanes. You might build a biplane "glider"... but it won't be a "sailplane".

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    Registered User jumpinjan's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Quote Originally Posted by HumanPoweredDesigner View Post
    But here is my twist: Make one of the wings much smaller corded than the other, so it just streamlines the spar cap inside it. The webbing is the struts and wires. The struts would be V's with the small wing at the point.
    Okay, just like a 1915 Nieuport 11? Well, do you know what that V-strut will do to your wings? It will put a twist into the bottom wing, just like the original N11 and the Albatros D.III & D.V
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    Registered User ultralajt's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    A biplane glider from past:






    Plans 1:2.5 scale: http://www.retroplane.net/forum/download.php?id=197

    This glider weight only 44 kg.
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    See Hoerner Fluid Dyndamic Drag section on boxwings.

    With such, induced drag can be remarkably low, as endplate effect reduces tip vortices. Distance between upper and lower surfaces make for very stiff and light structure. Drag wires not needed, as wings are supported at roots and tips.

    Think it over.

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    Registered User ultralajt's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Quote Originally Posted by Bart View Post
    Think it over.
    And calculate polar of actual design layout, to be dissapointed with result.
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Quote Originally Posted by ultralajt View Post
    And calculate polar of actual design layout, to be dissapointed with result.
    Amen. Soaring performance is highly dependant upon span, aspect ratio, and wing loading. A biplane has exactly one of those in the right ballpark.

    As others have mentioned, a low-wing-loading biplane "airchair" style glider would be a fun ridge-lift ship. For thermalling, it's going to give you sled rides in all but the strongest lift conditions.
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    Registered User HumanPoweredDesigner's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Quote Originally Posted by Dana View Post
    The early bipllanes had to be biplanes because they used such thin undercambered wing sections. These weren't more efficient-- a more modern section like the Clark Y or even thicker NACA sections are much better-- but they didn't know any better back then. Biplanes fell out of favor when the thin sections were abandoned, and structural techniques improved.
    Less efficient even at low speed? I know that undercambered wings are terrible for flying upside down, or at least would take more skill to do so. I also think undercambered wings are only good at specific speeds, and the more semetrical you get the more adaptable the wing is to different speeds and manuvers. But you are sure that a Clark Y could beat an undercambered wing at L/D even at low speed if both were sized to make the same lift?
    There are a lot of blogs out there saying this or that, so I could be wrong.

    What about laminar flow wings, which are thickest at mid cord and have a more even pressure distrubution: Do you think they can beat a Clark Y at low speed, level flight L/D? This is for a human powered helicopter blade tip.



    Quote Originally Posted by Dana View Post
    The fact that long span wooden sailplanes have been made proves there's no need for the aerodynamic disadvantages of a biplane.
    I'm glad to hear it has been done. I wondered if my designs were wrong when I had a wooden spar handling the loads that usually require carbon fiber. I now feel a bit more confident, though I'd like to see that spar and airfoil. I suspect that although CF has tremendous max compression strength, some designers have trouble taking advantage of it because of buckling, especially if they don't set it up right. I think it is easier to make wood work, even though it probably has less potential if both are done right.

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    Registered User HumanPoweredDesigner's Avatar
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    Re: Anyone ever wanted to build a wooden biplane sailplane?

    Dana,

    I think the NASA report you refer to said that biplane wings, even without struts, are less efficient than a mono-wing. Are you refering to a different report that said undercambered wings are less efficient than thicker airfoils, even at low speed? Many sail planes try to have have undercambered wings, but compromise with an undercamber in the back. Maybe sailplane technology has changed since that was written though.

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