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Thread: Joined Wing Stability and Control

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    Joined Wing Stability and Control

    I have been doing some research into alternative configurations for my project, a single seat, 500 mile range, 150mph runabout. I have done sketches for the normal low-wing monoplane layout (taildragger and tri-gear) and even a rough high wing layout. I would like to explore some other options, namely the joined wing concept. What attracts me to the joined wing is an increase in span efficiency and structural efficiency. Raymer suggests that wing weight savings of up to 30% are possible with this configuration. Having lower weight AND lower induced drag is appealing, since the engine sizing constraint is for the climb, where induced drag is dominant. Smaller engine means cheaper airplane. My questions are these:

    1)What is the lift distribution among the wings? Is the front wing like a canard, requiring a higher Cl in order to stall earlier? Is the rear wing instead lifting DOWN in reaction to the wing's pitching moment.

    2)What sort of structural issues would be expected? What are the moments involved? Where would flutter be an issue?

    3)Is this effort to get low induced drag going to end up costing in higher parasitic and interference drag, basically negating any gains (especially on the top end)?

    Advice, comments, and opinions are welcome, as are references. I am just exploring various configurations looking at different ways to arrive at the same design goal.

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    Re: Joined Wing Stability and Control

    You should do a google search and look at Julian Wolkovitch's papers first --he was the patent holder (now expired)although the concept goes back much earlier ( I met Julian in 1990 and would have been building prototypes for him if he had not died only a few months after I saw him (With Barnaby Wainfan from Kitplanes "Wind tunnel " column present --as far as I know Barnaby has never written about the joined wing despite his involvement with Julian and ACA associates with it --it is still the case that no manned joined wing has been flown or evaluated over any significant flight time -- one rough prototype crashed in trees and boxplanes are different (eg ligeti stratos)

    In answer I would say ,generally speaking, 1. Check literature. Yes. Maybe but should lift. 2. Complex to analyze (NOT amateur level --many resultants and oddball interactions ) Lower than cantilever. If splitting a wing the cross section quarters and stiffness 1/8 ths so it is important to properly "join' them to get the advantages .
    3. depends how you do it --

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    Super Moderator orion's Avatar
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    Re: Joined Wing Stability and Control

    Quote Originally Posted by highspeed View Post
    What attracts me to the joined wing is an increase in span efficiency and structural efficiency. Raymer suggests that wing weight savings of up to 30% are possible with this configuration. Having lower weight AND lower induced drag is appealing, since the engine sizing constraint is for the climb, where induced drag is dominant. Smaller engine means cheaper airplane.
    Remember that there is no such thing as magic aerodynamics and that most things do come with inherent penalties. One of the things to consider is that many of these technologies require point designs so any deviations from that narrow operational envelope will cause the design to lose the benefit. Furthermore, these benefits are very small so don't expect miraculous improvements in performance. Looking at practical implications of the aerodynamics alone, and assuming you design and build everything just right, your performance benefit may be on the order of a few percent. This may be significant at commercial aircraft scales but hardly noticeable in the GA environment.

    Quote Originally Posted by highspeed View Post
    1)What is the lift distribution among the wings? Is the front wing like a canard, requiring a higher Cl in order to stall earlier? Is the rear wing instead lifting DOWN in reaction to the wing's pitching moment.
    You can set this up any way you find beneficial. I've seen studies at boeing where the configurations were examined for military applications and both types of configuration were examined. I also did a similar design for a business class airplane about a year ago - in that one the wing flew in a conventional manner so the aft surface acted like a conventional tail.

    But as you do this keep one thought in the back of your mind - this configuration has been studied for some time but despite the potential advantages and the number of folks that really support this layout, no designs (except the small Australian design) of this type have surfaced.

    Quote Originally Posted by highspeed View Post
    2)What sort of structural issues would be expected? What are the moments involved? Where would flutter be an issue?
    The structure is no different than for any other configuration - just the reaction loads are different and there are some more complex concentrated load configurations out at the joint. The section moments are the same but just reacted a bit differently - the span-wise distributions are pretty much the same but the reactions and resolutions are a bit more complex (best to solve with FEA methods).

    I would assume flutter issues to be improved but one must be careful when doing the analysis since if there is a significant structural benefit from the standpoint of bending, the removal of material that might normally be there could create some secondary effects.

    Quote Originally Posted by highspeed View Post
    3)Is this effort to get low induced drag going to end up costing in higher parasitic and interference drag, basically negating any gains (especially on the top end)?
    The one negative I see is that since the wings are joined, the aft wing is relatively close to the forward one so allowable CG limits based on static stability criteria may be relatively narrow. Assuming a conventional configurational arrangement, there might also be a penalty since the aft wing (tail) is now probably substantially bigger due to the increased span. Given that and the details of the joint, the net structural benefit may be zero or even negative. Hard to tell without any analysis though.
    "To live is to learn; to learn is to live" (author unknown)

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    Re: Joined Wing Stability and Control

    From the sounds of it the joined wing configuration would probably not be worth the trouble and effort of designing and analyzing it. The few percentage points of improvement, if any, would be offset by a very steep learning curve. And there's no guarantee that the benefits would be even measurable. I'm not designing an airliner, as Orion pointed out. Novelty alone isn't enough. I find it telling that very few airplanes with this configuration have been flown.

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    Re: Joined Wing Stability and Control

    Hello HighSpeed I have been R&D Joined wings for nearly 4 years now. I started with simple paper models, then I build over 70 Balsa Hand Launched Gliders and now I,m near completing my 7th RC glider ( a more or less scale ultralight sailplane )
    There is tons of things to learn ( read ) , explore, analyse, test fly, modify, etc according to the design you wish to develop.

    I am working towards toy joined wings, UAVs and hopefully a full size FAI Class 2 glider or UL sailplane.

    Google Joined Wings and you will find several sites with some of my models, and concepts like for Human Power Aircraft JW
    as well as many other joined wings of several configurations.

    First check the different types of JW, like the J-3s ( tip joined wings ) the J-5 s, the J-9s, etc ALL of them FLY !
    with more or less stability and glide performance. Some weigh more , some have more aft wing flex.

    The ¨joints¨ is a structural issue than can be solved with several engineer options
    ( check for ¨A Study of Joint Fixativity in a Joined-Wing Aircraft ¨ by Stephen Smith et al )

    IF you have the time to do lots of R&D , go for it ! , and you can build any aircraft your heart desires ....the sky is the limit !



    http://design.projectwolfdragon.com/AIAA-45285-587.pdf

    Joined Wing Stability and Control-stagger-jws-002.jpgJoined Wing Stability and Control-alex-s-joined-wings.jpg

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    Registered User bmcj's Avatar
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    Re: Joined Wing Stability and Control

    Wow!

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    Re: Joined Wing Stability and Control

    @ALEX MORILLO Those are some impressive gliders. Thanks for the paper, it was enlightening and led me to several others. I will continue to pursue this line of design configuration for advancement of my knowledge, if anything. You wouldn't happen to have a copy of the paper describing the various configurations of joined-wing aircraft?

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    Re: Joined Wing Stability and Control

    ...sorry, I lost a way to acces the CRANFIEL CERES library, but here is the title to the paper--
    ¨ a Computational and Experimental Analysis of Joined wing Aircraft ¨ by HASHIMOTO M, Ishikawa M ,et al
    I credit him ( them ) for standarizing those 8 JW planforms, ... there is more like the ¨box wings¨ the swiss 2 FL, etc

    Check JW papers by Dr. ILAN KROO , one of the SWIFT designers and a very knowledgeable Stanford profesor

    Joined Wing Stability and Control-jay-8-f-3.jpg

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    Re: Joined Wing Stability and Control

    ..also this motor JW link ( your line )

    http://www.prandtlplane.it/temp/Deve...andtlPlane.pdf

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