bendy wings ?? how much bend is practical and what are advantages ? any of non ??

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by tunnels, Nov 5, 2014.

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  1. Nov 5, 2014 #1

    tunnels

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    Again I see wings with bend down when they are on the ground and bend up when flying !
    I look at pictures and see wings with lots of bend !. Ok on big planes because of size but what about smaller Planes ?
     
  2. Nov 5, 2014 #2

    BJC

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    All wings bend, it is just a matter of how much. :)
     
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  3. Nov 5, 2014 #3

    highspeed

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    It will vary from design to design, but a rough guide I've heard is 1" of deflection per g. This number came from Orion, a professional aircraft designer who, sadly, is no longer with us.
     
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  4. Nov 5, 2014 #4

    Dana

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    1" per g as a rule of thumb is meaningless without talking about the wingspan. Obviously large airliner wings flex a lot, much more than that. Small planes, much less than that. It's not generally an issue unless, or until, it causes issues with control linkage binding or something like that.

    Dana
     
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  5. Nov 5, 2014 #5

    BoKu

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    I came up against this question when I was doing structural design for the HP-24. The answer is a resounding "it depends." Basically, you want to make sure that wing deflection is not so great that it causes undesirable aerodynamic effects within the desired flight envelope. As to how much that is, the technical answer is the topic of entire books on aeroelasticity.

    For the HP-24, what I wanted to do was achieve wing stiffness somewhere in the middle of the range for similar sailplanes. But what I found is that wing stiffness is not something that sailplane designers or manufacturers tend to quantify in specifications available to the general public.

    What I ended up doing was building a collection of head-on photos of sailplanes in various attitudes, and doing some simple analysis to estimate the wing bend depicted in them. From that I was able to figure out a range of generally acceptable stiffness to shoot for.

    One important feature of wing bending is that it can potentially cause binding of the wing control surfaces (flaps, ailerons), depending on how they are hinged. On gliders, the control surfaces are often very limber in bending so that they can be deflected while the wing is bent.
     
  6. Nov 5, 2014 #6

    tunnels

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    With composites I have no problems at all with things that bend and move !
    General rule is if it doesn't move its going to break just when you don't really expect it to !!
     
  7. Nov 5, 2014 #7

    bmcj

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    And whether they return to the intended shape when the load is removed. ;)
     
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  8. Nov 5, 2014 #8

    autoreply

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    Span in meters times ar is roughly bending in mm per g.
     
  9. Nov 5, 2014 #9

    Victor Bravo

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    I have been fortunate enough to own and fly one of the airplanes near the extreme end of this discussion, my much beloved AS-W20 sailplane. It had a 15 meter span (49.2 ft.), and deflections of over 3 feet at the wingtips were not abnormal, and the structure had plenty more reserve left in it.

    This was the first sailplane to be designed to take advantage of varying the wing twist with the deflection, to create a dynamic based on bird wings flapping and providing thrust. If I recall correctly, this was once called "the Katzmayr effect".

    Again, if my feeble memory serves, that genius Dr. Waibel apparently designed the fiberglass layup schedule for the wings to twist nose-up as the wings deflected upward in lift and turbulence, and then twist nose-down as the wing deflected back downward. This would theoretically store some of the atmospheric energy (from a gust) in the wing as a spring, and then release that energy to produce forward thrust as the wingtip came down.

    The amount of mathematics and engineering that went into achieving this, without allowing the wing to have catastrophic aeroelastic flutter at speeds approaching 170 MPH... is staggering.

    But it worked ! The AS-W20's diving board wings absorbed turbulence from vertical gusts that would have otherwise caused huge airflow separations and resultant spikes in drag. This alone was very worthwhile when racing in rough air. When using the "dolphining" technique to make use of gusts and small thermals flying on course, the '20 always gave you the feeling that it was extracting a little more of that energy, and putting it back into performance. You could dolphin more aggressively in this sailplane than any other that I ever flew, without feeling like the pitch excursions cost you more in drag than you gained.

    Most importantly, the extreme flexibility of this glider's wings resulted in a much more comfortable, and less jarring ride in rough air. This may not seem important to a competition glider, but it reduced fatigue and allowed you to concentrate on other things, instead of being distracted or feeling like you were being beaten by thugs.

    When the glass wing Rutan Long-EZ was developed into the carbon wing Berkut, there was a very noticeable reduction in the "shock absorber" aspect of the fiberglass wing. Berkut pilots reported that flying fast through rough air was a lot more uncomfortable than in the older EZ aircraft.

    So in my opinion there are one or two benefits of flexible wings even for powered aircraft, but there are many other factors, torsional rigidity, control system binding as Bob said, etc. As you shorten the wingspan, all of this starts becoming more and more moot of a point. If you get out of a fiberglass wing sailplane after flying through choppy air and get into a strut-braced Cessna flying through choppy air, you will become very desirous of flexible wings!
     
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  10. Nov 5, 2014 #10

    Birdman100

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    No, this is not optical illusion, this is Eta sailplane in turn...
    marzinzik09.jpg
     
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  11. Nov 5, 2014 #11

    Birdman100

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    For sailplanes yes, too approximate otherwise. You have to count material type (big difference from carbon, glass, alu, steel, wood...) and airfoil (or spar) thickness...

    Bend up and bend down ratio is roughly body (fuselage, pilot, payload etc) weight to wing weight ratio per g....

    y (up)/ y (down) = weight (body)/weight (wing)/g
     
  12. Nov 5, 2014 #12

    Dana

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    I recall one of my aero professors saying that the B-36 bomber was the last large aircraft designed with a more or less rigid wing.

    Dana
     
  13. Nov 5, 2014 #13

    bmcj

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    You should have seen the Rutan Voyager after it took off with full tanks!
     
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  14. Nov 5, 2014 #14

    Birdman100

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    Couldnt find image on the web, but I can imagine... But, being that fuel is located at 1/3 semispan, and the fuel is 80% of take off mass, deflection is reduced as much as possible. Actually fuel is located that way for the structural reason...
     
  15. Nov 5, 2014 #15

    BJC

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  16. Nov 5, 2014 #16

    bmcj

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    Trust me, from a ground based perspective, it looked far more exagerated. I'll have to try finding my photos.
     
  17. Nov 6, 2014 #17

    BJC

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    I would have been very apprehensive Watching it accelerate at a snail's pace, then finally seeing the wings flex up, and get aloft.


    BJC
     
  18. Nov 6, 2014 #18

    tunnels

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    Twist and flex sort of go together and was going to be my next ask on this forum !
    I have done a lot of work using unidirectional glass over the years making and repairing all kinds of things and when I have demonstrated its advantages many learned and bright individuals within the marine and automotive glassing industry have simply walked away shaking there heads In disbelief usually saying it wont last because its to weak and flimsy ! when in all reality its out lasted 3 times over the same product from the same moulds with laminates designed by composite engineers, there products were twice the overall weight and all of there hulls never even finished one full season !!
     
    Last edited: Nov 6, 2014
  19. Nov 6, 2014 #19

    SVSUSteve

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    Flex is ok....twist, you have to be more careful with because if it twists too much, you can get flow separation and uncommanded control actions. Or, you know, it sets up a flutter and you become a series of data points in my research if you don't catch it soon enough.
     
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  20. Nov 6, 2014 #20

    tunnels

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    Re: bendy wings ?? how much bend is practical and what are advantages ? any of non ?

    Just after looking at some photos I oks like t last 1/3 of a wing has twist , maybe just t picture !!thank you understand !!!!what about flexible trailing edges on any or all ??
     
    Last edited: Nov 6, 2014

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