Ultralight struts/cantilever/additional weight

Discussion in 'The light stuff area' started by MrHopkins, Jan 14, 2020.

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  1. Jan 14, 2020 #1

    MrHopkins

    MrHopkins

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    Ultimately I am dancing around the questions "is it fair to compare the construction of Kolb wings to Thunder Gull wings?" What did the Thunder Gull design that other ultralights don't follow suit?

    Not necessarily the airfoils or aerodynamics but the actual construction choices. I think I am missing the point of struts. Kolb struts run below the wing. Minimax has two that are above the wing. Thunder Gull has none. Parasols have none, or only struts, so is that a grey area? Wouldn't attaching struts at the wing tip reduce moment arms?

    It took me a while to digest similar posts. I didn't understand if the answer is already here. While I am a strong reader, I might be a weak thinker.

    Less germane to the topic: I feel silly asking but when wings are called ladder based is that just a descriptor or are we talking literal ladders?
     
  2. Jan 14, 2020 #2

    Victor Bravo

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    Wing struts allow the wings to be simpler to build and easier to attach. Not always lighter than cantilever (no struts) but sometimes.

    Attaching the struts to the tips would make the wing break in the middle. There is a happy medium for where to attach the struts. You and I cannot guess at it, there is a lot of mathematics and engineering that goes into figuring that out.

    Thunder Gull uses a one-piece wing that is really neat, but it takes more room to build. In general, it's a little easier to design a wing with struts than it is to design one without struts.

    Yes there was one series of ultralight airplane from France that used real aluminum ladders for 3/4 of the wing structure. But most of the time we use the term "ladder frame wing" we are referring to an ultralight airplane wing that looks kind of like a ladder, but is made from round aluminum tubes.
     
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  3. Jan 14, 2020 #3

    lr27

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    I doubt if the math is that hard. Seems to me that it could be worked out with Schrenk's approximation and a spread sheet.
     
  4. Jan 14, 2020 #4

    Dana

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    If the lift distribution was rectangular on a rectangular wing, the optimum point for the strut to attach would be 2/3 of the way out to the tip. In reality, the lift distribution is more elliptical, so the optimum attach point is somewhat farther inward. That's for the wing itself, but it may be better to move the strut even farther inward to shorten the strut and improve its compression/buckling strength.

    Minimax struts are below the wing, attached to the landing gear. Perhaps you're thinking about the Volksplane, which has two struts above the wing.
     
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  5. Jan 14, 2020 #5

    TFF

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    Above the wing struts looks cooler, but costs a lot of extra weight. Struts in tension is better than compression. The thunder gull has twenty years of extra thought, and to looks to be much more complex of a design. MiniMax is all wood, Kolb is mostly aluminum. The TG is steel and aluminum. More potential mixing materials, but more complex to do so too.
    Are you designing or buying? Are you building or assembling? Do you have tools for the material of choice at your disposal, or only have a small set? These are just as important as what the thing looks like in the end.
    Is this to be a Pt 103 ultralight or rest of the world ultralight? Different design criteria.
     
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  6. Jan 14, 2020 #6

    stanislavz

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    Its all depends on mission and material choice.

    Wooden spar for cantilver wing is doable, but heavy and requures "fat" profile. Carbon rods with struts - looks crazy now - 600 mtow plane needs 0.2sqcm carbon rod in main spar with strut or 1cm without strut.
     
  7. Jan 14, 2020 #7

    Victor Bravo

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    Huh? Did you say something? I must have fainted for a second.
     
  8. Jan 15, 2020 #8

    BBerson

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    Not really any math, just follow the traditional layout in Fike's Practical Light Plane Design book.
     
  9. Jan 15, 2020 #9

    pictsidhe

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    Where you put the strut determines the in flight stresses. There is no ideal spot that minimises stress in all flight regimes. You aren't just going to fly straight in calm air. About 35-40% out is popular and works fairly well. Many aircraft have moved the strut further in and beefed the spar up to compensate.
     
  10. Jan 15, 2020 #10

    lr27

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    Seems to me that a strut won't save weight on an ultralight, but it might allow the use of a lousy spar, or easier assembly of a two piece wing. Plus, of course, on a part 103 the FAA will let you use more power.
     
  11. Jan 15, 2020 #11

    Martin R.

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    Your question is not silly at all; the French APEV used ladders on her Pouchel:
    Pouchel.jpg

    Here you will find the reason, why they were obliged to change:
    http://www.pouchel.com/english/index_eng.php?p=pouchel_eng.htm

    "..... Due of the success of the Pouchel (over 120 plans sets sold to members of the APEV), our ladder manufacturer became concerned about his legal liability and no longer agreed to sale us some potentially flying ladders ! Therefore, we launched the design study for the Pouchel II. ...."
     
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  12. Jan 15, 2020 #12

    Lendo

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    All depends on what size rods and what Tension and Compression values are you using for the rods and where are you getting the Rods from- hopefully not china.
    George
     
  13. Jan 17, 2020 #13

    lr27

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    Isn't it more about WHO is making the rods rather than their location? Of course it's a lot easier for Americans to visit the plant or talk to the quality people if the manufacturer is here rather than there. Still, I'd be surprised if there weren't at least a few good, reliable vendors over there.
    Seems to me that, for an ultralight, there's no reason not to use a really thick section. Mayne 20 percent. That ought to make things easier.
    If I recall the dimensions for the Sky Pup spar caps correctly, they probably weigh 16 to 20 lbs in total, and that's for a wing longer than most. I would guess you'd have to work hard to come up with a strut arrangement that would save enough weight to reduce the total much.
     
  14. Jan 17, 2020 #14

    lr27

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    P.S. Sky Pup wings don't taper very much, either. The spar caps could weigh less if they did.
     
  15. Jan 17, 2020 #15

    Tiger Tim

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    IIRC the Sky Pup’s ribs are all identical and it’s the spar that tapers a little bit in chord.
     
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  16. Jan 17, 2020 #16

    Aerowerx

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    Struts below the wing are in tension (except when sitting on the ground). Remember the wing is lifting up. The fuselage structure is in compression, so I don't think there is much chance of buckling there.

    Struts above the wing are in compression.
     
  17. Jan 17, 2020 #17

    Dana

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    Struts below the wing are in compression any time the plane is experiencing negative g loading, whether on the ground or in the air. Since the buckling strength of a long column is much less than its tensile strength, that's usually the limiting factor.
     
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  18. Jan 17, 2020 #18

    Hot Wings

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    With 18 inches of heavy snow on top that is a 30+ pound/ft^2 load...And snow doesn't follow the elliptical loading pattern of air either. :popcorn:
     
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  19. Jan 17, 2020 #19

    cluttonfred

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    Yes, that's right, the Sky Pup spars taper from several inches in chord at the root to just an inch or two at the tips with nose and tail ribs that are the same across the wing, so the chord gets a little shorter and the airfoil relative thickness increases as you get to the tips. It's an elegant solution to minimizing wing spar weight while keeping the ribs identical.

    wp845ea6ee_06.jpg

    Source: http://machnone.com/con3.html

     

    Attached Files:

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  20. Jan 17, 2020 #20

    rotax618

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    What is often overlooked in this argument is the distribution of the wing attachment loads to the fuselage, flight loads are only one consideration, landing and taxying loads and how they transfer into the structure has to be considered.
    Cantilever low wing seems less of a problem than a high wing, the High wing cockpit structure is less easily braced, struts can help triangulate and brace that structure.
    Using 2 struts per wing gives the wing torsional resistance, this has to be considered in your weight calculations.
     
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