Plywood stitching construction method

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Mar 2, 2019.

  1. Mar 2, 2019 #1

    rtfm

    rtfm

    rtfm

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    Hi,
    I've been reading up on how to build boats using the stitched plywood over bulkheads method. It results in an extremely light yet strong craft, and I was wondering how applicable this method might be for wooden aircraft construction.
    https://www.christinedemerchant.com/stitch-and-glue-boatbuilding.html

    I've designed an airframe (using DevFus), and it seems like an ideal candidate for stitch and glue fabrication.
    Wasp_profile_and_bulkheads.png
    The idea would be to run stringers front/aft, and then bond/stitch 2mm plywood strips lengthwise to these. I'm going to test the method on a scale model and see how it works out.

    Comments?
    Duncan
    PS This is not an actual project - more of a side-interest. I am currently working on the fuselage of my AeroMax.
     
  2. Mar 2, 2019 #2

    lr27

    lr27

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    I've finished up* a couple of boats that use this method. Generally, they don't use stringers where the sheets join. You just stitch the edges together, fill the inside corner with something to round it a bit, then tape. When that's set, you can sand down the outside corner and tape that. For 2 mm ply, you'll probably be using fairly light tape. As I recall, 6 oz. tape is about right for 6mm ply.

    That Wasp drawing looks very curvy. Can you really get the plywood to take that shape?

    *Someone else started them, then abandoned the projects. I'm not sure it saved very much time compared to building from scratch.
     
  3. Mar 3, 2019 #3

    deskpilot

    deskpilot

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    Hi Duncan, glad to see you're still experimenting. Perhaps, one day, you'll finish one of your designs (it it hasn't already happened and I missed it)
    What happened to this I remodeled for you?
    Duncans tractor gyro.jpg
     
  4. Mar 3, 2019 #4

    rtfm

    rtfm

    rtfm

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    Hi guys,
    Doug - I have your little model safe and secure on my laptop - but I've gone off the idea of a gyro. Pity - the design was very cool. Tractors rule!

    As far as reasons for using the glue and stich method is concerned: it isn't for speed of construction, actually. Just a construction thought-experiment. The plywood only needs to bend in one direction, since the bulkheads are not round, but with a dozen or so flat surfaces. I'm just intrigued by the construction process, and will give a scale model (out of balsa) a go first, to see where the issues are. And to see if it is strong enough.

    No stringers? Interesting. Just bulkheads. Mmmm. Certainly worth a trial.

    As for finishing a project - the Aeromax is progressing...

    Regards,
    Duncan
     
  5. Mar 3, 2019 #5

    BBerson

    BBerson

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    I think the idea of the Pygmy Boats stitch and glue kits is no jig required. The computer designed/cut parts in the kit are just stitched together and form a 3D structure.
     
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  6. Mar 3, 2019 #6

    rtfm

    rtfm

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    Hi,
    The operative term is "the computer designed/cut parts". What computer design? I'm doing this by the measure and cut method. So I would definitely need bulkheads. The bulkheads are easy to cut, since DevFus does that for me. The plywood planks, on the other hand, are a different story. Once the bulkheads/stringers are in place and bonded firmly, I would have to make templates of each plywood piece, and cut that by hand. An interesting project, but probably not optimal. Once the templates have been cut, however, then reproduction should be fairly straightforward.

    Duncan
     
  7. Mar 3, 2019 #7

    BBerson

    BBerson

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  8. Mar 3, 2019 #8

    Riggerrob

    Riggerrob

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    Stitch and glue plywood boats are moncoque shells with a bare minimum of bulkheads and only have stringers around hatches.

    Cut to fit sounds extremely labour-intensive.
    Many CAD programs have “sheet metal” sub-programs that allow you to draw the outside mood lines, then “flatten” sheets of metal, plywood, fabric, etc. then cut them with lasers or CNC routers.
     
  9. Mar 3, 2019 #9

    rtfm

    rtfm

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    Hi,
    I'm caught between technologies... On the one hand, I have DevFus, which easily created the bulkheads, and even produces full male or female molds to cut foam plugs. But it doesn't do what you have described Riggerrob. And I don't know how to move from what DevFus gives me to what you describe.

    Cut and fit IS labor intensive, but can you think of a way round this?

    Duncan
     
  10. Mar 3, 2019 #10

    cluttonfred

    cluttonfred

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    Stitch-and-glue boat construction uses wire loops or plastic zip ties to hold the edges of the plywood together so that a fillet of filled epoxy reinforced with fiberglass tape can join them permanently. The wire or ties are then cut off, the corner rounded, and another layer or two of tape added to the outside. Tack-and-tape is a similar but easier method when building simple shapes that uses wooden battens at the edges of intermediate frames to locate all the parts, then resin and tape is applied directly without wires or ties. True stitch-and-glue can create more curved complex shapes but it can be challenging at larger scales.

    More than 20 years ago (!) I started building little plywood boats as a substitute for the aircraft building I would have liked to tackle but didn't have the time or the money or the space. All of the boats I built were variations on the tack-and-tape theme designed by the late, great Philip C. Bolger and promoted by the late, great Harold "Dynamite" Payson in his Instant Boats books. I would recommend Bolger's books, especially Boats with an Open Mind for anyone interested in designing anything, not just boats, as it contains great insight into form vs. function in all kinds of ways.

    For me, the most exciting application of this type of construction in boats and potentially in aircraft is in creating simple shapes quickly. Yes, some CAD programs can create those "flattened" shapes for you and, if you keep it simple, you can also do it manually with geometry. In effect, you create your own kit of parts that when assembled "want" to be a boat (or fuselage) shape.

    yleaf_me.jpg yleaf_build.jpg yleaf_plan.jpg

    Check out this page from my *old* web site (the baby in my wife's belly in the photos is now 19 and in college) about building the simple Yellow Leaf pirogue (flat-bottom canoe). Often the curvy shape is actually achieved with parallel-sided "planks" of plywood with the ends cut off at an angle. Flat-bottom skiffs (pointy on one end, squared off on the other) have long been built that way along the East Coast of the USA and in many other places around the world.

    Many builders refer to a fuselage without wings, tails, engine, or landing gear as the "canoe stage" of building. It's not at all hard to see how a simple and elegant plywood fuselage with or without traditional longerons could be made this way. You would build essentially 2/3 or 3/4 of a flat-bottom canoe for a single-seater or a tandem two-seater or a backwards flat-bottom skiff for a side-by-side two-seater!
     
  11. Mar 3, 2019 #11

    Aerowerx

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    03ac396bc2b55c35b2e366aeebf9d66f.jpg

    Build it like this, except for no spacing between the strips

    Two layers at some angle to each other.

    This has been discussed before here on HBA. Not the lightest construction, because of the glue between layers, but quite strong.
     
  12. Mar 3, 2019 #12

    FritzW

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    The elephant in the room is that the fuselage you drew in post #1 can't be made using the stitch and glue method. Your (very pretty) fuselage is the poster child for compound curves and you can't do compound curves with stitch and glue. It's all got to be flap wrapped.

    ganagobie_n1949.jpg ...if you put a wasp'ish raised tail cone on a Ganagobie you have something very similar.
     
  13. Mar 3, 2019 #13

    cluttonfred

    cluttonfred

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    Teal1.jpg Teal2.jpg Teal3.jpg

    Here's a good example of the "curves from straight planks" I described earlier, the little 12' Bolger/Payson Teal rowboat/sailboat. It's 42" across at the widest point, so it's not hard to imagine a stretched out version say 14' even on the same width. The greater length on the same width would reduce the angle of the "rudder post" though an angled hinge line would still seem logical.
     
    Last edited: Mar 3, 2019
  14. Mar 3, 2019 #14

    Aesquire

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  15. Mar 3, 2019 #15

    nerobro

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    Stitch and glue isn't.. all that light. Once you stitch the parts, the stability of the sheets comes from buttering the corners with epoxy. That's not light.

    Hmmpf. If you've planned your design well. I could see stitch and glue being a decent way to build your plug... Maybe even a "leave in" plug? Some thickness to provide stiffness when you fiberglass the inside? I dunno.
     
  16. Mar 3, 2019 #16

    cluttonfred

    cluttonfred

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    I was thinking more of monocoque-style construction and just use epoxy and cloth to hold the edges together. It would also be possible to use traditional longerons to tie the edges together.
     
  17. Mar 3, 2019 #17

    BJC

    BJC

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    Or just stitch composite panels together, then laminate the seams.


    BJC
     
  18. Mar 3, 2019 #18

    rtfm

    rtfm

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    Hi,
    Here is a rough sketch of a part of one of the bulkheads (which will not be solid, of course, but will have the centre section removed, leaving a 2 inch perimiter). Notice, if the curve of the bulkhead is retained, we DO have compound curves for the plywood to bend around. However, simply by drawing straight lines between the longerons, we end up with flat wrapped plywood. The challenge is to accurately draw/cut the individual strips of plywood. One could assemble the bulkheads/stringers first, and then use this structure to draw each strip. Shouldn't be too difficult. Once the strips have been drawn, cut and verified, one would have accurate templates for any additional constructions.

    And no big epoxy fillets. Bond the plywood to the 15mm square stringers/longerons with T88 - very light, and very strong. I have seen videos of "how to build using the stitch and glue method" where they use epoxy bulked out with microballoons! CRAZY! A one-stop method for making weak joints. Plywood to longeron/bulkhead joins using only T88 is going to be way stronger (and lighter).

    Bulkhead_#8.jpg

    DevFus (I'm not sure if you are familiar with the package) does a great job of drawing the bulkheads, as well as the building jig. Here's a screenshot...

    Building_jig.jpg
     
  19. Mar 3, 2019 #19

    lr27

    lr27

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    If you poke around the boatbuilding world, you will find free programs that figure out the plywood shapes and even show how to plot them on plywood sheets with minimal waste. Alternatively, you can find an aircraft with the right sort of fuselage shape and buy a paper model of it!


    I'm pretty sure the epoxy and glass used to join the sheets is lighter than using a stringer. Plus more of the fibers run the right way. (On a stringer, none of them run the right way.) The epoxy used for the fillets can have microballoons in it and probably doesn't need to be much heavier than the wood involved, especially if that wood is birch.

    Seems like the diagonal plywood strip method wouldn't have to be any heavier than plywood. And it, or something like it, has been used for aircraft many times. Weren't a bunch of Lockheed fuselages made that way?
     
  20. Mar 4, 2019 #20

    Dan Thomas

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    Way back in the '70s I built two small boats with stitch-and-glue before I had heard about anyone else doing it. Those boats were heavy for their size, as I had to use thicker plywood to get the required stiffness. I'd have been better off (lighter) making a light frame and thinner ply. The resin and glass on the inside does add weight. I did the entire outside in light glass and resin, for waterproofing and weather resistance.

    I also designed and built a frameless 11' boat, and it was REALLY heavy due to the need for much heavier ply.

    Famous aircraft designer once said that lightness was not simple. It most often requires a complex structure. I've seen that in aircraft, having once started a Maranda with its 1/4" plywood wing and tail ribs. Very quick and simple to make, but much heavier than ribs made of 1/4" spruce capstrips and small 1/16" birch ply gussets, as my Jodel and taylor had. Lightness means placing the weight only where it will do the most good: along load paths.
     

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