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Thread: Plywood stitching construction method

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    Registered User rtfm's Avatar
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    Plywood stitching construction method

    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/...tbuilding.html

    I've designed an airframe (using DevFus), and it seems like an ideal candidate for stitch and glue fabrication.
    Click image for larger version. 

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    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.
    ====================
    Brisbane, Australia

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    Re: Plywood stitching construction method

    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.

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    Registered User deskpilot's Avatar
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    Re: Plywood stitching construction method

    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?
    Click image for larger version. 

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    Fly safe.

    Cheers, Doug

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    Registered User rtfm's Avatar
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    Re: Plywood stitching construction method

    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
    ====================
    Brisbane, Australia

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    Re: Plywood stitching construction method

    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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    Re: Plywood stitching construction method

    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
    ====================
    Brisbane, Australia

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    Re: Plywood stitching construction method


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    Re: Plywood stitching construction method

    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.

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    Re: Plywood stitching construction method

    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
    ====================
    Brisbane, Australia

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    Re: Plywood stitching construction method

    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.

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    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!
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    Re: Plywood stitching construction method

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    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.
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    Re: Plywood stitching construction method

    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.

    Click image for larger version. 

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ID:	78766 ...if you put a wasp'ish raised tail cone on a Ganagobie you have something very similar.
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    Re: Plywood stitching construction method

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    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 by cluttonfred; March 3rd, 2019 at 02:31 PM.
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    Re: Plywood stitching construction method

    Elephant? Ha!

    The weasel in the room is cloth covered is lighter still. http://www.gaboats.com/boats/
    This construction may seem familiar?

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    Re: Plywood stitching construction method

    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.

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