Pop rivets holding 1.5 mm plywood on 15x10mm spruce longerons . Good idea ?

Discussion in 'Wood Construction' started by Speedboat100, Oct 4, 2019.

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  1. Oct 5, 2019 #21

    Speedboat100

    Speedboat100

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    I change all the longerons into Black Arder. This is a non flying aerodynamical part of a bigger test model that is supposed to brake the Betz's limit in near future.

    Thank you again for all the commets.

    https://en.wikipedia.org/wiki/Betz's_law
     
  2. Oct 5, 2019 #22

    Speedboat100

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    Backside has now a washer....which seems to cure the cracking phenomena.
     
  3. Oct 5, 2019 #23

    Dan Thomas

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    Staples or rivets or nails won't help much at all if the glue fails. The shear and tensile strength of the glue is far higher than any mechanical fastener in wood other than a nut and bolt, which is why standard aircraft practice with wood is to make joints by gluing in shear or by bolting. Wings and tails are attached with bolts, as are engine mounts, control surface hinges, landing gear, and so on; somtimes wood-to-wood, usually wood-to-metal.

    I did some tests on one-inch-wide 1.5mm Baltic Birch strips that I had scarfed-jointed at 15:1 and glued with epoxy. I pulled on them with a calibrated hydraulic affair and all of the strips failed at between 1100 and 1500 pound of pull, and most didn't break at the glue joint. NO mechanical fastener would have taken anywhere near that. A pop rivet or anything else would tear out instantly if the glue failed. Those fasteners wouldn't save your skin if your glue joints came apart in flight.

    1.5mm ply bends like a blanket across the outside grain if you steam it. I used a spray bottle with water and a clothes iron. Spray the water on the wood, and run the hot iron over it. The water steams and pops and sizzles and softens the outside plies. The center ply is difficult to get the steam into, so it remains rather stiff and the ply has to go on the leading edge parallel to the span. I steamed a strip of the that ply like that and spiral-wrapped it around a pencil without it cracking. MIL-spec aircraft ply is designed to take that sort of treatment.

    Then you wrap the limp leading edge ply around the leading edge and hold it there with straps. Let it dry thoroughly, then glue it down.
     
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  4. Oct 5, 2019 #24

    Aerowerx

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    Then it would be time to scrap the whole thing, I think.
     
  5. Oct 5, 2019 #25

    Speedboat100

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    Yes Dan Thomas has it right...the pop rivets alone cannot do much. I could also use screws to fasten the campered part plywood into the wooden longeron.
     
  6. Oct 5, 2019 #26

    Speedboat100

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    I have to try this...otherwise the whole bending affairs seems very tricky to do.

    ---

    This piece is a fairing to reduce 95% of the tower turbulence that affect the VAWT blades.

    This is very simple aerodynamics...but no one has ever tried it before in this kinda system ( makes you wonder ).
     

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    Last edited: Oct 5, 2019
  7. Oct 6, 2019 #27

    Tony Spezio

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    With all my years in aviation I would say NO. Pop rivets for aluminum to aluminum yes if pop rivets are aluminum.
     
  8. Oct 6, 2019 #28

    Speedboat100

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    I can only find one thing positive about pop rivets..they make the craft look like a metal bird with silver ( metal ) paint job. And it sorta does reinforce the part where the steam bending may have deteriorated the glue of the plywood.

    Down sides are...more weight, more complexity...more turbulent flow.
     
  9. Oct 6, 2019 #29

    Dan Thomas

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    MIL-spec birch plywood can take boiling water for 30 minutes without glue degradation.
     
  10. Oct 6, 2019 #30

    Speedboat100

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    But can it take bending at the same time ?
     
  11. Oct 7, 2019 #31

    Dan Thomas

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    That's how it's often done. Boiled in some container, or as I described, sprayed with water and the iron run over it to boil it and force it in. Then it's bent. Try bending it dry and see what happens; you'll bust it if the radius is small enough. It has to be wet and hot and soft.

    From Wiki:

    Structural aircraft-grade plywood is most commonly manufactured from African mahogany, spruce or birch veneers that are bonded together in a hot press over hardwood cores of basswood or poplar or from European Birch veneers throughout. Basswood is another type of aviation-grade plywood that is lighter and more flexible than mahogany and birch plywood but has slightly less structural strength. Aviation-grade plywood is manufactured to a number of specifications including those outlined since 1931 in the Germanischer Lloyd Rules for Surveying and Testing of Plywood for Aircraft and MIL-P-6070, the latter of which calls for shear testing after immersion in boiling water for three hours to verify the adhesive qualities between the plies and meets specifications.

    See that? I was wrong. It's not 30 minutes; it's three hours in boiling water that the glues have to withstand.
     
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  12. Oct 17, 2019 #32

    Speedboat100

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    Pop rivets seem to do the trick that nuts and bolts would take a lifetime to install.
     

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  13. Oct 17, 2019 #33

    narfi

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    If you are just tabbing wood feet, why not use epoxy and glass tape? It would look a lot cleaner and your brackets wouldn't rust.
     
  14. Oct 18, 2019 #34

    Speedboat100

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    I found this way much faster.
     
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  15. Oct 18, 2019 #35

    narfi

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    Fair enough :)
     
  16. Oct 18, 2019 #36

    wktaylor

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    Wood is very susceptible to moisture intrusion... and is also sensitive to the installed fastener material/finish. The small brass nails used in construction have relatively small cross-sections and are naturally corrosion resistant.

    Why not use high quality glue and temporary fasteners, such as clothes-pins or wood-worker's clamps [that look like clothes pins] for lap-joint clamp-up during Adhesive cure?
     
  17. Oct 18, 2019 #37

    Speedboat100

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    I just simply needed more strength to this.
     

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    Last edited: Oct 18, 2019
  18. Nov 6, 2019 #38

    Speedboat100

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    I think I will try the pop riveting tomorrow on this.
     

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  19. Nov 7, 2019 #39

    Speedboat100

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    I almost blew it.

    Actually the pre drilled holes and pop rivets saved the operation. Not like I planned, but still.
     

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  20. Nov 7, 2019 #40

    Speedboat100

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    In pricipal I am very happy how this turned out.
     

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