ETLB Squawk Forums — Archive

Since I can only afford to by plywood locally, due to shipping costs, and I would rather use aircraft Birch than Okume plywood, I need to splice plywood in order to get the length that I need for the fuselage sides.
The plywood that I can get is 4’x4’, so I obviously can’t produce a piece that is 53 ½” long without splicing. I know that it will have to be a 12:1 scarph joint. That is no problem. I am just wondering what is the best place to locate the scarph joint. I would assume that the red line in the picture below is where the scarph joint should go. Mostly due to the front of the fuselage being curved in slightly.
Secondly, would you put a plywood “doubler” on the inside of the scarph joint? I was thinking maybe a 6” wide doubler on the inside of the fuselage. Would seem to me that it would make the joint that much stronger. Maybe just make the doubler 1/16” ply instead of 1/8”?
Any assistance would be greatly appreciated!

I think a backing plate would be overkill if the scarf is well done. ANC-18 would be a good reference. The red line would be my choice for the location of a joint. Fiberglass tape is another option for reinforcement.

While 12 to 1 is recommended, 10 t0 1 is approved. I would put a 3" wide doubler on the back side.

It does make sense to place scarphs where there is little or no bend.  If there is any flaw in the scarphing that puts the least stress on it.  Provided the scarphs are perfect without ragged feather edges or rounding or cupping of the surfaces there really isn't any need for doublers or anything like that.  Provided a sanding type of scarphing system is used with a good solid jig you should have good results.  If you have any reason to question the scarph, rather than add a doubler I would put one thin layer of either glass cloth or carbon fiber and epoxy, the lightest you can get, on either side to ensure the distribution of stress over a wider area at the surface.  That adds the absolute minimum weight and the scarph will then surely be stronger than it needs to be.

Tom

Of course you can get a 53"panel out of a 48" sheet. You will need to buy two and use your remainder elsewhere.

I am encouraged by this post.I approached the PFI < forest institute> locally if they could make some high quality plywood.They have a 3 foot wood peeler but only a
2 foot square hot press.The forest institute was supposed to be a research institution but has not lived up to it.
Local plywood manufacturers don't care about making  less then 100 sheets.
Presently my mulberry log is being softened in hot water. I have also located phenol glue which is manufactured locally by
wah noble.

If it all works out I will get 22 inch square sheets of 1/8 EXPERIMENTAL plywood comparable to mahogany.

The post suggests that scarf and butt joints are allowed. The only thing I have to do is cobble a jigsaw puzzle out of the 22 inch squares.

I also have the 1600 cc minimax in mind as I have the engine which I run once a year just to remind me.Altough I am a little wary of the high stalling speed of 45 mph,
In that case the 1100 would come to the rescue as I have an 800cc 1/2 vw also which I have to upgrade with 92mm pistons/

KK

for the piet I went to a local lumber yard and they got some baltic birtch it comes in 5ft x 5ft pieces and is every bit as good as ACS plywood which i order a couple of 4 by 2 sheets of before i found the birtch, I even torture tested, ie soaked, boiled hit with a hammer, it held up just as good.

I will have quite a bit of ply scrap left over mostly from mahogany. I also have a 4x 4 ' sheet of 1/4 birch.along with a bunch spruce and 1/8 birch ply scraps. Not enough to build a whole lot but enough to help. I live in upstate south Carolina pm if you want it, will not ship.will help you load it on your pickup truck at the end of my driveway.