T. van Vuuren wrote:I could also not find the correct length wood for the spars or longerons when I built the L-18 Cub ( in Sabie nogall ), so I simply made good old fassioned scarf joints as per the AC-43 regs. I did several tests (even with joints tht were only 50% the minimum specs) and every time the wood failed outside the joint. BUT - stay with what the plans say!!!
To make a scaf joint in something like a 15mm by 25mm stringer, you will take either the with or thickness of the wood and multiply by a factor of 15 to get a 1/15 slope.Cut the wood with that angle (both pieces) and do the final shaping with a hand plane.Constantly check the fit and if there is no gap, you can glue and clamp.
I use Epidermix 372 epoxy ,but there are MANY products on the market.Just chat to AMT in Kempton, they make epoxies
Never sand a joint to final stage, the dust can clog up the wood fibers and lead to a weak joint.Always make a test block with a new bach of glue and remember to keep a log of the bach #'s
I also laminated the spar caps of the cub as I was not totally happy with the idea of not being able to "see" inside the wood caps.The spar is made up of an upper cap (20mm by 70mm IIRC) and a slightly smaller lover cap with A grade ply web.
I cut strips of 15mm by 20mm to do the cap laminations and stagered the individual length scarf joints as not to have them all in the same place. Even if there was to be a "vrot kol" in one strip, the other will bear the load.Just check the wood carefully.
Remember, epoxy likes to have a slightly corse surface to stick to, so I usually scrape the area to be glued with a 32 tooth hacksaw blade to make fine scrapes in the glue face.
Also remember that the grain run-off for any wood is a min of 1/15. This means that if you were to look at a spesific line of grain, it must run from the top of the plank out the bottom of the plank in no less than 15 times the with of that plank, if that makes any sence to you
Laminating to wood is really not hard, just time consuming, but atleast you can be sure it is all fine.OR just buy spar quality spruce and save a lot of work I was to cash straped to do this when the $ was 13 rand!
Dont forget woods like Douglass fir and clear Oregon pine, they are also very good and easier to work.
Hope it helps a little.
I will try and find pix of how I laminated the spars and post it. NB!! if you laminate spar caps, do it against a straight edge on the tabe, otherwize you could laminate beautifully curved caps for a frencs Flying flea!
A tip on glueing, don't clamp it so tight that the wood starts to creek, actually leave some glue in the joint!
Thanks Theuns, your advice is much appreciated (also on another thread). Will probably at some stage contact you and "pay for your time" as Noel suggested.