Author Topic: Scarfing the Spar booms  (Read 956 times)

Offline WI_LEagle

Scarfing the Spar booms
« on: December 23, 2018, 02:38:03 PM »
To obtain good wood for the Spar booms most need to ship them.  Motor freight of full length would add about $300.  UPS allows for upto 108" in length.  https://www.ups.com/us/en/help-center/packaging-and-supplies/prepare-overize.page#contentBlock-13

Where is it recommended to put the scarf with respect to the attachment end of the spar?

Regards

Matt

Offline MichaelINWA

Re: Scarfing the Spar booms
« Reply #1 on: October 31, 2019, 12:11:43 PM »
A 10:1 scarf joint will retain at least equal structural integrity across the span as long as the grain is the same direction between the two pieces. (Read: it doesn't matter where)

Edit: it doesn't matter in terms of strength, often the joint is stronger than the surrounding wood which changes properties of flexibility and stiffness, for these reasons some would choose to centralize the "hard spot" though in this case without finite element analysis of the completed spar structure it'd be hard to gauge what differences if any may exist.

There's several species of wood suitable to build with, I'd encourage you to look at what's in you're area if possible. Ash and Doug Fir are sometimes available in the lengths needed at the big box stores, just gotta dig for the right pieces.

Offline Kamcoman77

Re: Scarfing the Spar booms
« Reply #2 on: October 31, 2019, 05:54:54 PM »
Most manuals state you should not put a spar splice in close proximity to any wing attach or strut fittings. If it were my plane, I would try to put the splice outboard of the wing struts. If you used one 9-ft and one 6-ft spar cap overlapping from the 8-ft mark & the 5-ft mark you would end up with a 13-foot spar with a 12-inch splice one bay outboard of the strut fitting bay.

Offline Vince Carucci

Re: Scarfing the Spar booms
« Reply #3 on: November 01, 2019, 09:23:59 AM »
Generally speaking, due to the bending moment on the wing, the spar cap-strips will feel compression or tension only. The webbing (1/8 plywood) carries shear forces in the spar. Both shear and bending moment are greatest near the strut bracket. That being said, it might be best to splice spar parts as far from the strut bracket as possible.

Offline Tom XL-7

Re: Scarfing the Spar booms
« Reply #4 on: November 01, 2019, 10:24:29 AM »
My spar caps are full length. my plywood is 8'. For my ply I am going with 34" then 90" finally30" that is inboard to out. Obviously wanting most strength at the strut attachment. offered up as consideration for centers of spliced spar caps if you are doing that. Just don't stack your ply joints in same location. My ply piece will have an extra splice two as opposed to one but I think it is worth the effort. When I build things I try to do my best. Thinking that after reality seizes the day it will be good enough. When I was very young I did things good enough only to discover the cost of redoing or repairing sometimes.

Offline 914pete

Re: Scarfing the Spar booms
« Reply #5 on: November 01, 2019, 12:33:03 PM »
Following this discussion. I've put off ordering spruce to start my spars because of the shipping expense going with full length spar stock.