Because of the length of the wings spar caps they will either need to be custom cut and shipped (expensive) or scarfed together out of 36" stock. I'll be scarfing together my spar caps out of douglas fir. Tom wrote up a nice piece on "Flightline" describing how he scarfs a joint. Several important things to remember about scarf joints and spar caps as pointed out in Tom's posting:
First of all, a scarf is a diagonal or sloped joint used to marry two pieces of stock together to make a larger or longer part. In boat building we use them for gunwales and side and bottom planking and in aircraft for stringners and leading or trailing edge stock and spars and spar caps.
Second, its best if possible to make the scarf joint fall on or cross over a structural framing member such as a rib or bulkhead.
Third, if possible and without making the scarf too long, have the ends of the joint pass over at least two structural members.
Fourth, its best not to have the upper and lower spar caps scarf-jointed at the same location so the lower cap can be scarfed on the inner panel. I would chose this method, greater distance to the first scarf on the upper, shorter distance to the first scarf on the bottom with good reason - most failures occur in compression and in the case of the wing, this would be the upper surface. Also, the greatest shear loads on a wing panel occur in the first 10% of the span and diminish geometrically further away from the junction of the wing with the fuselage.
In the case of Mike's 1-23, the rib spacing is less than three inches and the stock he is working with is 3/16" x 1/2" douglas fir. With the 36" long parts, Mike can make the first scarf between ribs 5 and 6 which are the second and third ribs of the outer tapered wing panel. Using a slope of about 5 degrees for the joint will enable him to pass by both of these ribs by about 3/4" to 1" which will yield the highest strength.
By arranging the scarf joints as pointed out above, this will yield the highest margin of safety. In addition, make the joints on upper and lower surfaces fall at least three of four rib bays away from each other so loads can be distributed well away from any inherent weakness in the spar caps caused by scarfing.
So With Tom's advice in mind - the first thing I did was to locate the scarf joints. Each wing will have 4 scarf joints - 2 for the upper spar and 2 for the lower spar. Locate your scarf joints on your plans:
Next I selected the wood:
Now I layout the pieces for rough fit.
Setting up to cut the scarf joints. I did it a touch different from Tom's method. I taped the stacked parts together then used a mitre back saw to do the cut. Worked fine.
Rough cuts before sanding and final fitting:
Joint clamped and glued - I used Titebond III:
Rough scarf joint - repeat 3 more times for a set of upper and lower spar caps.
Final product two straight 96" spar caps:
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