I just had a conversation with Mike about scarfing the joints in the spar caps for his current build, the SGS 1-23H at quarter scale. Because spruce stock from hobby suppliers generally comes in 36" and 48" lengths, two or more pieces must be joined to make a spar cap continuous for a wing panel greater than those lengths. That includes just about any sailplane with greater than 96" (2.4m) span.
Here are a few key points to keep in mind and the procedure for making a scarf:
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 stringers 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.
Arranging the scarf joints as pointed out above, 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.
Now all of this said, it should be remembered that if one uses a high quality epoxy or Titebond IITM to cement the scarf joints following the procedures above and making the jointed surfaces as accurate as possible, no problems with these joints will ever be experienced. The joints will be strengthened further by the use of shear webs against and across the edges of the joints and the balsa sheeting applied on the wide or flat surfaces of the joints. In all cases, make sure you have good wetting of the joints with the adhesive chosen, and a good system for clamping the joints till cured to achieve the best quality bonds.
For cutting the joints, mark one piece with the slope or angle of the scarf and then stack the two parts to be joined one over the other, end-to-end and cut both parts through at the same time, making sure to keep your razor saw in perfect 90 degree or vertical alignment perpendicular to the working surfaces. Test fit the joints once cut and if necessary, align the two scarfs together mated against their flat sides and run a small hand plane over the joints to square and true them.
After testing the fit to your satisfaction, be sure to wet both surfaces with just enough cement to make sure there is at least a little squeeze out under clamping pressure but not so much that the parts will slide against one another. To prevent this, before cementing place waxed paper on the edge of the workbench and clamp two stop blocks on the bench positioned at each end of the parts to keep the clamping pressure from moving them away from each other longitudinally. Have at least two small clamps prepared to clamp the joint once aligned in your shop-made "jig". To make sure that the fine tapered ends of the joint receive as much clamping pressure as the center section, use two small cauls or blocks of wood between the clamps and each part to spread the force throughout the jointed surfaces. If necessary, place another piece of waxed paper on top of the joint and another clamp to hold down-pressure keeping the joint tight to the workbench and the two parts in face alignment while the glue cures.
Finally, after the cement has cured to initial hardness, remove the three clamps at the joint and using a scraper or razor blade, remove as much of the excess cement from both sides before it reaches full cure and final hardness. Then set the part aside to cure for another few hours before cutting to final length or making the next scarf joint, if required.
Mike will illustrate these proceedures in his build log.