The long wing Model 110’s have a 32 foot wingspan. The front and rear spars need to be long enough to provide the 32 foot length plus the required overlap at the center scarf joint. Plus it doesn’t hurt to have a little extra length just in case something goes wrong when cutting the scarf. I was able to purchase 17’6” spar blanks directly from the mill in Canada. I spoke directly with the person who would be selecting and cutting the wood. This worked out great. The front spar is 1” thick and the rear spar is 1.25” thick. It took a while for boards which met my requirements to become available, but it was well worth the wait. They were absolutely beautiful, with clear straight grain well exceeding the requirements of AC 43.13 Chapter 1.
You have to cut the scarf joint first because everything else will be measured and set as a relative difference from the center line. The front and rear scarf joints have the same slope, but the rear spar scarf is longer due to the larger width of the spar.
I used a technique described in one of the Monocoupe Newsletters to fabricate a jig that when used in combination with a router would make the scarf joints. The jig is just boards cut, glued and screwed together to provide the required “ramp” for a router to slide on top of and thus cut the required angle.
I made a custom router base out of 1/4” thick Lexan. The base needs to be wide enough to allow the router to span the jig from side to side. Because the Lexan has a tendency to flex, the base was reinforced on both edges with 1/8” thick aluminum angle screwed to the base. Make sure everything is held down real good. The jig is screwed to the table and the spar blank is clamped using a large clamp to both the jig and the table. You can’t really see it in the pictures but the very last 3/4” of the spar is actually screwed to the jig to keep the end of the spar from moving around. That last 3/4” of waste is trimmed off after the scarf is complete. The block of wood screwed to the top of the jig at the far end provides a hard stop for the router and defines the end of the cut.
Using a brand new sharp 1/2” flat router bit, slowly start to remove material. I never took off more than 1/16” on any given pass and most passes were probably less than that. That means lots and lots of passes with the router. I actually had to reposition the spar half way through the process due to the depth ability of my router. Just take your time and go real slow. I did these cuts in August in Arizona with the hangar temps pushing north of 100 degrees. I was using a HobbyAir forced fresh air breathing mask and full coverage eye protection.
I used the original spar to layout the ribs and lightening pockets for the new spars. Each spar has 4 lightening holes per side, thus a total of 8 per spar. Each pair is a different size. A variable size jig was made to guide the router to the proper dimension. Just like when making the scarf joint, the lightening pockets were cut with many, many passes of the router, each taking off about 1/32” of wood. Around the edges of the pocket I used a rounded bit. At the bottom of this page is a picture of that same lightening pocket in a more “finished state”. This is where the gas tank is located.
The wing has one degree of dihedral. I worked up a simple Xcel spreadsheet that gave me the vertical rise dimensions at every foot from the center line. I made up a 16 foot long table and laid out the dihedral with pencil lines. I put clamping blocks every foot. Glue the joints and clamp everything into the jig. See the picture at the left. Notice the small pieces of spruce glued back into the lightening pockets where ribs will be located.
The spars have steel bushings where the lift struts attach. They are 5/8” OD, drilled through the center for the 1/4” AN4 attach bolts, and are 1 7/16” long. There are three at each attach point in the front spar, as seen in the picture to the left. There is one in each rear spar lift strut attach point. Here is a link to the spar bushing drawing.