|Adjusting rack for spar centers...|
The top wing is built in one piece, this means making a bench at least 24ft long.
Tips for those planning on building something like this:
2 x 4's often have a slight twist in them, so before you start levelling get those concrete bags down to keep the legs down and solid.
I used bolts and T nuts as per the Skybolt Manual at biplaneforum.com on all legs with a lock nut. Use small squares of steel plate under each bolt head to spread the load.
Using a small protractor type gizmo is not accurate enough for the 6.5 degree cut of that size. 6.5 deg on those small things can be 6.71 etc, a big difference by the time you get to the end of the run. Use the fall calculator table in the Charts chapter of the Skybolt Manual.
Level length ways with a large level to the best you can get, then level width-ways at intervals going down the table, this must be consistent, otherwise you are building in an inherent twist in the wing. I invested in a electronic level for this which 'beeps' when 0.0 is reached...handy when grovelling under a bench.
I tensioned builders string down the length of the table [10 pound weights] for a visual flatness guide in 3 places, then adjusted where neccessary, and in a couple of places I added some shims under the 2x4 where the screws had pulled the mdf down to the 2x4 contour. Then finally, one more time worked down the table width with the level.
If my table is not level length-ways, it will only be out about a 1/16, but it is flat and is dead level width ways.
If you want to be absolutely sure on level lengthways, use a water level. This is just some clear tubing with water in it [water of course, finds its own level].
Spar racks were then fabricated using mdf. These are for fixing the wing to the bench during construction.