That's right, it's finally time to put part 1 to rest. I've finished* building ribs.
Tonight I started on trying out a method for profiling the wing spars. First, I made a "good enough" attempt at resawing a length of 2x6 to get a 1" thickness. It's a perfect 1" thick on the edges, but the fence that came with my table saw is definitely not machinist-quality, nor exactly 90 degrees, so the center of my faux spar is a bit concave. Good enough for what I'm doing, where the edges are what matter. If I need to do aircraft-quality resawing, I'll figure out how to do it on a bandsaw or planer.
I pulled out the trigonometry, and decided that the table saw blade would have to be set to 1.79 degrees (which is atan 0.03125) for the 1/32" cut on the bottom, and about 10.6 deg for the 3/16" cut on top. My first attempt at this was to use my inexpensive digital protractor to get the table saw blade to the right angle. I came quite close on the first attempt, seeming to achieve the desired 1/32" on the bottom edge, and 3/16" on the top edge, according to my unmagnified view with a machinist's rule and a speed square.
The spar's back length (4 7/16") is quite accurate, but the front is a bit short, 4.167" when it should be 4.219", so obviously my measuring is off somewhere. The too-short front is clearly shown when trying to fit a rib on my spar-ette, and it looks like my top cut angle is too steep, with about a 1/32" gap at the front of the spar, and the back of the spar about 1/32" too tall to fit most of the ribs, according to a random sampling. I believe the first jig I used was not set up quite right, as a rib from the second jig fits exactly right.
This of course raises the question: what's 1/32" between friends? If I profile my spar so it fits the majority of my ribs perfectly, it will have a gap of about 1/32" with the four wing tip ribs (the only ribs built on the second jig). Is a gap of this size a real problem, or the expected level of slop in a biplane built in some nerd's garage?
* Probably
Tonight I started on trying out a method for profiling the wing spars. First, I made a "good enough" attempt at resawing a length of 2x6 to get a 1" thickness. It's a perfect 1" thick on the edges, but the fence that came with my table saw is definitely not machinist-quality, nor exactly 90 degrees, so the center of my faux spar is a bit concave. Good enough for what I'm doing, where the edges are what matter. If I need to do aircraft-quality resawing, I'll figure out how to do it on a bandsaw or planer.
I pulled out the trigonometry, and decided that the table saw blade would have to be set to 1.79 degrees (which is atan 0.03125) for the 1/32" cut on the bottom, and about 10.6 deg for the 3/16" cut on top. My first attempt at this was to use my inexpensive digital protractor to get the table saw blade to the right angle. I came quite close on the first attempt, seeming to achieve the desired 1/32" on the bottom edge, and 3/16" on the top edge, according to my unmagnified view with a machinist's rule and a speed square.
The spar's back length (4 7/16") is quite accurate, but the front is a bit short, 4.167" when it should be 4.219", so obviously my measuring is off somewhere. The too-short front is clearly shown when trying to fit a rib on my spar-ette, and it looks like my top cut angle is too steep, with about a 1/32" gap at the front of the spar, and the back of the spar about 1/32" too tall to fit most of the ribs, according to a random sampling. I believe the first jig I used was not set up quite right, as a rib from the second jig fits exactly right.
This of course raises the question: what's 1/32" between friends? If I profile my spar so it fits the majority of my ribs perfectly, it will have a gap of about 1/32" with the four wing tip ribs (the only ribs built on the second jig). Is a gap of this size a real problem, or the expected level of slop in a biplane built in some nerd's garage?
* Probably
