Light Compression Ribs

There are 3 structural bays in each wing.  Each bay is made up of it's portion of the forward and aft spars, a diagonal drag wire and an anti-drag wire, and at each end of the wires there is a compression member running between the spars.  The diagonal wires are squeezing the spars and the ribs.  The spars can take the squeezing but the ribs are too week so the compression members take this squeezing to protect the ribs.  The job of the ribs is transferring the lift from the fabric to the spars, without being needlessly heavy.

 Because the loads are higher where the wing struts and bracing wires attach, the compression member there is heavier, stronger, than at the other locations.

 For some of these lightly loaded members they just glued sticks to the sides of the ribs.  These light compression ribs are at the outboard end of the first bay, on the upper and lower wings, and the outboard end of bay 3, near the tip, on the upper wing.

 The outboard ribs on the upper wings have are shortened because of the ailerons.  The compression member goes between the spars so the rib looks different but the problems of fitting the stick are the same.

The ribs used as compression ribs were made with gussets which were trimmed.  That way the sticks can be glued directly to the side of the sticks making up the truss structure of the ribs.

 The exception to this is the gusset running between the upper and lower cap strips.  It was added on the Model TEN.  The compression stick on the NINE were made from 3/8" x 3/4" spruce.  On the TEN they made them 1/2" thick to allow the stick to be notched to fit around the gusset.  The notch was quick and easy on the table saw.

 I made the gusset notch first, then cut the sticks to length.  I left ends of the sticks slightly too long so I could trim them to a tight fit between the spars.

The front spar is tipped forward 6 1/2 degrees and the aft spar is tipped aft 3 1/2 degrees so the ends of the sticks are not square.  For making the saw cuts and for grinding the ends to length I made an angle block from a piece of plywood.  From the CAD file I figured out the front angle is tipped forward from square 7 deg. 32' 31" and the rear is tipped back 2 deg. 27' 29".  It sounds complex but it's real easy to layout precise angle cuts on the block using trigonometry.  The tangent of the angle is the rise over the run.  Draw the angle line on the block, cut close to the line, then grind just to the line on the belt sander.  I should have taken some pictures.  Ok, I'll try taking some pictures and post them.

 The angle block is held against the miter fence.

I draw A line near the end, not to grind to, just so I can see how much I'm removing with each pass on the belt sander.

The stick being ground is held tight to the angle block. Take gentle light cuts.  Who cares if it takes several trial fits to get the length to a tight fit.  As I grind the ends I choose which end to grind, forward or aft, to keep the middle gusset in the notch.

I've added a block next to the one for the rib to spread out the load from the end of the stick onto the spar web.  At the other compression ribs the spar is not routed so the block isn't needed there.

Keep slowly working the ends until the stick is a nice snug fit.

The stick was glued and clamped to the rib sticks and center gusset.

The compression rib at the outboard end of the first bay is the same concept with a few twists.  There are sticks on both sides of the rib and the the sticks rest on the wire clip for the drag wires.  The sticks also have to fit between the 2 bolts holding the wire clip to the spar.

In this case the wire clips are made of 0.095" thick steel, which means the sticks are that much shorter on each end.  I started with the sticks sawed to the same length as the finished stick at the tip rib.

I used the angle guide to hold the stick so the end would be flat on the table.  I scrap piece of 0.095" steel was used as a guide to draw a reference line for grinding down the ends.

Again I'm not grinding to the line but to a tight fit.  The line is just a helpful guide.

Just like with the outboard rib stick, I started grinding each end on the belt sander, check the fit, grind a little more.  I stopped when the stick easily fit with about 1/8" of the wire clip still showing above it and forced it tight from there.

Same process for the stick on the other side of the rib.

The original sticks were 3/8" thick vs 1/2" for these, but even the 3/8" sticks needed the end ground down a little to clear the bolt heads and nuts.  The bolts a 1 1/2" on centers, the bolt head are 7/16" across the flats, and the rib stick is 3/8" wide.  That leaves 11/32" for each stick.

On the rear spar the bolt heads are next to the sticks.  At that end of the stick I tapered the end in 5/32" for about 1".  They fit fine with the bolt head flats vertical.

On the forward spar the nuts are next to the sticks.  The nuts are taller than the bolt heads and they have washers under them so it needed a more gradual taper to clear them.

 For these I made the taper about 2" long and it all fit fine.

The sticks were glued to the rib sticks and the center gusset.  The easy way to hold them in place was just with some clamps.

The day after I took this picture I went to Harbor Freight to get more clamps.  They had them on sale for $1.99 for the little ones and $2.99 for the big ones.  I like the rubber pad on this style clamp because they don't leave a mark like using C-clamps so you don't need shims to protect the wood.