Apparently the heavy compression rib at the strut fittings wasn't strong enough for the loads on the upper wings. WACO added what they called a Secondary Strut 3 9/16" inboard of the rib, just on the upper wings. Because the spars are not parallel the strut wants to slide up the spars. The factory tried a few methods to hold this stick in place before they settled on some "U" shaped blocks of plywood, glued to the spars above the sticks.
The stick is a piece of spruce 1 1/8" wide by 7/8" tall. The stick is not parallel to the cord line nor to the compression sticks on the other ribs so we need another angle guide to get a tight fit to the spars. The forward end tips forward 6 deg. 37' 31". The aft end tips aft 3 deg. 22' 29".
For me the easiest way to make these angled joints fit tight is to make an angle guide from a piece of plywood. I normally would have made this block a little wider but this was the piece of scrap I had. The first step is to saw the top and bottom so they are parallel and so the sides are perpendicular to them. The 2 ends of the stick have different angles so I've drawn a centerline perpendicular to the top edge. This way I can use the guide for both ends, forward end on one side, aft end on the other.
Now we're off to math land. We're going to use some trigonometry.
The tangent of an angle is the distance the line rises, or in this case falls down the edge of the board, divided by the distance along the the top edge of the board from our centerline. The distance along the line is the hypotenuse of the triangle, we're not using it, don't care how long it is for this.
Our guide block is 3 17/32" from the centerline to the edge. For the forward spar angle, right side, the tangent of 6.625278 degrees is 0.116151. I'll skip the algebra of why, we just multiply the tangent times the distance to the edge and we have the distance down the edge 0.410158" or 13/32". My scale is in 64 ths of an inch. The same process gets us the distance down for the aft side which in this case is 13/64".
I sawed close to the line but not to it.
A gentle touch on the belt sander allows me to grind the block to just split the line for a nice accurate guide.
Both sides done.
To use it I hold the guide block against the miter. I sawed the ends of the stick close to the line but not on it. Then I grind it on the belt sander until I get a very tight fit between the spars.
I've drawn the location of the stick on each spar. The idea is to position the stick to the lines and then glue on the blocks to hold it in place. There are no dimensions on the drawings for how far the stick is located up the spar from the bottom. The photos show the lower edge of the spar at the top, easier to work with the wing upside down. The location on the old spars are clear on the rear spar, the bottom of the stick is 3/8" from the edge of the spar.
On the front spar it's not clear on the old spars where they positioned the stick. I've placed it 19/32" from the bottom edge. This gets the edge of the stick tangent to the lower edge of the wing ribs. I didn't want it pushing out on the fabric. The old wings may have done that a little, not clear.
There are some more blocks of spruce that go around the fittings, for attaching the fabric, they will close the bottom of the retainer. It doesn't matter though because the stick can not move further down because of the angle of the spars.
I've done all this with the wing upside down. From the upper side you can see how close the stick is to the bottom of the spars.
There is a box that goes on the other side of the heavy rib, for the end of the aileron control horn to travel into. I'll do the blocks around the fittings when I build the box.
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