Spar doubler and strut angle fit-up

More progress on the spars and associated parts for the wings.

While using the bandsaw and bench top sander for other parts, I cut out the fuselage side wing attach plates.  The inside corners are 6.4mm radius which is too tight to cut on the bandsaw, so I used a 1/4 inch bit to make the holes, starting with a centre punch for accuracy:

With the holes done, I cut the rough shapes out with the bandsaw.  The rest of the shape will be formed using the bench sander.

Here are the pair awaiting final shaping with the bench sander.  These attach to the back top corner of the cabin frame.  The rear spar of the wing attaches to the small round ears at the top left and right.  The ears get drilled when the wings are installed.  They are thick (0.188 plate) - quite heavy but very robust.

The 8 wing side slat pick-ups, 2 wing rear strut pickups and cabin attach plates are almost ready for prime: 

Wing flapperon attach arms are all rough cut out awaiting final bench and hand sanding:

I cut the spar web doublers for both wings to size and bent the top flanges.  The inboard and outboard sides of each doubler are not the same length, so before bending I checked and rechecked the dimensions and orientation and labled them on both sides to avoid any confusion to how they orient on the spar:

It's hard to see in the picture, but the right wing spar web doubler fits nicely on the spar in the correct location (both left and right spars are back to back and the one closest to the edge of the bench is actually upside down).  If you recall, I specifically didn't drill all the holes in the spar caps in this area as I planned on doing them with the doubler in place to keep things accurate.  To keep the upper flange of the doubler in line with the top of the spar I used a couple of pieces of straight tool steel to line everything up:

With everything double checked and aligned, the doubler is clamped to the front face of the spar:

With the doubler in place I scribed the rivet line along the spar cap to get a couple of the holes started.  While I was at it, I laid out the rivet line that follows the inboard edge of the doubler.  The more I can secure the doubler as I go, the more accurate the following steps will be.

​No issue with the layout being 10mm from the edge of the web doubler, but the plans call for 12 A5 rivets at 25 pitch along this line and it has to be into the web between the spar caps on the other side.  So according to math, this means I need 325mm of space between the spar caps:

(12 rivets + 1 extra space at one end) x (25 pitch) = 325mm.

I tried several times to see where I was going wrong on the spacing.  The total distance between the spar caps as measured on my actual spar (which is perfectly accurate as to the plans) is 255mm (as shown in the red dimension line I added to the above picture).  

With 255mm available and using the same math I can extrapolate the required pitch to make 12 rivets fit on a 255mm line by rearranging the equation to solve for pitch:

(255mm line) / (12 rivets + 1 extra space at one end) = 19.615 pitch

​​So..... not exactly the same and not an easy round number for pitch.  20 pitch is the closest, but that means I'd need:

(12 rivets + 1 extra space at one end) x (20 pitch) = 260mm.

But that assumes I need a full 20 pitch space between the edge of the spar caps and the first and last rivet in the line.  So what I propose to do is keep them at 20 pitch for ease of measurement, but shorten the space at each end between the last rivet and the spar cap line enough to miss the spar caps, perhaps make the space at each end 18mm.

Man, I never thought all the math and equations I disliked in school so much would eventually come in handy!  What I can confirm is there is no physical way to place a line of 12 rivets on 25 pitch on 255mm.  I've sent an email off to Zenith to see if 20 vs 25 pitch is acceptable and I imagine it is but I'll wait for confirmation before drilling holes here.

With the first couple of holes drilled through the doubler, I flipped the whole spar over and back drilled through the spar caps from the backside of the spar on the confirmed rivet spacing.

With the doubler in place and secure enough, I started to fit the front strut angle.  I'd already cut the angles to length when I was cutting the spar caps.  Now I needed to figure out how to trim the upper end.  As usual, I needed to pay close attention as the drawing can be hard to interpret and I'm working on the right wing strut angle (the plans show the left one as the example).  I sketched out what I figured was correct, trimmmed it close and placed it in approximate place on the doubler to check if it made sense and would be oriented correctly:

I flipped the spar back over so the forward side was facing up.  In order to ensure proper placement of the strut angle, I extended the rivet hole lines from left to right and up and down from where the bolt holes will attach the ends of the strut angle.  The angle does not follow the edge of the web doubler, it actually starts at the top outboard edge and crosses the spar doubler lower edge and spar cap just inboard of centre:

I scribed a line on the lower side of the strut angle as per the measurements on the strut drawing.  I also scribed a line across the bottom flange of the lower spar cap.  When the two lines are aligned, the strut angle is clamped in the correct position.  Lifting the spar upright, I back drilled from the spar cap side, through the web, the web doubler and the strut angle.  The assembly is clecoed togther and the overhanging corner of the strut angle will be trimmed after:  

Flip the spar end for end and follow the same procedure a the top end bot hole.  Clamp and back drill through from the other side.

Maeasure again to confirm fit and all is good.  Lay out the rivet spacing.  Use a spring punch to pilot the holes, drill to A3 and cleco:

Stand the spar on the bench upside down.  Here you can see how the strut angle doesn't follow the angle of the web doubler edge:

Mark the excess strut corner for trimming:

Remove the strut, rough trim with the bandsaw then bench sander to clean things up.  Final sanding during debur will make this really clean.

That's it for this update.  While I wait for an answer from Zenith regarding the web doubler rivet spacing, I'll get the lightening holes done in this wing and flange them and I'll start to fit the strut attach pickups.  I'll probably drill what holes I do know out to A5 as well as the top and bottom A4 bolts in the strut angle.

As always, thanks for following along.

Onwards....