Rear Wing Spars

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Rear spar parts prior to assembly.
The 0.032" channel used for the rear spar comes pre-formed from Sonex. The top and bottom flanges are not bent symmetrically, and you must be sure to orient it correctly. One end also needs a slight taper cut into it.
There is a fair amount of laying out to do on these spars, including the location of each rib, pilot holes for each one, and the pass-through holes for the aileron and flap control tubes.
To simplify laying out the holes, I fabricated a template from some left over channel. I layed out the holes in such a way that all I needed to do was place the template on the spar channel and the holes would be in the right spot.
I then marked and drilled them at a rapid pace.
The pass-through holes are more oval than circular, and have a width of 1 1/8". My UniBit only goes up to 1", but it was a fast way to remove the majority of the material.
I next turned to my rat tail file to cut out the rest. This file has very smooth teeth, and cuts right through the channel with a minimum of grabbing and chattering.
I was able to work the hole right up to my line.
Next I used my half round file to smooth out the straight sides and generally even things out. This also proved quick and easy, and left only minor dressing with a scotchbrite pad to complete the part.
Because each spar has so many dimensions to lay out, I took great care on the first one so that I could transfer dimensions directly onto the second one. This works by placing them back to back (as seen in the picture) because the left and right spars are mirrors of each other. Laying out the second spar was especially fast.
The tip section of the spar is made as a separate piece. I believe this was purely for ease of construction so that the builder (or Sonex) is not forced to order 10 ft long aluminum to fabricate the channel. Seen here is the pass-through hole for the aileron counterweight. As recommended by another builder, I increased the opening by 1/32" on each side to provide some additional clearance for the counterweight.
To open this hole up, I first drilled the corner radiuses with my UniBit. This will allow me to get my hand nibbler in and cut out the center.
The nibbler doesn't like to start properly on a curved edge (like in a hole), so I first file a flat spot using a square file. This allows me to position the nibbler close to the edge of the hole and trim very close to the line.
The nibbler makes a neat and clean cut from hole to hole. It doesn't change direction very well, but for straight cut does great. This nibbler is a $5 model from Harbor Freight, and definitely comes in handy!
After the center is removed, the edges are filed to the line, and finally dressed with a scotchbrite pad.
As with the spar channel, the dimensions of the first piece were transferred to the second one.
Completed spar tip sections.
The spar tip is attached to the main channel with a 0.063 splice plate. The plans provide a full size drawing for it, and you have to transfer the dimensions and cut it out.
I decided to drill all the holes first.
Then, I cut out the plates using the band saw.
The inside radius was a little too tight for the blade to cut out cleanly, but a little dressing up with a file completed the splice plates nicely.
The aileron control tube pass through hole gets stiffened up with a doubler.
Fitting the spar tip and splice plate.
Rear spars complete.


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Updated: 3 July 05