SPAR carry through doublers

Some quality time in the shop this weekend.

The spar carry-through channel and doublers are critical components that define symmetrical mounting points for the wings and cabin frame.  Kit builders truly have an advantage here.  Their components come pre-drilled match up perfectly.  I originally decided to buy the main channel from Zenith then fabricate the doublers myself once I figured out the order of operations to match drill the other two parts (the factory part is already drilled).

Ron and I had the plasma cutter out for some other fabrication work (engine stand for his O-200 engine) so we decided to give it a try cutting my doubler blanks from 063 aluminum sheet.

We'll need some more practice using the plasma cutter on aluminum (material feed rate, amps, air-pressure) to get nicer results, but with a bit of clean-up, the first doubler angle turned out acceptable.  This doubler runs along the upper rear of the spar channel.  Here is a picture of it lying flat on the bench in front of the channel and my smaller test piece to confirm bend angle with:

With the test piece confirmed correct, I used a piece of channel as a straight edge to mark the bending line:

Used the heavy bender to form the doubler angle to the correct 65 degrees closed angle.  My shorter test piece was perfect, but it took some extra effort and convincing to bend the doubler as 063 is almost too stiff a material to bend at that length with the bender we have.

Once confirmed as correct when matched in position on the spar channel, I clamped the assembly together and elevated it on the bench to allow the clamps to be where I wanted them.  Both arms of the doubler angle must lay flat against the spar channel in order to ensure the match drilling works properly.

For balanced drilling, I started in the middle and worked alternatively left and right to drill the rear facing rivet holes to A3 (eventually will be upsized to A5 here): 

With the rear side of the doubler angle matched to the spar channel, I clecoed it in position on the upper skin (remember, this is upside down on the bench right now) to check for fit.  The sub-assembly is held in position with the outermost A5 clecos that can be match drilled from above.  All good so far:

Rear arm of doubler angle is perfectly flat against the channel and also sits flat against the skin under the doubler:

To finish the doubler match drilling on the underside, I flipped the top skin over, clecoed the top fuselage doublers to the skin.  Then I elevated the skin onto square tubing (not shown in picture) and added the spar channel sub-assembly under the skin:

The outermost A5 clecos through the top skin doublers, the skin and into the sub-assembly holds things in position.  The I used a deep c-clamp and some scrap wood blocks to hold the "sandwich" together for match drilling:

I also added a long thin strip of scrap 063 on the front facing edge as well - this temporary spacer  represents where the front doubler channel would be once I have it in place, keeping the sub assembly square to the skin and the holes in lined up perfectly vertical for match drilling.  I worked from centre out to the sides, drilling though the existing A5 skin holes in the skin, then the doubler and into the existing spar channel holes:

Rear facing spar channel doubler angle match drilled perfectly.  This orientation of the fuselage skin on the table confirms the top doublers are positioned correctly to the drawn skin edges as well.

Doubler is in perfect position flat against the skin and the spar channel.  Eventually this will be drilled out to A5 across the spar channel and A6 rivets at the shoulders of the top fuselage doublers at the wing mounting points.

Very happy with how this first doubler turned out.  The front facing doubler which is actually another channel, will need to be purchased from Zenith however.  We just don't have the ability to fabricate this in the shop accurately enough to ensure good match drilled holes.  I believe we could try to do it, but the effort to do so could lead to unacceptable bend and match hole accuracy - not something I willing to save a few dollars on.

​More to come, thanks for following along :)