Project Update - Fabricating my Scratch Built Ribs
Step-By-Step Rib Building Process.
Here's a photo of the final ribs.
Recall, that I am working with a building partner who is also scratch building
a cub. I made a set of forms (template and die set) to experiment making a
three piece hydro-formed rib which we pressed using a hydraulic press.
Although the rib turned out very nice, I elected to build ribs using the single
piece cap strip, and aluminum intercoastals (truss style), as I could more
easily make these in my home shop with basic tools, and I liked the
single piece rib.
The photo above shows our three piece stamped rib,
my trussed rib, and an original Cub rib.
I drew the traditional USA35B cub rib profile, printed, and glued the print
onto a smooth board (shelf board). I built both left (shown here) and right
hand rib fixtures, as I am using the "L" shape cap strip material, so I wanted
symmetry on each wing regarding the direction of the "L" in the ribs.
I then cut the rib profile from another shelf board and over layed it onto
the print. I then cut a nailed the little blocks onto the board to locate the
intercoastals. Here's a photo of the right hand rib form.
I bought 6061-T6 aluminum cap strip material from Carlson Aircraft (on the web).
I elected to have Carlson machine the flange off at what would be the nose
of the rib (they know where to cut it), so the material may be folded as shown for
mail order delivery.
I used a metal shrinker to set the bends into the cap strips. Insert the material
and squeeze to "set" the bend; keep moving along to bend where needed.
Inexpensive shrinkers can be bought for non-industrial use.
Make sure the final rib fits nicely into the form. Cut the trailing
end of the cap strips to final length. I over lapped the ends and
married them together for a clean and tight fit.
Fabricating Intercoastals - I then sheared a bunch of aluminum to
width, and cut pieces to the approximate required length. I then pressed
a bead down the center of the part using a small arbor press. A friend of mine
milled a slot into a piece of flat steel stock, and welded a piece of round
stock onto a second piece of flat stock to fabricate a bead former. I
did not have easy access to a roller style bead former, and this worked
very well in my home shop.
I then bent flanges on the intercoastals as shown using a small bench top break.
Cut and sand parts to final shape. Place as shown into the fixture,
mark and pre-drill the holes, then drill the final hole size, and add clecos as
you go to hold everything in place.
Ensure the cap strip is always tight against the form when inserting
the intercoastals, and make sure the intercoastals are tight against
their form blocks to maintain consistency. Use clamps when needed.
Pop the rib out of the form, take the whole thing apart to de-burr all
of the holes. Place it all back into the form for pre-assembly, again
using clecos prior to riveting.
Pop the rib out of the form again, and replace a cleco with a rivet, and
continue. You will notice that I drilled some holes in the wood form where
the clecos extend into the form. This is done to allow the rib to set
flat in the form when the clecos are in place. I used solid aluminum rivets
and a rivet squeezer.
I fabricated all of the full length ribs first, then modified the form
slightly to make the shorter ribs as shown.
Before I went too far, I wanted to make sure that the short ribs
were matching up well enough, so I made a short sample of a
false spar and placed it as shown. I was very happy to see
how well everything fit up.
The final stacks of ribs. To reduce complexity and weight, and
having the desire to build light, I will not have flaps, therefore
I have more full length ribs than a cub with flaps has.
I learned these processes by scouring the web, looking at kit plane designs,
and from other builders on Supercub dot org. I hope others can
benefit from my posts as well.
Thank you for looking.
John