Wing Ribs

I built the wing rib jig such that I could clamp the whole rib flat rather than having to clamp each joint. I also choose to nail each gusset to the braces as per Bernie’s original proposals. I intend to build around 32 ribs. These should be enough for the main ribs, all end (butt) ribs and the center section ribs. The center section ribs will require some major trimming when the center section is assembled.

image001Jig with capstrips ready for gluing

image003Jig clamped up with bolts.

I first cut out gussets from 1/8th inch Birch ply. I purchased two 4 foot by 4 foot pieces from Aircraft Spruce. I marked up the board as shown and cut each circle of ply using a 21/4inch hole saw mounted on an electric drill. This method uses more ply than cutting each rib square but in my view the job looks far more professional with round gussets.

image005Making gussets from 1/8th ply

I first drilled a hole in the center position. This allowed the hole saw to be placed directly on the ply to cut the hole. I made two partial hole cuts from both sides of the ply in an effort to make a clean cut. Some were good whilst others were a bit rough. To clean up the gussets, I mounted a bolt in the electric drill, placed the gusset on the bolt and tighten down the nut. A few spins and a clean with some sandpaper soon had the gussets looking better.

image007Clean up of the drilled gussets

image010Gussets marked out as above for drilling.

Next was to cut the spruce capstrips to size for the ribs and braces. I worked out how to get as much from one 72inch capstrip as I could.

image011Enough for a few ribs in the “Production Line”

Brace No. Normal (32) End (6)
1 1.125
2 4.875
3 4.750
4 5.125
5 7.875
6 5.500
7 7.875
8 5.625 11.125
9 7.875
10 5.375
11 7.875
12 5.250
13 7.000
14 5.000
15 4.750
16 12.250
17 1.313
18 2.250 14.188
19 2.500 57.625 14.375
20 61.000 71.750
21 59.000 72.000

Table showing how many braces can be cut from each capstrip.

I purchased 100 72inch lengths from Aircraft Spruce. From the above I should need three strips per rib, with only 96 for all the ribs!

I soaked four of the long capstrips in a bath of water for about half an hour and then clamped them up in my capstrip bending jig. I can get four at a time in the jig so that was good for a few days. After letting the dry I could use them in the rib jig.

image013Capstrips in the bath for soaking…

image015…then into the bending jib for shaping.

I lined the rib jig with clear cling film. I experimented with this because I first thought that the cling film was too tight and required cutting to keep the braces flat. Doing it this way caused some of the cling film to enter the gaps between the rib and the braces, particularly around the spar fittings. Probably if I build another jig I would not make the spar section full size. After all it is only a guide. I decided to keep the cling film tight.

image017Cling film lined jig

I inserted the top and bottom capstrips in the jig with some excess length at each end; I then marked the correct size of each brace and using my disc sander, sanded them down to a tight fit in the rib jig. I started at the front and continued to the back of the rib.

image019Ready for the gussets

Next I pre-nailed all the gussets. Amazing what you can do with a “LeatherMan”. After everything was prepared I mixed up the T-88 epoxy glue. A couple of inches are all that is needed for the first gluing session. I mixed mine on an old ceramic tile and used plastic knives (suitably shaped) to mix and apply the glue. I applied glue to the joints and then the braces. The gussets were nailed in place and helped keep the braces level. I started from the front to the first spar brace, next was in between spars and finally the rear of the rib. After all the gussets were in place and inspected, I clamped on the top “sandwich”, tightened it all up and left overnight to set. I made a sample joint from some off-cuts of capstrip and ply so that I could test the glue constituency. I then thoroughly cleaned all the gluing utensils in hot water.

The following session all I had to do was to take the top off the jig and remove the rib. Careful levering with my fingers was all that was needed. When removing the cling film check that none has crept into the joint. Clean up all the joints with a sanding block.

image021Ready for the second set of gussets

I prepared the next set of gussets, mixed the glue and fitted the remaining gussets to the rib as before. I left to set for a few hours before sanding and cleaning the completed rib to get rid of any gussets overhang and old glue.

image023Completed ribs and aileron inboard ends before final trimming.

Did you know….there are 15 pieces of capstrip, 17 x 2 gussets and 64 x 2 nails, 177 parts, to one standard rib!

Repeat 32 times!!!!!

image025Butt rib gussets are almost full size

image027Butt ribs showing aileron quick connect mounting

image029All over the floor in the correct mounting order