ProCub Rear Fuselage Assembly Part 3

Continuing our assembly of the rear fuselage of a ProCub.

Start by gluing the three horizontal incidence wedgies together.  Do not exceed a glue zone of 3″.

Horizontal Incidence Wedgies

Glue the three horizontal stabilizer incidence wedgies together. Do not exceed a glue length of 3″, as they will be spread apart when installed on the fuselage.

In order to install the longerons, a channel must be made in each corner of the fuselage.  Using a router and a 1/2″ square end ball bearing bit, route a 1/2″ channel (square channel) down each of the four sides of the fuselage.  The router does not (and should not) go all the way to the end of the foam; the channel should just be long enough to accept the carbon fiber 1/2″ round longerons.  You will be routing through the 12mm plywood hard points.

Longeron Route Attachment

Routing a 1/2″ channel on each edge of the rear fuselage. Ignore the masking tape; it was used to check fit of the longerons.

Longeron Route Attachment 2

The routing channel ends 1″ from the front end of the 12mm birch plywood hardpoints.

Longeron Route Attachment 3

The routing channel ends approximately 3″ before the rudder post.

Longeron Route Attachment 4

The routing channel ends before the rudder post.

Lightly sand all four carbon fiber longerons.  220 grit sandpaper is appropriate.  Prepare and mix foam compatible laminating epoxy (for instance, West Systems or AeroMarine) and paint it into the longeron notch.

Longeron Route Attachment 5

The carbon fiber longeron has been sanded; the epoxy has been mixed.

Longeron Route Attachment 6

Another view of epoxy, already mixed and ready to paint into each of the longeron notches.

Longeron Route Attachment 7

A longeron glued in position and held in position with tape until the glue is thoroughly set.

Do this for all four longerons.  We then used two straps, firmly attached around the front and the rear of the fuselage, to ensure that the longerons would not lift away from their placement in the notch.  (Not shown.)

A day later, the straps and tape are removed, and 3M 2216 flexible epoxy with stiffening flox is used to fill the gaps.  We only put the 2216 for 18″ at each end of the longerons.

Longeron Route Attachment 8

Placing 2216 epoxy into the notches for 18″ at each end of the longerons.

Masking tape is excellent for ensuring that the 2216 epoxy does not drip.

Longeron 2216 gluing

Using masking tape to ensure that the 2216 glue does not drip. Pull it very tight, and overlap it from one end to the other.

The horizontal stabilizer incidence wedgies may now be glued to the top of the rear fuselage.  Use pins and wieghts; carefully spread the wedgies so that each side is flush with the fuselage.  Use gorilla glue.

Horizontal Incidence Wedgies 2

Horizontal incidence wedgies glued in place.

Remove the pins and weight; sand each side flush with the fuselage.  Use lightweight spackling to fill the gaps.

Horizontal Incidence Wedgies 4

Wedgies sanded to match fuselage sides. Spackling placed under horizontal incidence wedgies.

If you’ve already built your horizontal stabilizer, this would be an excellent time for a test fit.

Longeron 2216 gluing 3

Test fitting the horizontal stabilizer and elevator.

Longeron 2216 gluing 4

A view of the rear fuselage with carbon fiber longerons attached.

To be continued.

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