.
72.
a. Construct the firewall by gluing the two 5/32" plywood F-I formers together
with Sig Kwik-Set epoxy. Use a heavy weight to hold the two pieces
perfectly flat while drying.
b. When dry, use "Cross Section at F-I" drawing (Plate 1) to draw the vertical
centerline and thrust line on the firewall.
c. Carefully position your engine and engine mount on the firewall and mark
the location of the mounting bolts. Drill the firewall at the marks. If you are
using blind nuts, they should be installed now and glued firmly in place.
NOTE: The engine shown being fitted to the photo model is a Zenoah G-38 with
a B & B Specialties engine mount and muffler. This mount uses four bolts
installed from behind the firewall; therefore, blind nuts were not necessary. The
two upper bolts are easily accessed through the opening cut into the plywood
top, PT-I, in Step 71.
73.
a. Epoxy the firewall in place on the fuselage. The thrust line should be level
with the top of the spruce longerons and balsa crossbrace, NOT THE
TOP OF PT-I!
b. Reinforce the firewall/fuselage joint with two lengths of 1" triangle stock,
one on each side. (See Photo 88.)
c. Carve and sand the bottom edge of the firewall at the same angle as PB-l.
74.
a. Glue the lite-ply formers F-4 and F-5 together.
b. When dry, glue the F-4/F-5 assembly to PT-2 in the position shown on the
plan. Use a triangle to square with PT-2.
c. Glue formers F-2 and F-3 to the top of the fuselage in the positions shown
on the plans.
75.
The position of F-IA may have to be adjusted vertically on the back of the firewall
so that the stringers will be perfectly straight from front to rear. Trial fit several
stringers and trim F-IA as necessary to make them straight. When satisfied with
the fit, glue F-IA to the back of the firewall.
76.
Glue five 1/4" sq. balsa stringers in place between formers F-IA and F-3. Trim the
stringers flush with the back edge of F-3.
77.
Protect the balsa stringers and the edge of F-2 with masking tape, then use a long sanding block to bevel the top edge of
the firewall until it is flush with the outer surface of the stringers.
78.
a. Three sheets of 1/8"x4"x21" balsa are provided for the front deck sheeting. Start with a full-width sheet in the
center. It should just cover about half of the two stringers on either side. The rear edge of the sheeting should just
cover F-4, leaving F-5 exposed for the rear stringers and apron pieces to be installed later. It will probably be
necessary to soak the outer surface of the balsa with water or alcohol to help it bend around the formers. While wet,
gently form the balsa with your fingers to about the right contour, then glue it in place, using tape or pins to hold it
until dry.
b. Prepare the sheeting for the rest of the front deck by soaking and forming as you did before. When formed to about
the correct shape, hold it in place and mark the bottom edge for trimming. It should be trimmed so that it overlaps
the edge of PT-1and PT-2, even with the top edge of the top longeron. Use an easy-to-sand glue, such as Sig
Bond, to glue the sheeting in place. Notice that the sheeting tends to bow outwards between formers F-3 and F-4,
but most of this is cut out anyway for the cockpit opening.
c. When the sheeting has dried, trim off the excess at the front flush with the firewall. Use a long sanding block to
sand the sheeting smooth, being careful not to sand away too much material.
Summary of Contents for SIGRC61
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