One of my favorite planes is the RCM Basic Trainer. Don’t let the name fool you. Just because it’s called the Basic Trainer doesn’t mean that you have to be a beginner to enjoy it. This is a four-function airplane with ailerons, and it’s pretty good looking, too. Everybody really ought to build one of these. The flat fuselage top and bottom make it a perfect project for learning to build a straight fuselage without a jig.
After you get the fuselage sides and cabin bulkheads cut out, glue these four pieces together to make a basic box. Measure the diagonal distance from corner to corner in the cabin/radio compartment. First measure one way….
Then the other way….
If your two measurements are the same, then your fuselage is square. If not, squash the fuselage diagonally until the two dimensions match. You may have to put a squeeze on it hard enough to crack the glue, then reglue it in the right position.
After you have the radio compartment square, put the planking on the bottom of the fuselage, only in the cabin area. Then draw a centerline on the bottom of the plane using a straight edge.
Now it’s time to pull the tail together. I’ve found that if you do the nose first you’re more likely to get it crooked, and it will have a severe effect on the tail. But if you do the tail first it’s easy to get it straight, which sets you up for the nose to be straight. Just use the line on the bottom of the fuselage to line up the tail joint. When you pull the tail together and the line points right at the joint, it’s straight. You can use a long straight edge instead of your eyeball for greater accuracy.
When you’re gluing the tailpost, in addition to being centered the joint also has to be vertical. Glue just the tip of the bottom corner, then flip the fuselage around and hold it in front of you with the tail pointing at your eye. You should be able to tell if the tailpost is parallel to the vertical sides of the fuselage. Shift the tailpost seam as needed until everything is straight, and then glue the whole joint.
After you get the tailpost joint worked out, go ahead and plank the bottom of the tail section. Whenever I buy balsa wood, they always send me some flat sheets and some warped ones. Save the flat ones for wing sheeting and fuselage sides, and use the warped stuff for cutting up into little pieces for fuselage planking. Here’s a warped sheet of 1/16″ balsa, in case you wanted to see one.
After you get the aft planking done, it’s time to install the firewall. Mark a centerline on the firewall and use a ruler to line it up with the centerline on the bottom of the fuselage. A thin flexible metal ruler will be able to bend and follow the curve of the fuselage bottom.
After you get the firewall lined up on the centerline, epoxy it and clamp it in place. After you have that done, attach the chin sheeting. Now you have a straight fuselage, and you didn’t even need a jig.