Those who have built balsa airplanes for years or decades have a long list of shortcuts they have learned. Here are a few of mine which I think will be helpful for new builders.
Make a paper dihedral template
When you build a wing with dihedral, the root ribs need to be leaned inward at a specific angle to give the wings the correct dihedral angle. This is easy to do with a dihedral template, which you can make from a piece of paper. All you need to know is the length of one wing panel, and the amount each wing panel will be raised from the building board. This can be tricky, so make sure you get this part right. Let’s say your plan calls for a 72 inch wingspan and 2.5 inches of dihedral at each tip. In this case our numbers would be 36 inches and 2.5 inches. If your plan says 48 inch span and dihedral 4 inches at one tip with the other wing panel flat on the table, then that’s a 24 inch wing panel. We want to imagine the wing sitting on the table like a V, so we have 2 inches at each tip. If you’re building an 84 inch plane with a 12 inch center section and 4 inches of dihedral at each tip, it gets a bit complicated. You have to make two dihedral joints where the panels join the center section. Imagine one joint sitting on the table like a V with the root ribs standing vertical. The center section is angled up on one side, and the wing panel is angled up on the other. This cuts your 4 inch dihedral at the tip to 2 inches. So your necessary data for our formula is 36 inch panel and 2 inch dihedral. If you find any of this confusing, send me an email and I’ll walk you through it.
Now you need a sheet of paper, a ruler, a pencil and an X-Acto knife. We’re going to make a scale model of your root rib angle on the lower left corner of the paper. Let’s use the example with a 48 inch span and 4 inches of dihedral at one tip. The numbers we’re looking for are 24 inches and 2 inches. Use a handy scaling unit that will fit on a sheet of paper. I have a ruler with centimeters on one side, so I just count centimeters. Measure from the lower left corner of the paper 2 cm to the right and make a mark on the lower edge. Measure from the corner 24 cm upwards and make a mark on the left edge.
Now place the ruler on these two points and cut with an X-Acto knife.
Discard the sliver of paper you cut off.
The angle you have created on the corner of the sheet is the angle your root ribs need to lean inwards. To make it easier to see which way to use the template, cut the other corner off so it’s round on that edge. That way you’ll never use the wrong corner by mistake.
Place one edge of this paper flat on the table, and position the rib with the other edge.
If you are working with numbers that are just too big for a sheet of paper, such as a 96 inch wingspan and 5 inches at each tip, you’ll need to scale things down even further. You can’t measure 48 cm on a piece of paper because the paper isn’t that big. Just divide it by some convenient number, but make sure you divide your dihedral number also. You started with 48 and 5, so divide by 2 and get 24 and 2.5. This is small enough to fit on a sheet of paper.
Use sanding dust to fill gaps
Sometimes you glue things together and they just don’t fit. You can use gap filling CA glue, but it tends to run away somewhere before it sets, leaving a gap. So you fill the gap again and shoot it with accelerator. Typically when this is done you are left with a protruding lump of CA that needs to be sanded away. One potential solution is to stick everything together with thin CA, then sand the area around the joint. Use your fingers to push balsa dust into the crack. Brush away the excess so nothing protrudes past the surface, then use thin CA to glue the dust in the crack.
Put your wing spar where it belongs
You have to balance your plane with the correct center of gravity before you fly it. The center of gravity should be marked on the plan sheet. Most of the time it will be right on the spar, or at the rear edge of the spar. This makes it easy to balance the plane because you just put your fingers under the spar and hold the airplane up. Some designers arrange things so the spar and the CG do not coincide, which requires you to mark the CG somewhere else to balance your finished model. Make it easier for yourself. Take note of the CG location on the plans, and when you’re cutting out the wing ribs place the spar notches at the designated center of gravity. I’ve built a lot of planes like this over the years, and I always just move the spar to where it belongs.
Plastic covering for your plans
Just about every construction article will tell you to put something transparent over your plan sheet, then build the plane on top of it. Lots of people use plastic food wrap, which is just about the worst thing, or wax paper, which is almost as bad. The best thing to use is the clear plastic backing from a roll of Monokote. Of course you’ll have to build a plane first and then cover it, and then you’ll be ready for your next building project. This plastic can be used repeatedly. I always save the backing from the wing covering on my largest building projects, which are big enough for just about anything I build.
Put wing sheeting on with the grain going in the correct direction
When you put sheeting on the leading edge of a tapered wing, don’t orient the grain so it’s parallel to the spar, like this:
Orient the grain so it is parallel to the leading edge, like this:
When the grain is diagonal to the leading edge, it tends to make a series of bumps and dips along the edge. When the grain is parallel to the leading edge it tends to lie flat, so it’s easier to make a wing with a nice, smooth surface.
Use glue dots for your center of gravity marker
Put a dot of medium CA glue on each wing at the designated center of gravity location, and give it a shot of accelerator. You can easily feel the little dot with your fingers as you hold the plane up to check the balance.