Airfield Models - Making Upright and Diagonal Bracing that Fit

December 19, 2021

Making Upright and Diagonal Bracing that Fit

The most difficult part of building a truss is cutting proper angles on the ends of the braces while ending up at the correct length. In fact this is one aspect of construction that I found extremely frustrating until I found a nearly foolproof method.

Attempting to cut angles on sticks using a razor, hobby knife or razor saw will often lead to unsatisfactory results. Attempting to hold the part in hand while sanding isn't any better.

The diagonal bracing is even more difficult because it has two double-bevels that not only must be the correct angle, but that angle must be at the correct angle to the centerline of the brace.

I purchased a small disk sander specifically for doing this type of work. I have had very good results with it. However, I realize that most modelers don't have a disk sander so I found another method that any modeler can use.

I used the method exclusively to prove it's validity and found it to be better than my disk sander.

Making the Braces

Note: Always make the longest braces first and work your way to the shortest. There are two very good reasons for this:

If you make the short braces first you may end up with a bunch of short scraps that aren't long enough for the longer braces.

If you have to reject a brace because it ended up too short, you can use it for a shorter brace.

Make a template to ensure the uprights and diagonals will fit perfectly.

I skipped another step. You may think that you see uprights in this photo but you actually don't. You're either imagining things or maybe you can see into the future.

What you should see is a plywood sanding template. Cut a piece of scrap plywood that aligns perfectly on all sides. Take measurements from the plan to ensure the angles are correct.

Take care making the template because it will determine how well the joints of the uprights and diagonal bracing will fit.

The upright braces are the easy to make by placing against the template and sanding.

Make a sanding block having faces that are all square to each other. I used 3/4" square pine.

Use spray glue to attach medium (220) sandpaper to one side and fine (400) paper to the other.

Note: This sanding block will come in handy for all kinds of purposes, so don't think of it as a limited use item.

Place the upright brace against the side of the template and gently sand the one end.

Balsa sands away at a much faster rate than plywood so the template should easily last through the project. If you happen to sand too much of the template away, correct it before sanding any more braces.

Fit the sanded end of the brace and mark the other end with a single-edge razor blade.

Fit the sanded end of the upright to ensure the angle is correct. Lightly mark the other end with a single-edge razor blade. Use the template to sand the end of the brace to fit.

Be conservative and leave the brace over-length at first. Sand a little away at a time. Fit often.

The bracing should be a good fit — not too snug and definitely not loose.

Several vertical braces fit and glued in place.

Here you can see several upright braces glued into place.

Cut diagonal braces over-length and center them over the corners of the existing joints.

The template you made will make short work of this task and for a change, the diagonal braces will actually fit properly.

Cut the piece over-length as shown. Lay the brace over the fuselage side aligning it with the plan.

I normally ignore the plan and instead align the brace so that it centers on the corners made by the upright and longeron.

Mark the cut lines with a single-edge razor blade. Take care aligning the blade such that it is parallel to the sides of the corner.

Carefully align the razor over the longeron and existing upright and use it to lightly mark the bevels.

You do not have to mark directly above the joint. In fact, it's best to mark slightly over-size.

It is important that the angle of the cut lines exactly match the corner in relation to the centerline of the diagonal piece. Read that a few times until it makes sense.

Align the brace over the correct corner of the sanding template.

Place the diagonal brace over the correct corner of the template ensuring that the marked lines are aligned properly with the template and that the end of the brace is centered over the template corner.

Sand the end to shape. Although I didn't do it, a piece of sandpaper spray glued to the template would have helped prevent the diagonals from sliding around while I was sanding.

Fit the brace. Be sure to check that the other end is centered over the joint.

At this point only one end is sanded to shape but look at both ends. The opposite end should be too long but should center over the existing joint.

If the other end isn't centered properly then note which way the diagonal must rotate. Take the diagonal back to the template to correct it.

When you are satisfied with the first end, mark it with pencil so that you know which side is up and which end is which. It's easy to flip the part around when adding glue or whatever. Once glue is on the ends, it is more difficult to tell which end is which.

Fit the second end just as you did the first.

Repeat the above steps for the other end.

Note that if the fuselage has straight outlines, but also tapers, then the angle on the ends will always be the same on braces that are aligned in the same direction.

However, the diagonal braces are not parallel to each other and therefore the angle on the end will not be at the same angle to the centerline of the brace.

I know that sounds confusing, but once you start making these pieces you'll understand what I mean. My point is that you can't stack up all the diagonals and cut them at once. They won't fit if you do that.

If the diagonal is still too long then you can make adjustments while sanding away the excess. Always use the template.

If the length of the diagonal is correct, but one end or the other doesn't fit properly then there's not much you can do. Your choices are to live with it or try again with a new part.

This was my first time using a template and I only had to discard two parts. I have a higher reject ratio using a disk sander and a much higher reject ratio making braces when cutting and sanding by hand.

A properly fitting brace slides into place with little force. It is neither too tight nor too loose.

This is how the diagonal should fit. The diagonal slips in place easily but is not too loose and not too snug.

Do not force a tight brace into position. It will create undesirable internal stresses and may weaken the longeron by crushing the wood fibers. It will also be forcing the glue joint apart of the upright braces.

Use the double-glue technique whenever you glue end-grain wood.

Double-glue the ends of the braces using carpenter's glue. Put some glue on each end and set the part aside for a minute. Put some glue in the existing joint as well.

After giving the glue some time to soak into the end grain, add a little more glue and put the part in place. Ensure everything is flat on the board while the glue dries.

Make a scooper from a toothpick or bamboo skewer by cutting a long bevel on the end. Use it to scoop up all the glue that oozes out.

Your work will look especially neat now that you have made perfect fitting joints using nicely sanded pieces and there is no visible glue.