WEEKENDERS, JULY ’21: THE WOOD STUFF

Alex Whittaker looks at some simple sheet balsa wood working techniques.

Perversely, although the flying field is open, the wind is light and the sun is shining, I have been back at my bench.

As usual, instead of finishing the almost completed model in hand, I suffered a rush of blood to the head and have begun a random new project. This is typical of me, and most unwise.

I soon get to the stage where I have to send myself outside the shed and give myself a good talking to. I fully accept that starting yet another project, with at least two others already blocking the bench, is totally irrational. However, I have long ago come to the conclusion that I quite like defying myself.

SPLICE IS NICE

I bought an old kit from our Club Swapmeet a couple of years ago. It had groovy foam veneered wings. The trouble is that the obeche wood covering had cockled, lifted and become bumpy. Now it was possible to carefully remove some obeche from such wings, cutting back to the unbuckled area and then to splice in some virgin veneer. However, in Lockdown I was fresh out of veneer!

Now, a while ago, many US ARTF kits were sold with foam wings that used balsa sheeting instead of veneer. I decided to do the same. So, the key task was butt-jointing quite a bit of balsa wood sheeting. Now this is an easy process, if you know how. However, there are probably as many ways to do this simple job as there are modellers to tell us. However, how I did it might be of interest you.

FIRST CATCH YOUR BALSA

We seem to be in the midst of a mini balsa supply crisis. Mainly due to the rise of the ARTF, but also due to Lockdown balsa shortages, the average model shop might only be able to offer a restricted range of balsa wood compared to days of yore. Unless you are lucky enough to live near somewhere like Webbs of Frodsham (with the sadly missed Frederick ‘Ribstock’ Webb’s famed Upstairs Balsa Emporium) then gone are the days when a knowledgeable member of staff would sift through a shed load of balsa with you for that elusive speckled red rib stock.

Responding to the times your model shop will almost certainly stock the main sheet thicknesses, but often only in 36” lengths rather than the 48” sizes that were more common some years ago. Sometimes we will have two nice sound bits of balsa sheet that are just a bit short for our requirements. This means that we all have had to improve our wood-splicing skills. Or else restrict our home-brew wings and fuselages to 36” sections!

The main sheet balsa splice I use is the classic, simple ‘V’ cut or ‘chevron’ splice. I just overlap the two pieces of sheet balsa to be spliced, pin them down, and mark out the splice with a deep Biro groove. I use a schoolboy’s plastic set square to accurately scribe this 90-degree angle centrally across the wood. Then I cut out the splice with an absolutely vertical razor saw, using a good blade, running in the bottom of the Biro groove. Pull strokes, rather than sawing away, seems to work best.

Now, you’d think that this carefully laid-out approach would guarantee a perfect splice every time, would you not? Well, up to a point, Minister. Speaking as a savage, I can assure you I often need to sand the ‘arrow’ bit of the splice (not usually the open ‘V’) to get a truly snug fit. I use one of those ace ‘cutaway’ Perma-Grit sanders.

When it comes to gluing up the splice another good tip is to apply masking tape to the joint, fold the joint back on itself on this tape hinge and then run a bead of thin cyano or PVA glue down the mating edges. Then just press the joint back flat, letting the tape hold it. You can add a weight to hold the joint fair. This ensures a tight, flat joint every time.

By the way, it is a largely overlooked but amazing fact of physics that cyano glue does not seem to stick to this el-cheapo paper masking tape. You can always pull it off cleanly. Just be sure to pull it back lengthways on itself. Conversely, the more expensive car shop stuff often sticks like babysick to a blanket. Don’t forget the same glue-and-tape technique may be used for tail planes, rudders or fins made up from butt-jointed flat sheets.

BRING ON THE STRIPPER

With the modern ‘rationalised’ range of balsa wood sizes available these days buying a cheap and effective SLEC Balsa Stripper makes even more sense. Check out: www.slecuk.com/balsa-stripper?search=Balsa%20Stripper

This nifty device allows you to cut model shop balsa wood sheets into any size of strip you like. It costs a paltry fourteen quid. Essentially it is a scalpel blade mounted on a holder that can be drawn along a metal channel mounted on a simple DIY chipboard base. The channel ensures a straight cut, and you can add spacers to the blade mount to get exactly the cut you want. It is ideal for cutting parallel strips like rib capping strips, longerons or balsa wing spars.

Medium stiff balsa seems to work best for me, since it seems to come off the balsa stripper without any bends or twists in the wood. I have found that softer balsa wood can be cut square by the stripper, but internal stresses in the wood (perhaps relieved by cutting) can put a puzzling bend or twist in the finished article. Using stiffer or denser wood seems to cure this perplexing phenomenon.

SLEC BUILDING JIG

Whilst we are at it, let’s not forget the equally handy SLEC Building Jig. This handy set allows you to hold fuselages square and steady, thus avoiding building a ‘banana’.

Now then, when building a slab-sided sports fuselage from sheet, I find it important to choose my wood ‘in twos’. As is usual with balsa sheet there will be a slight hollow to one side of the wood. It is important to have these hollows face each other. In this way each opposing fuselage side (or set of strip longerons) should then cancel out any bend as you glue them up. I will routinely check the bendiness of longer items and try to ‘balance’ them equally within the structure. For example, you don’t want two soft longerons on the starboard side and two hard longerons on the port side. Failure to do this will almost certainly result in the dreaded aforementioned ‘banana’.

I find that the cheap and cheerful SLEC Building Jig helps cure this problem. It allows you ‘dry jig’ up a fuselage and check it before gluing. The jig is also most useful for wings and sub-assemblies, especially on scale models where there is a lot of fussy glazing around the cockpit, so you make your structures square and symmetrical. It costs a trifling eighteen quid, so it is far from extravagant.

It comes as a little kit. All you need to find is a flat board (chipboard / MDF etc.) between 12mm to 15mm (1/2″ to 5/8″) thick and 381mm x 1220mm in size. A printed self-adhesive grid is supplied, which you stick onto your board. Holes are drilled where indicated and captive nuts are drawn into the underside using the bolts provided. The moulded angles are slotted, so that by using the bolts and moulded washers the angles may be accurately positioned as required. 56 captive nuts are provided, together with 10 plastic angles, complete with locking bolts and washers.

Additional angles, captive nuts, bolts and washers are available cheaply. I would buy these when you order the jig since they really add to the versatility of what is an essentially simple but clever device.
I

use my jig for all my fuselages and many of my wings. It is ideal for very odd shaped scale fuselages, since you can position the plastic angles almost anywhere you like. Full instructions are given but, really, you’d need the IQ of a toaster to get it wrong…

TRAMMEL A PARALLEL EDGE

Sometimes you will wish to mark-off, then cut, a parallel strip off a balsa or ply sheet. I find that if I mark it out with a pencil, ruler and set-square, I never seem to get it right. What’s needed is a marking out guide: a means of trammeling the mark to deliver the blade and cut to exactly the right place.

A good dodge is to tape a piece of known-sized, square and true hard wood on top of the balsa sheet, carefully butted-up hard to the edge of said sheet with masking tape, then make your cut. The tape stops things slipping. So, a 1/4” inch strip of true and square beech would give you a clean parallel line exactly 1/4” inwards from the edge of the balsa sheet.

I have now collected a few sizes of such engine bearer stock to perform this trammeling task. Of course, you can stack them to give you multiples of quarter inches. Somehow, 1/4” x 1/2” beech seems to be the most useful size to me.

SPAR SLOTTING A RIB

Before we leave working balsa sheet, I must mention how I cut spar slots in balsa wing ribs. I used to saw both the slot sides with a razor saw and then dig out the slot with my scalpel. I would then file the bottom of the slot flat with a flat swiss file. With my savage bench skills, this could produce slots that were slack, wonky, and less than accurate. Then I discovered that Perma-Grit sell rib slotters.

Essentially, you use the square slotter to ‘sand’ the slot down into the rib. Very effective. I have two. The one with a red plastic handle likes file is not suitable for tight slots, but the other handle less one they make from a blank of tool steel is absolutely first rate. It produces really tight spar slots. The one I bought is 1/4” x 1/4” square, which suits my beech bearers. I have since discovered that Perma-Grit market a range of Coarse and Fine spar slotters. I must buy a set!

Before using the spar slotter, I razor saw the forward edge of the slot to slightly less than full depth. Then I begin sanding the slot in the rib with the Perma-Grit slotter. One side of the spotter is smooth steel, a ‘safe edge’, with no sanding medium. Therefore, it butts smoothly into the sawn slot edge nicely, but does not cut on that side. It works like charm, but of course you have to improvise some sort of handle. I use a bit of masking tape. It is also good for those tight areas where you want to sand in a neat square or rectangular slot, say to admit a beech bearer into a plywood firewall. It is a satisfyingly precise tool:
www.permagrit.com/model-building/