Back in the old days, many plans had a materials list included on the drawing – and it’s been suggested that it would be good to reintroduce the idea. However, even if future plans do incorporate such a list there are thousands of old ones that don’t, leaving the builder to work out what materials are needed.
The materials list of yore was only ever a loose guide, because some people use more wood than others. For example, one individual might cut a set of wing ribs from two sheets of wood by carefully interleaving them, whereas someone else might use the ‘solid block’ method of cutting them out, which uses far more wood. The other problem was that the list only usually told half the story. For example, it might call for six sheets of 1/8 x 3” balsa. But what grade are you going to buy? Soft? Hard? And how much of each grade do you need? This essential grading information was usually missing.
I’ve penned a few designs over the years, but I couldn’t tell you how much wood most of them need because I don’t buy on a model-by-model basis; I like to keep a large stock of wood to hand so I can build a model without having to buy any more. I only estimate the wood needed on those rare occasions when I farm out a plan to a club member. When stocks are low I order from Hobbystores at Watton who still supply Balsacraft wood, the quality of which is superb. When the selection arrives it’s exactly what I’ve asked for, and the price is about two thirds of what you’d normally pay – don’t ask me why!
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There’s no science involved when it comes to working out the amount of wood needed, in fact it’s remarkably easy. Providing you’re aware of the stock sizes of wood available and you have your full-size plan to hand then you shouldn’t have any problems.
It’s probably easiest to work out the wing requirement, so we’ll start there. Let’s take a simple panel of 58” span and 12” chord with a maximum thickness of 11/2”. It has leading edge sheet, a normal four-spar arrangement, a solid trailing edge, and is equipped with strip ailerons.
First off, we’ll need four strips of the specified wood for the spars; two strips each for the leading edge and trailing edge. The in-flight stresses imparted upon these spars means that the wood used must be rock hard, and if not of ‘stock’ size it might have to be stripped from sheet. If you were working to my plans these would be termed as being made from ‘medium’ wood.
Next, measure the width of the leading edge sheet to determine whether you need 3” or 4” stock. For our example wing you’ll need four sheets, medium hard, the density of which should be such that you can bend it across the width of the sheet. Each wing panel is 29” span, so the stock sheet length of 36” will leave 7” long offcuts. Don’t discard these to the balsa scraps bin as they can be used as part of the centre-section sheet; measure up what’s required for the centre-section and make what you can from such offcuts. Less waste equates to less money in the long run!
Is the trailing edge sheeted? If so, add that into the estimation. Any cap strips also need to be calculated, though these might come from excess t.e. sheet or the balance of the centre section sheet. Are the strip ailerons a standard size of t.e. stock? If not then you might need to buy an additional sheet of wood from which to carve them.
Estimating wood for the wing ribs may be the hardest part. The basic technique is to take a rectangle that will contain each rib and see how many times it goes into a sheet. In our example, 12 x 11/2” will go into a standard sheet six times. Dispense with the rectangles, interleave each rib and you might get eight out of a sheet. Use that figure as the dividing factor for the total number of ribs needed, and that’s how many sheets of balsa are required for the rib stock; e.g. 32 ribs ÷ 8 ribs per sheet = 4 sheets. The wood used for ribs should be stiff in all directions; commonly called ‘quarter grain’, it has a speckled appearance.
Finishing off the wing estimation are all the extra little bits such as wing tips and gussets, however these might come from offcuts from other parts of the model and may not call for the purchasing of more wood.
By way of example we’ll take a conventional fuselage with slab sides and a rolled sheet turtle deck. The sides will be the largest item, and with any luck they can be cut from one sheet of wood each; just check the width to be sure. As a bonus there may be some left over for the odd former.
The sides must be hard balsa and the grades must match, otherwise you’ll end up with a banana-shape fuselage; dissimilar densities will bend differently. The rolled sheet turtle deck may use the same thickness of wood as the sides but it must be soft and sufficiently pliable to bend across the grain. The bottom of the fuselage will also need to be sheeted, so don’t forget to add that to the list. Formers can be calculated in much the same manner as the ribs.
A built-up fuselage will need large amounts of strip, using hard grade for both the longerons and the cross braces. Determining the amount required for the longerons is easy enough, but the cross braces can fool you. They’ll take at least as many strips as the longerons, and probably a bit more. You could spend hours measuring, but strip is cheap, so buy plenty. Again, anything left over is a bonus for your next project.
The cowling may well be built up from sheet, and the main consideration here is to check for the maximum width so you know whether 4” wide wood is needed.
The majority of R/C models use all-sheet tail surfaces, which are usually made from medium grade balsa. Simple measuring is all that’s needed to estimate the wood for the job and depending on the shape of the parts it may also be possible to do some interleaving and so save a little timber. Some tails are built-up, using a sheet outline with ribs in the middle. The outline can be cut out in sections from a sheet, careful measuring being the only way to work out whether or not you can get it all from one sheet.
Okay then, that should take care of all the balsa parts, now we need to work out the ply.
The main use of ply will be for the doublers, so measure the length and see if you can get away with a 12” piece. If not, then a 24” length is needed. Plywood formers will come from a stock piece of 12” square, which will also cover things like dihedral braces etc. Some designs may need nothing more than a single 1/4” ply former; you could laminate two pieces of 1/8” ply but you’re bound to use more 1/4” ply on future projects so bite the bullet and buy the right stuff for the job.
ODDS ‘N ‘ENDS
So that’s the wood aspect sorted, but to complete your shopping list there’s some hardware to consider: an engine mount of the correct size and the bolts to mount it with, spinner, wheels, horns, clevises etc. The only thing that needs any measuring in this area is wire; a typical model will get away with one length of a fairly substantial gauge for the undercarriage and some lengths of somewhat thinner gauge for the pushrods.
So that’s just about it. You should now have a list of all the wood sizes and grades you need, plus the hardware. When putting such a wood pack together I always add some extra for the mistakes that inevitably occur. Perhaps replacing a part that’s been cut with the wrong grain direction, broken, or even lost. Never be afraid to overestimate, there’s nothing worse that running out of wood on a Saturday evening when the model shop owner’s tucked up at home with a cup of cocoa.
YOU CAN DO IT
Anyone can learn to estimate the wood needed, it doesn’t take a degree in calculus and it won’t be the end of the world if you get it wrong. Better still, rather than buy on a model-by-model basis, build up a good stock of wood and other materials. Then, when you see a plan that you want to have a go at, you can start right away. You know it makes sense!
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