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Memoriál Rudolfa Drnce 2017

Mr Tompkins in Miniscaleland - chapter 2: selecting material

Mr. Tompkins

Every miniscale model building instructions start the same, it’s kind of miniscale “once upon a time”: Get light but strong balsa. There is no such thing. The correct miniscale balsa comes from the tree planted by seventh son of the seventh son of miniscallemodellerfather, it has negative weight and regarding strength it outcomes boron fiber. It is exactly the material which desperate miniscale modeler searches, the one who can very carefully sequentially look at all 1mm balsa sheets they have got in the shop, including those which are in the stock and those which are not in the shop yet to be leaving the shop after several hours search with a single sheet of balsa, while he is not convinced yet that it is junk material for patterns only.

The truth is that not all the balsa is the same. My up-till-now classification into hard, soft and normal is not sufficient when building tiny airplanes. So I listened up ISim’s lecture and made a trip to the shop. What remained in my lousy memory was this: Balsa with straight brain is good for spars, leading edge and trailing edge. Mirror balsa is good for fuselage formers. Balsa really has to be selected carefully, it should be light and strong. We need only tiny amount of material, so it is worth to spend the time searching for the ideal sheet. It makes no sense to buy a balsa with no need for it. However, I have to calm down dear colleagues beginners, on my BV215 I finally made trailing edge from completely wrong kind of balsa.

My gurus told me one more thing: hair does not lift! Selected sheets must be carefully sanded with subtle sand paper, even if you think that they come from the shop perfectly smooth (which is not often). However my personal feeling is that hair maybe does not lift, but it definitely weights nothing, so for the admirers of hairy chest: the fuselage formers can be in Tarzan style.

North American XB-70 Valkyrie made of potpron

North American XB-70 Valkyrie

What else apart from balsa do we need? I am personally not very fond of “new” materials, in particular various styrens, pylens and godknowswhat-ens. However, during one cloudy Sunday afternoon I was thinking how to made small rapier powered model of North American XB-70 Valkyrie, beloved airplane of my dad. I know that this contradicts everything from previous chapter full of advice regarding simple originals and holding with verified plans drawn by somebody smart, but what the heck. Valkyrie has long and slim fuselage, wing is deep, wingspan small, it seemed impossible to make the balsa design. I tried to make at least the wing of balsa sheet. It was nicely stiff, but even for me (and I don’t pay attention to the weight that much) it was heavy as devil (metaphorically speaking off course, estimated weight of the devil – about 87 kilos was not reached). So full of resistance and reluctance I went to hobby shop, and bought one of Ens (potpron in particular, which, as I see it now does not have ens at the end) 4 mm thick, and sanded the wing directly. Fuselage was made of balsa keel with pron on both sides, sanded in hand into Valkyrie elegant shape. Approximately. The model did not fly yet, so I can’t reveal the usual happy-end, but with all my teeth grinding I have to admit that there may be something about those Ens. One advantage is clear – you don’t have to select, as it’s all the same.

So after spending the whole day in hobby shop we are equipped with a piece of heavy and not very strong balsa, down the bag one can find a piece of plastic of inorganic origin and in the next chapter we will look what tools we will need.

And at the end of this chapter we again have a quiz. On following images, which one is balsa?

Balsa ? (foto Center for Tropical Forest Science) Balsa ? (foto Center for Tropical Forest Science)

A: Balsa ?

B: Balsa ?

And the correct answer is ........................... A again !

Professors notes...

ISim's notes:

My first visit in Ruja’s tiny cellar workroom was remarkable by several apperceptions: first it was the slight smell from the conduit mixed with the odor of natural gas (moreover the mercaptan used for “marking”) and secondly the shelf with material, which I could not identify at the first look. At the second look it was balsa, but definitely not suitable for anything with wingspan less than 150cm. So there we started.

Many may be surprised with the fact that balsa as a tree has no growth rings. Forget the pictures which show you the cut of the trunk looking like a pine with plenty of concentric growth rings. Balsa tree in trunk cross-section has radial beams instead of rings. Depending on the angle the balsa sheet is cut from the trunk the sheet has its characteristic appearance and properties. You can google more about balsa. Nowadays offers of nice, selected and brushed balsa trepan into getting the sheet with “super light” print and start building. But light may differ…

For spars select balsa with visible longitudal grain, which are uniform and parallel to the sheet edge. No matter how hard you try optically select the balsa in the shop, the real quality is usually revealed at home, after cutting the first spar. When balsa strip cutter is correctly set and trimmed, the product of our work should be straight spar with rectangular cross-section constant along the spar. If the balsa moves from the cut, is not cut by the blades but breaks just in front of the blades, there is something wrong.

For formers, ribs and similar parts, in which the load direction is not clearly along a axis, select balsa with “mirrors”. Such balsa is stronger along multiple directions, the balsa sheet will be more resistant to bending along the shorter dimension compared with previous case. When sanded carefully you can obtain nice shiny surface, as there are no cavities from grain.

Apart from the position where the balsa sheet was cut from the place and way of growing is also important. You will find “stone” balsa – hard and heavy and “gingerbread” balsa – sparse wood which breaks in irregular shapes. Quite often you can recognize gingerbread balsa (thin sheets) in the shop, as the sheet is tattered on some portions, as it is from the paper and not weed. Something can be recognized from the color of the weed – balsa sheet should not be grey, should not have stains on it, but those imperfections are clear on the first look.

Even if the balsa selecting is important, the most essential during the building is the “dimensions feeling”. That is something you will get only by building more and more models. According to this feeling you change the dimensions shown on the plan. If you have harder and heavier balsa, you can use smaller dimensions compared to the plan. Smaller spar also drinks less paint and the final result can be better when compared with the super light balsa of higher dimensions, which sucks paint as mushroom.

Regarding hair, apart from the fact that the bones where one can see the hair (or even the saw marks) on the ribs and formers looks ugly, the hair definitely do not increase strength and on contrary it sucks paint or glue and increase weight. So today’s motto of Father Fur is: “shave the hair”.

Felda's notes:

Where are the times when it was pure luck to get any balsa at all, one could not select and all the sheets were hairy. Today the situation is different, we can get sheets nicely sanded, with various widths and often presorted by weight. However, there is one big BUT – usually it is balsa I would call “industrial” – it has certain standard properties, among those is roughly the same quality of the whole sheet, sufficient strengths, good workability but unfortunately also higher weight. It is ideal building material for large RC models (mainly for mass producers), but usually not suitable for miniscales. We have to focus on sheets labeled as SUPERLIGHT balsa, i.e. less than 100g/dm3. And we have to select very carefully as such balsa is more expensive and for the processor nothing is easier than to put such label into balsa which might be very light, but with bad mechanical properties – so called “dried bladder”. This balsa has properties of elder pith, it breaks under slightest load both along and across the grain. But even such material can be used in minimodels – I use it successfully for various nonstructural filling, plank fuselages, etc. We have to keep in mind that it can not be used for structural elements, and that even the covering paper can deform it after stretching.

But back to original theme of this chapter. Selecting the right balsa is an alchemy, it depends mainly on experiences. And smaller the model more important is the selection of the right balsa (this holds doubly for free flight). Balsa cut types are nicely drawn and described in literature, but no hobby shop (exceptions possible) has sheets of type A, B or C. For the main spars I use heavier but really strong balsa, it makes no sense to consider weight, the strength is determining factor. I make all other parts from the lightest balsa available, formers and full ribs from mirror balsa, additional spars, stringers and strip ribs from balsa with mirror on slim side of the sheet. Special selection is necessary for balsa used for lamellated parts, here all we can do is to cut the strip, water it and test if it can be bent into required radius or it breaks.

What I wrote in previous paragraph is easy to write, but hard to actually do. You have to be prepared for the fact that the sheet which seems to have all the necessary properties and looks just great is unusable when spars are cut from it as the spars are simply catastrophic. And on contrary some sheets look unusable and spars cut from it are perfect. Also remember that usually we get spars with different qualities from each side of the sheet.

From all above we can logically conclude that balsa with disappointing properties which we could not used for what we planned it to will aggregate at home. But that is not a reason to despair. Balsa with different properties is suitable for various models and their design nodes so hopefully we can use that balsa in the future. And moreover we need many different jigs (patterns, supports, gauges, ...) and tools (sanding blocks, sanders of different shape, ...) during the building and balsa which can not be used otherwise will become handy.

And as the chapter is called “selecting the material” and miniscale modeler does not use balsa only, let me mention more building materials which can be successfully used. The choice is wide and its width grows with the size of model to be built. Balsa is usually enough for peanut size models. But for M-mins (1/20) with wingspan of 60 cm we can start to combine the materials. Main wing spar is a good example. It can be balsa centre web, or better balsa T or I profile. Higher transverse strength for larger models can be reached if we use top and bottom booms from ply strips, bamboo chips, pine spars or if we laminate balsa center web on top and bottom surface with carbon fiber.

And I can not avoid “modern” materials, mainly various types of polystyrene. Building from classical white construction foam polystyrene is fortunately over, but materials such as construction isolating polystyrene and thin sheets of pron (depron, potpron) are usable extremely well. When combining those materials with balsa we can successfully take advantage of good properties of all materials used. We just have to keep in mind that during covering the polystyrenes usually do not stand common covering glue – diluted glue paint; and balsa swell when covered with water diluted disperse glue. Therefore the model must be divided into independent parts not only with respect to the design, but also regarding the technology according to materials used and those parts must be covered independently and in suitable way.

Warning at the end – EPP based materials are totally unusable for miniscale models. It can stand almost anything, but that may by why the workability is extremely difficult and we will never get the surface with necessary quality.

And recommendation at the end – good model must have its pilot and construction isolating polystyrene is for me the best material for sculpturing. More information on pilot making will appear in the Details chapter