Times change. Over a decade or two ago I recall lusting after the latest model from Graupner - the Cumulus. It was then state of the art with white foam wings covered in balsa but now no it's longer available. The fuselage was some sort of nylon material and therefore very robust. Carbon fibre, Rohacell and Nomex were unheard of at that time. The latest all moulded models these days have carbon fibre skins and Kevlar™ reinforced glass fuzzes, the sandwich material may be the expensive Rohacell foam or even Nomex honeycomb but more often than not it's soft balsa.
So what are these materials? Read on then and we'll have a look at a few of the composite materials used in models today. I'll start with woven materials for the reinforcement of resin matrixes.
Everyone reading this will have encountered fibre glass fuselages but very few will have made one for themselves from a mould. Moulding does give you experience and the ability to judge and appreciate the efforts of others. If you were going to do some moulding which materials would you use and why? Cheque book fliers miss so much in this respect. For those uninitiated souls and beginners, here's a few pointers on the cloth available for aeromodelling. Later articles will cover foam and resin addtives.
There are several different types of glass fibre - the one we usually use is called E-Glass - a description going to back to the early days when glass was used primarily for E-electrical insulation. This is not the strongest glass available, that is given by a S-Glass not so common here but you may see it advertised for sale in the States. A-Glass is a third type of glass, the lowest strength/quality often used in chopped strand matt for simple filling. Since the fibres are not woven and therefore have no coherent direction, chopped strand adds only a little strength to resin matrixes. Even so it is still a useful material for filling although a little heavy. In fact, glass is the heaviest of reinforcement materials with a density (S.G. if you prefer) of ~2.5g/cm³.
I love carbon fibre - it's easy to work and provides exceptional strength. It does have the drawback that it's expensive, an order of magnitude more than glass fibre, especially the lighter weaves; it's also very brittle and snaps suddenly with little bending beforehand. It's lighter than glass too coming in at ~1.8g/cm³. Areal weights are generally 100-200 gsm although I see that woven carbon is now available in Germany at 60 gsm - whether this is a result of a loose weave or some lighter variant of carbon fibre itself I do not know. Certainly, it's a useful addition to the range of cloths available for tailplanes, HLG's and the like.
This is the yellowish material fuselages are made of, or reinforced with. I personally try to avoid using it because of the fuzzing you get when trying to sand exposed material. It has the benefit of being rather good at sustaining impacts (cf. bullet proof vests and body armour) and is therefore ideal for fuselages. It is no where near as brittle as either glass or carbon fibre. It's the lightest so far of the materials with a density of ~1.45g/cm³. It's also often known as Aramid fibre . The type used for models is Kevlar 49 - different types such as Kevlar 29 and Twaron have different properties.
Spectra, the latest in the line of modeller materials, is the lightest material so far with a density of 0.97g/cm³ less than water. It is a long chain polythene chain molecule with a high impact resistance and the highest strength to weight ratio so far. Like aramid fibre it has the drawback of fuzzing when worked on. I have used it and like the idea of using it - but working is problematical as I say. Surprisingly little used. Spectra is the US name, in Europe it's called Dyneema. If you're into fishing or kite flying you will have come across these two fibres already.
WARP AND WEFT
Many modellers still ask which fabric they should use for moulding, unclear as to the relative merits of plain, twill and satin weaves. Unaided, pretty soon they realise that plain weaves are not much good for moulding fuselages - don't get me wrong, it can be done but there are better choices. What is the difference in these weaves, then? Let's begin with Uni-Directional materials. Simply the fibres or tows of fibres all run in one direction, UD comes with a few weft threads of glass or aramid woven in or even some sort of backing material to keep them in order. Fine they may be for some wings but not for surfaces where strength is needed in more than one direction so we choose a woven material. The following weaves are in order of their drape, strength and generally, expense.
- WARP = Lengthwise threads, i.e. they run the length of the roll of woven cloth.
- WEFT = The transverse, or crosswise, threads woven into the warp threads.
- DRAPE = The way the cloth drapes over or fits into compound curves.
- AREAL WEIGHT = Weight of cloth generally in gsm (gram per square metre)
The simplest fabric to understand is a Plain Weave; it's simply over and under in each direction. (That is to say; the weft is carried over all odd-numbered warps and under all even-numbered warps. For the next pass of the shuttle, the weft passes over the even-numbered warps, and under the odd.) Sometimes it's two over and two under. It has the advantage of giving strength in both directions with a 90º bias but the disadvantage of halving the UD strength (since half the cloth is at 90º). Another disadvantage of the plain weave is that the cloth is bent over and under which imparts some shearing distress on the fibres therefore weakening them compared to UD fibre which lays flat. Plain weaves will curve over simple one-way curves but with compound curves there can be a problem. Try pushing some in a mould and you'll find out - you'll end up with wrinkles and will have to snip the cloth here and there if you're not careful. The finest plain weave glass is about 25gsm. (German Plain = Leinwand)
Twill Weaves are better at handling compound curves than plain weaves. Look closely at the diagrams and you'll see the way the weave is formed. (The twill is formed when the weft passes over warps 1 and 2 and under warps 3 and 4, and in the next pass, the shuttle of the loom passes over warps 2 and 3 and under warps 4 and 5.) There are many variations on the theme so you will see twills listed as 1/2, 2/2, 1/3, etc. Twill fabrics are characterised by their appearing to have diagonal lines running either right or left on the fabric face.The first advantage is the fewer number of times the fibres go under and over one another. A variation of the twill is the Crowsfoot or cross twill. Twill weaves feel generally tighter, or more closely woven, than plain weaves. For most purposes a nice 100 - 160gsm twill is the best choice. (German Twill = Köper.)
In the Satin Weave, the weft floats or skips over as many as 12 warps before being woven in. The next pick repeats the float, but on a different set of warps. Compare the weft over the warp threads in the diagram below. You may see satin fabrics labelled as 5 or 8 harness satin weaves indicating the degree of float. The one here is a 5 harness - the weft floats over 4 warp threads. In Germany these weaves are called Atlas verbindung. These are probably the best fabrics to use for complex moulds but can appear to be tightly woven and therefore difficult to wet out with resin. Since the threads have less crimping than plain and twill, satins make for the strongest use of the fibres.
In an attempt to harness the stiffness and compressive strength of carbon fibre with the impact resistance and tensile strength of aramid fibre, hybrid fabrics are also available. These come in many forms - the carbon/aramid twills have rather nice yellow and black patterns to them too. I actually have some rather nice UD carbon and UD Dyneema with a 45º bias.
As its name suggests, prepreg is pre-impregnated material i.e. impregnated with mixed epoxy resin. It comes with a backing paper and has a limited shelf life and must be stored in cool conditions. Pre-preg carbon has the appearance of sticky liquorice. This material is NOT suitable for modelling since autoclaves and high temperatures are needed to cure the resin. Newer types use thermoplastics instead of Epoxy resins and are like stiff plastic until softened by heat, e.g PEEK (polyethyl ether ketone).
Special type of woven cloth - a woven tubing if you like - rather like a Chinese finger puzzle. It's sometimes called braid. Ideal for spars, fuselage booms, fuselage pods, engine nacelles, etc where no seam is required. It effectively forms a spiral weave for cylindrical objects and pulls round compound curves. I used it with the lost foam method to make an HLG fuselage. Available, only from Germany and the USA as far as I know, in glass, carbon, aramid and carbon/aramid hybrid forms. Many diameters from 5 mm up to 150 mm before stretching.