Unfortunately many people make the mistake and assume that it's the capacity of a battery that determines how it will perform in an RC receiver pack -- it most certainly is not.
It's all about how much current the battery can deliver without losing voltage and that has nothing to do with capacity, it is related to the internal resistance of the cells being used. (Basically, the more square shaped the cell is, the lower its internal resistance. Ed) The higher the internal resistance of the cells, the more the voltage will drop for a given load. If the resistance is too high at a given current, the output voltage will fall so low that your receiver and/or servos stop working. What's worse, generally speaking the higher the capacity of a given type of cell (AA or AAA for instance), the higher its internal resistance is too. That's because in order to deliver more capacity it's necessary to use thinner plates and thinner plates are not as good at conducting electricity as thicker ones. So, in buying a higher capacity cell (for a given physical size) you're sometimes actually shooting yourself in the foot because they perform far worse than medium-capacity cells. As for AAA cells, steer well away from them. They are absolutely the worst thing to use for RC receiver packs Most of them have a very high internal resistance and therefore can't deliver the currents needed to run a 4-servo setup. In a 4-cell pack you run an enormous risk of your receiver stopping and even a 5-cell pack is very marginal. As far as AA cells go, steer clear of the hi-capacity ones (anything higher than 2000mAH) In my testing, the brand name (Sanyo/Panasonic) 1650mAH cells are about the best you can use -- if you insist on using AA cells. And don't bother with a 4-cell pack, you must use a 5-cell one to provide adequate headroom for your receiver. However, as I've said before, I really, really recommend that people switch from NiMH packs to LiFePO4/A123 ones for a multitude of reasons. I have yet to hear of anyone using A123 packs to have *any* power issues (unless they're also silly enough to use a regulator). If you build them yourself then a 1100mAH A123 pack can cost well under $30, weigh just 3/4 of a 4-cell AA pack at 90g, be recharged from flat in 15 minutes and last about 2,000 charge cycles. What's more, unlike a hi-capacity NiMH pack, the A123s will actually deliver their rated capacity. In testing, the higher internal resistance of hi-capacity NiMH packs shows that much of the stated capacity is mitigated by the higher internal resistance when decent loads are applied. If you need more than 1100mAH then there are also 2300mAH A123 packs available. These packs can deliver 100 amps without blinking an eye -- that's how good they are.

Model aircraft construction in polystyrene offers several advantages over conventional methods. Cost is far lower, structures are less complex and tend to absorb more damage in a severe crash, sparing vital engine and radio components. Time taken is often less, and there is a fair amount of “artistic” effort involved in shaping components, which some find very rewarding. The parts count on any particular airframe is usually far lower than from conventional building techniques. Different shapes and designs can be experimented with quickly and cheaply, and most important of all, the airframes have a high degree of “flyability”. From experience I construe this to be largely because they are light, and have on average thinner wing sections than conventionally built aircraft. Thin wing equals less drag.
The Raw Materials
Sourcing polystyrene is not difficult. Local D.I.Y and Building Merchants are the best suppliers. Sheets come in eight foot by four foot sizes, and a variety of thicknesses. 20- 30 mm is best. White Polystyrene foam is far cheaper, lighter, and more available than blue foams. It just requires a little more reinforcing in strategic areas to render it fit for purpose. Take a craft knife to the store, and measure and cut your sheets in half to transport them home in the family car. One sheet provides material for several models. Brown paper is the next ingredient. Rolls of brown wrapping paper are available in many stationary outlets. Other wrapping paper may be used. Avoid silver “metallised” papers, as these can cause problems with RC receivers. Balsa and spruce for wing spars, plywood for dihedral braces and engine mounts, PVA and epoxy glues are the other major requirements.
Tip: Avoid balsa cement, Ados type contact glues, and Cyanoacrylates, (Superglues) as these all attack polystyrene foam.

69” own design Zero

Recently I was reading a very interesting anthology of New Zealanders in flight. One story was about 22 year-old pilot in Southland early in 1914 flying a brand new Caudron. A local writer at the time enthusiastically espoused this “fascinating piece of modern mechanism, with its powerful Anzani motor and minimum of gravity attraction.” The quaint phrase “minimum of gravity attraction” appealed to my slightly off beat sense of humour. I even contemplated writing it on one of my models, but thought better of it. However, it does beg the question, how do you design and build a model with minimum gravity attraction?
An aeroplane in flight has two primary forces that must be balanced in order to remain flying. They are weight and drag. To balance these forces we need lift to counteract the weight and thrust to counteract the drag. Weight is easy to determine, just put it on the scales. But drag comes in a variety of guises, such form drag, parasitic drag, induced drag, etc.
Theoretically, weight is easy to control. It really is simply a matter of not making any component heavier than it needs to be to do the job for which it is intended. Understanding basic design principles helps here. Strength does not always equate with weight. A primary consideration is: where is strength required? Three basic principles help in making these decisions.
1.    Tie the motor, wing and undercarriage together. These points are where live loadings act upon the aeroplane. The inertial weight of the motor, during aerobatic manoeuvres and landing will try to rip it from the fuselage. The wing, of course, supports the entire weight of the model during takeoffs and landings and airborne manoeuvres, sometimes multiplying the apparent weight of the aircraft by a factor of up to 10 or even more, depending on the violence of the manoeuvre. And during landing the undercarriage will have forces trying to push it upwards and rearwards.
2.    Brace with triangles. If it is an open structure, install well fitted cross bracing from one corner to another in the open parts. Converting open squares or rectangles to triangles creates a truss-like component, with maximum strength for minimum weight.
3.    Eliminate stress risers. A stress riser occurs when there is an abrupt change of mass of a load bearing component. A good example of a stress riser is a 6mm ply or hardwood main spar dihedral brace, connecting to balsa spars. When the wing is stressed, the point that it wants to bend is concentrated at the end of the brace. The centre section won’t bend, it’s extremely strong, but the loading becomes concentrated right where the brace stops and that is where it will break. Strong bracing is necessary of course, but fair the ends off so that the transition from the brace to the spar is gradual. This spreads the loading over some distance, instead of concentrating it at one point.
A great deal of weight can be saved simply by making each part carry its maximum load, by both sizing and fitting the component correctly. Don’t allow haste to let you make bad joints. It can be tempting to use more than enough glue to fill an open joint, instead of cutting a new part. The extra glue adds weight, but the joint will be compromised to some degree because glue on its own across an open joint will not be as strong as a properly made joint. Free flighters in the past used to quote the adage; “look at what didn't break in a crash and then lighten that”. If it didn’t break it must have been too strong, therefore too heavy, or it would have broken along with everything else. When you do have a crash, look for what gave way or broke, and ask why it broke that way. See what you could have done to make it stronger but lighter.
Just adding wood to a structure does not necessarily make it stronger either. My black and white Stearman (that I crashed on its second flight) was made from an old kit produced in the 1970’s or 80’s. The designer must have had a very complicated mind, because there were bits of wood all over the place, creating backups of backups. Although it flew nicely, it was heavy, and at the end of the day, all that over-engineering did nothing to save it in the crash. In fact, it was one of the very few models that I have taken straight home and put in the rubbish bin (after removing the gear, of course). Everything except the tail plane was broken; even the wing ribs were shattered. The kinetic energy of all that mass was absorbed by smashing all the wood into little bits. My new Stearman is 10 percent bigger and will weigh at least 2 lbs (1 kg) lighter, simply because it has less wood, but each part does its own job (he says hopefully).
How do you deal with drag? Some drag is inherent with the ability to provide lift. Induced drag is directly the result of lift. However, parasitic and form drag can be either designed out or reduced dramatically simply by employing fairings, reducing cross sectional area and, without embarking on a discussion of laminar versus turbulent airflow, by making the surface as smooth and slippery as possible.
Even induced drag can be reduced. Since it is a product of lift, the idea is to make the aeroplane fly without creating more lift than is necessary. As it gets close to its perfect centre of gravity the drag of an aeroplane reduces markedly. When the CG is too far forward, more up elevator has to be applied to keep the nose level. This increases the wings’ angle of attack, creating more lift, thereby causing more drag. And it’s not only the wing drag that increases. Raising the elevator creates a downward lifting moment in the tail plane, to offset the extra weight up front, so increasing drag there as well.
Choice of airfoil also has an effect on how much drag you have to deal with. An airfoil with lots of camber will create more lift and also more drag than one with less camber. Of course, less camber requires more speed or less weight, so everything is a tradeoff; there’s no free lunch, as they say.
Finally, just as weight can be added or reduced by the designer/builder, so drag can be added or reduced to some extent by the way the pilot flies his aeroplane. Most aeroplanes have a “sweet spot,” a mixture of trim and speed at which it flies at highest speed for least power. In full size you can find that by climbing to just above the selected cruise altitude, set the throttle to the correct rpm’s and nudge the nose down very slightly until it picks up speed then level off; you get “on the step.” To a degree we can do the same with a model by adjusting trim and throttle to get on the step. Unless, of course, you open full throttle at take-off and keep it there for the entire flight, in which case minimum gravity attraction may not be of any interest to you, until the unthinkable happens!

Composite construction is not new. The Ancient Egyptians made bricks by reinforcing clay with chopped straw. And we are all familiar with steel reinforced concrete, combining the compressive strength of concrete with the tensile strength of steel. ‘Composite’ simply means composed of separate parts. However, the field of composite fabrication that we are interested in is reinforced plastics. In aero-modeling this has been used variously, but especially in producing fuselages, cowls and wheel pants, etc. In this series of articles we will look at the materials and how they are used. First, let us consider the materials themselves.
Generally, there are two types of plastic: "thermosetting," which hardens permanently after application of heat and ‘thermoplastic,’ which becomes soft when heated and hardens as it cools. Thermoplastic can be softened and worked many times, whereas thermosetting plastic can only be processed once. A composite structure consists of a thermosetting resin used in conjunction with some type of reinforcement, such as woven fiberglass cloth. The most common types of room-temperature-curing resins are polyester, epoxy and vinyl ester.
Polyester resin is a general-purpose resin suitable for a wide variety of applications. Methyl Ethyl Ketone Peroxide (MEKP) is used as the catalyst. The MEKP reacts with cobalt in the resin and generates heat, causing the resin to set. Varying the amount of MEKP (from ½% to 2% by weight), will affect the set-off rate. Increasing the catalyst will cause a faster reaction – a useful factor on cold days or when the laminate is very thin, with little bulk of resin. In thinner laminations, the surface may remain tacky and not cure properly if left exposed to the air. So general lay up resin contains styrene wax, which covers the surface as the resin cures, acting as a barrier to the air. Styrene wax must be sanded off after curing if subsequent adhesives are to be used for fitting out.
Epoxy resins are more critical in their resin / hardener volumes (usually 4 parts of resin to 1 of hardener) and cannot be varied. Adequate temperatures (at least 20°C) must be maintained during the curing process. However, epoxies provide greater strength and dimensional stability (they do not shrink as much as they cure) and adhere to other materials better than polyester resins. Although epoxy resin systems are more costly than polyester resins, they are highly recommended for use with aramid and carbon fibres.
The third type of resin, vinyl ester, possesses qualities that for the most part fall between polyester and epoxy resins. It is tougher than either of them and has superior temperature resistance (good to 150°C). Like polyester resin, it is catalyzed with MEKP, but vinyl ester has a shorter shelf life (about three months).
Of course, the resin is only half of the composite, it needs to be combined with a fabric that will drape easily over the contours of the mould, and add necessary strength to the part being produced. There are many reinforcing fabrics available that are used with the resins discussed. The three most commonly used are fiberglass, aramid fibre and carbon fibre. Each possesses different qualities and advantages. All three are usually available as tow or roving and woven fabrics. Additionally, fiberglass is available as a chopped strand mat (CSM), which consists of short, randomly oriented fibers held together by a binder (CSM cannot be used with epoxy as it will not dissolve the binder).
Most general-purpose applications utilize glass fibre cloth. Glass fibre is formed when thin strands of silica-based glass is extruded into many fibres with small diameters suitable for textile processing. Although it lacks the light weight and strength of carbon or aramid, it is considerably cheaper to produce. Fiberglass cloth comes in a wide variety of styles and weights, making it ideal for many applications. There are two main types of glass fibres: E-glass and S-glass. E-glass is more commonly used, while S-glass has a high tensile strength and is stiffer.
Carbon Fibre is synthetic thread that has been heated to such a degree that total carbonization has taken place. Each carbon filament is made out of long, thin sheets of carbon similar to graphite. These filaments are stranded into a thread. The very fine threads are either left as long fibres along the roll length (uni-directional fibre) with good strength in that axis, or woven together to form a fabric cloth giving multi directional strength, with the appearance most people associate with carbon fibre parts. Various woven styles of cloth are available such as plain, twill and satin weaves. Carbon Fibre's weight, stiffness and strength properties are why it is commonly used in aerospace, sports equipment, motor sport and boat building. Carbon fibre costs the most to purchase, but it offers exceptionally high strength and stiffness, in combination with extremely light weight. Aramid fibre (Kevlar) also offers light weight, along with excellent abrasion resistance. The name is a shortened form of "aromatic polyamide". They are fibres in which the chain molecules are highly oriented along the fibre axis, so the strength of the chemical bond can be exploited. It has a very high strength-to-weight ratio, said to be 5 times stronger than steel of equivalent weight. It is, however, difficult to cut and wet out with resin. For finishing purposes, fabricators often use a surface layer of lightweight fiberglass cloth in Kevlar laminates, because Kevlar is virtually impossible to sand once cured. Before we consider how to make best use of these materials, we should consider the matter of safety in composites. Reinforcing fabrics can be a problem when cutting. The small fibres will travel through the air, so a dust mask should be worn to avoid breathing them in. Vacuuming the work table will help to reduce the amount of airborne particles. They can also be irritating to the skin; some people seem to be more susceptible than others to this problem, but sensitivity can also be developed over time. Skin sensitivity will show as itching, a rash, or both, and varies in intensity among individuals. The best way to protect your skin is to wear gloves and long sleeves when cutting or handling the reinforcements.
Resins are liquids that become solids through a chemical reaction. Always operate in a well ventilated area, both when working with the resins as well as when they are left alone to cure. Respirators are often used when working in a closed area where ample fresh air can not be circulated. To prevent resin from contacting your skin or eyes wear protective goggles and gloves. Polyester resin and acetone (used for cleanup) are flammable liquids. Each of these products has a flash point lower than 38°C. This means that vapours of these products can ignite at temperatures lower than 38°C if presented with a direct ignition source. The lower the flash point, the more highly flammable a material is considered to be. Consequently, when working with any of these products, you should have plenty of fresh air and avoid all sources of ignition. Be particularly careful with larger quantities of catalysed resin, as heat will build up quickly and the resin can self ignite.
MEKP is an organic peroxide. Although it has a flash point higher than 40°C and is not considered a flammable liquid, it has a unique fire hazard. If MEKP ignites it produces its own oxygen which makes it quite difficult to extinguish. Consequently, MEKP should be stored in a cool place away from flammable liquids and away from direct sources of ignition. Some epoxy hardeners are considered corrosive and must be transported as hazardous chemicals. Special attention should be given to keeping these materials out of eyes and away from skin.
Next time we will look at how to use these materials in model applications.

Last time we looked at the common basic components of composite construction, with a brief consideration of safety. Now let us consider how to use these materials. Normally a mould is made in which the desired part is formed. First, briefly consider the types of moulds that can be used.
Male moulds are usually cheaper and quicker to construct. This is a form that is the final shape of the part, and the part is fabricated over its outer surface. Each part produced will have a rough outer texture which requires laborious finishing. The part will also “grow” during lamination, but if the mould is intentionally made slightly smaller the part will grow into the desired finished dimensions. Male moulds are generally used when only one or two parts are required.
They can be made of any material, but for a one-off part ordinary white polystyrene foam is suitable. Cut and shape the component to its finished dimensions, allowing for the thickness of the glass laminate when it is laid up. The foam can be worked quite accurately by cutting to rough shape with a fine saw or knife, then sanding with progressively finer grades of sandpaper. Polystyrene is ideal if there are undercuts, or negative draught, because the mould is sacrificial.
Once you are satisfied with the shape, give at least two generous coats of water based polyurethane varnish, allowing it to dry between coats. The varnish will fill most of the holes that cannot be sanded out due to the cellular nature of the foam. When the polyurethane is completely dry apply a generous wet coat of polyvinyl acetate (PVA) mould release with a piece of sponge. Allow the PVA to dry thoroughly, then lay-up the fibreglass and resin as you would normally do. Either polyester or epoxy resin can be used, but remember that polyester will dissolve any unprotected foam. Once the resin has cured dig out the foam, using a knife or chisel. You will find that the foam will come away, leaving the layer of varnish. Now simply peel this flexible polyurethane skin off the part; it comes away easily and cleanly, leaving the finished product with no residue at all.
Female or cavity moulds cost more, but there are advantages. Finishing time is significantly reduced because every part comes out with a smooth outer surface. They also lend themselves to use with core materials, used to provide a thicker sandwich without adding a lot of weight, because the outer skin is always a smooth regardless of how inconsistently the core is used inside the part. Female moulds are formed over a plug, which is the exact shape and dimensions of the final part. The plug may be made of almost anything, so long as its surface can be finished to the standard you desire. It can be an existing item or something fabricated specifically for the purpose. Materials commonly used in plug construction include wood, plaster, metal and polyurethane foam.
Compression moulds are excellent for producing precision parts and are made by using matched male and female forms. The moulds are loaded with reinforcement and resin before they are closed and tightened. Excess resin is squeezed out, reducing voids, and parts emerge smooth on both sides. Compression moulds can also be modified for use with resin transfer infusion, in which the moulds are assembled with the reinforcement dry. The resin is then transferred by evacuating all the air, forcing resin completely into the mould. The major advantages of compression moulding are, 1) both surfaces of the part are finished exactly as required, 2) the thickness of the part is to exact tolerances, 3) the resin content is very low, making a strong moulding. For the kind of work we do however, compression moulds would be unnecessary due to the large amount of work and cost involved.
For the most part, we would use a female, or cavity, mould. A primary key to success in mould construction is proper preparation of the plug, which is the original pattern used to create the mould. Any imperfections in the plug surface will be transferred to the mould, and then to future parts made from the mould. The plug must have a finish at least as good as the part you wish to produce.
The final part will be a faithful replica of the plug, so it is at this stage that you will need to work carefully, especially if working on a scale model. The basic shape can be cut quickly down to within a few millimetres of the final size. Then carefully bring the plug down its final contours. If the plug is made of wood, it should be possible to paint it and buff to a finish. If other materials, such as foam, are used you will want to make the plug slightly smaller (just a millimetre or two) since a reasonably thick coat of resin will need to be applied. Note that foam is good for getting the shape quickly, but following that a layer of light glass cloth – say 3 ounce – will be laid up over the foam. Be sure to use only epoxy resin as polyester will cause an immediate and dramatic disappearance of your plug. Once the lay up is cured, you can skim a body filler compound over the surface, and sand to a smooth finish.
With a choice between wood and foam plug, I frequently opt for wood, because in the end I think it is faster. However, if your workshop does not have much woodworking machinery, you may find carving foam to be easier. It’s just that more time needs to be spent finishing a foam plug. In either case, what you want to end up with is a plug that is to the shape and size you require, with an acceptable surface finish. I prefer a two pot primer (polyurethane, acrylic or epoxy) sprayed on. This can be sanded and polished to an acceptable finish, but if you want a mould with a perfect high gloss, then you will need to follow up the primer with two pot gloss paint. It mostly depends on whether you will use a coloured gel coat or paint to finish the final product. I like paint, since I can work on the part; joining, drilling holes, etc, before painting. A painted part is also lighter.
How you intend to release the mould from the plug and subsequent parts from the mould will affect the overall design. The first factor to consider is the draught angle of the mould. This is the angle of the sides of the mould compared to its base. On a mould with positive draft, the sides are wider at the top than they are at the base, allowing easy removal of the part. A mould with negative draught is tighter at the top than at the bottom, making removal of the part impossible. Shapes with negative draught must be made in multiple piece moulds. Each piece has positive draught for easy release, yet they all bolt together forming the negative cavity. Parts such as fuselages require at least two piece moulds.
Where the mould is to be split an up stand, or flange, is temporarily fitted to the plug. This is made from plasticine, shaped to the desired contour, or from strip plywood held in position using plasticine or modelling clay. An aid to location of split moulds after removal from the plug is to place on the up stand large dome–head upholstery tacks spaced at a suitable distance. These will form recesses in the mould flange. When the up stand is removed and the mating part moulded, a protrusion is formed. On future assembly, the parts will locate together accurately.
Once the plug has been prepared construction of the mould can commence. First, a mould release agent needs to be applied to the plug. This is an essential step, since it will allow you to separate the mould from the plug. If the mould doesn’t release properly from the plug, both mould and the plug could be damaged or destroyed.
The two most common release agents employed are the traditional combination of parting wax and PVA release film. Generally four or five coats of wax are applied with an hour wait in between the coats, as directed on the tin. After the final application has dried and been buffed, the PVA can be applied onto the plug. For best results, the PVA should be sprayed, but is satisfactorily applied by lightly wiping on with a piece of sponge.
Next time we will look at laying up the mould, and then finally making the part.

Last time we looked at making and preparing a plug on which to make a mould. Let’s now consider how to lay up and prepare the mould. Assuming that you are making a fuselage, you will need to fit a parting dam along the centre line, as mentioned before. A piece of ply or hardboard is ideal; if you have made the plug in two halves, it is simply a matter of fixing a flat sheet to the flat centre line faces. Allow a margin of around 25mm and screw, not glue the sheet to the plug; it may be helpful later to be able to unscrew it when removing the finished mould from the plug. If the plug is a complete piece, such as an existing fuselage, mark out and cut a flange that fits snugly at the centre line and attach it to the plug with plasticine or something similar. Once it’s in place, you will need to fill any gaps to prevent resin from running in behind the dam. I often use ordinary linseed glazing putty for this as it can be tooled easily with a wet knife. Any locating keys or dowels for re-aligning the segments of a multiple–piece mould should be added to the parting dam. Dome head upholstery tacks are suitable for this. These locating keys leave an impression that is filled when the second half of the mould is laid up, allowing both pieces of the mould to be accurately fitted together. Apply four or five even coats of a proprietary parting wax, allowing time to dry thoroughly between coats. Don’t be tempted to use car polish or furniture wax. Then apply an even coat of PVA (polyvinyl alcohol) release agent, with a piece of sponge, making sure that you don’t miss any areas. This forms a membrane which, coupled with the wax, should give a clean release from the plug. Once the PVA is thoroughly dry, you are ready to start making the mould. But first, check the temperature of your workshop. It is not good practice to make a mould with the ambient temperature below 20ºC as the gelcoat must gel within 30 minutes on the plug to achieve good thorough cure. This also applies to the laminating resin, so ensure that the minimum temperature in the mould making area is 20ºC throughout the mould manufacturing period. This is not a problem during summer, but in winter you may require heating to maintain a minimum temperature. Remember that you are dealing with flammable and accelerant materials, so it is best not to use a heater with an open flame close to the work area. The first layer to apply is a polyester or vinyl ester tooling gelcoat. This gives the best working surface when later laying up the actual part in the mould. General purpose gelcoats can be used, but they do not have the resilience, chemical resistance and heat distortion level required for multiple use. Tooling gelcoat can be obtained in small quantities so it is worth using. However, if you envisage a very limited run of parts from your moulds, you may decide to use ordinary gel coat. Mix it thoroughly with about 1½% MEKP (methyl ethel ketone peroxide) and brush onto the plug fairly thickly – at least 0.5mm thick. Being thixotropic it will not run off vertical surfaces. Once the gelcoat has been applied, it’s important to stabilize it within 1.5 to 5 hours with the first layer of reinforcement. Wait until the gelcoat has set off but is still a little bit tacky on the surface. Brush on a generous layer of laminating resin and allow it to tack. Use an ordinary isophthalic polyester resin. Then apply one layer of 225gm chopped strand mat (CSM), followed by one layer of 450gm CSM. Using the brush, stipple the resin thoroughly to fully saturate the mat. The first layer of 225gm CSM will minimise print–through of the glass strand pattern (225gm CSM can be easily made by splitting 450gm). This thickness of laminate will develop sufficient exotherm to ensure adequate initial cure, without shrinking, which causes print-through. Be very careful to apply the first layer of reinforcement without trapping air bubbles. All air pockets directly beneath the surface coat are prone to chipping after producing a part or two, and the mould surface will need resurfacing. Once you are satisfied that the mat is saturated, wipe resin out of the brush and stipple it dry over the laminate to remove any excess resin. Remember that maximum strength comes from a high glass to resin ratio, not the other way round. Rather than cut the CSM to shape, it is better to tear it. The frayed edges blend well without trapping air like sharp scissor cut edges do. The flanges will need some strips cut to the proper width to butt into the corner of the parting dam to exclude air. However, this is about the only area where cut edges are needed. Work out all trapped air pockets so that the mat is tight against the plug surface, and it must be uniformly saturated with resin. Air bubbles and dry areas will appear milky against the dark tooling gelcoat. Stipple with the brush to work air pockets out of the mat and, if you have one, use a grooved saturating roller to help compact the laminate. Rollers are not very expensive and if you plan to do an amount of composite construction, it’s worth buying one (or two). Watch for bridging (lifting) of the fibres across sharp corners and in textured areas. Any air bubbles remaining after the resin gels must be carefully cut out with a sharp knife and a matching patch laminated in place. Once the initial layer has cured, preferably overnight, lightly sand it in preparation for additional layers, following the same procedure as with the initial layer. The laminate is built up to design thickness by successive layers of CSM. Apply one or two 450gm layers at a time, allowing to cure between, to ensure that laminates do not develop excess exotherm. For most moulds that a modeller would make, four or five layers should be enough. As a general rule, a mould should be a minimum of twice the thickness of the part it is to produce. It is essential through the laminating process that a high glass to resin ratio is maintained, to ensure minimal shrinkage. Avoid the temptation to facilitate wet–out by using more resin, which will increase shrinkage. Adequate wet–out is properly accomplished by rolling, forcing the fibres deeper into the resin, rather than simply putting on more resin. The same principle will apply to the part that you make in the mould; resin–rich equals weight without strength and it will shrink more. Once all of the layers are in place and have properly cured, the parting dam can be stripped off the back of the new mould flange and discarded. Use clean rags to wipe away any excess clay that might remain on the surface. Take care not to scratch the plug while doing this. Apply fresh mould release agent to the newly exposed flange, as this will be the form against which its mate will be constructed. Then lay up the other half mould, following the sequence described above from surface coat to final reinforcement until all the segments of the mould have been built. The most important facet of satisfactory mould life occurs when the mould is first made. The mould must be allowed to cure on the plug for seven days at 20ºC or more, to ensure minimum shrinkage and distortion during cure. When all the pieces are complete and cured it is time to trim the edges of the mould and drill any holes for clamping bolts. Drill the holes first so that if any part of the mould pre-releases while trimming everything will still line up later. Trimming is best achieved with a hacksaw, followed by grinding or sanding. Finally, it is time for the moment of truth – releasing the mould from the plug! Release wedges can be used to help coax the mould off the plug; I often use tilers’ plastic wedges. Whatever you do, don’t use screwdrivers and putty knives, because they will score the mould surface. Insert the wedges around the perimeter of the mould and gently tap them into place, progressing evenly around the edges. If necessary, light blows with a rubber mallet can send vibrations through the mould causing separation. But be careful, heavy pounding can fracture the mould. Once the mould is off the plug, wash the PVA off with warm water and inspect its surface. If all is well and nothing was damaged during release, the surface should already be very smooth. PVA can leave a slight texture behind but, if necessary, this can be quickly removed by wet sanding with 800 grit wet and dry paper, eventually moving to 1000 grit, then 1200. Rinse the bucket and the mould surface before moving to the next grade of paper, so any remaining grit from the previous sandpaper is removed. Once completed to your satisfaction, buff the mould surface with a fine abrasive polishing compound. Next time we will consider how to use the mould to make a part.

In previous articles we have considered the type materials available for composite construction, how to make a plug and how to form the mould from the plug. At last the end is in sight, we will now look at using the mould to produce the final part. The mould is off the plug and you are ready to start using it. Apply wax and PVA release agent just as you did during the mould construction process. It’s good practice to apply a few extra coats of wax on a new mould for the first few lay-ups, until it becomes ‘seasoned.’ If you are doing many lay-ups, eventually you can dispense with the use of PVA, relying on wax alone to give good release. At this point you must decide what type of resin to use. Primarily the choice is epoxy or polyester. Epoxy is lighter but more sensitive to higher temperatures. It is also the most expensive. Polyester is easier to use, and does not have to be heavy; it is all a matter of how you use it. For my part, I use polyester for general work and concentrate on keeping resin volume as low as possible. This has a twofold benefit; it keeps the part as light and strong as possible and keeps the weight down. If this is your first foray into the world of composites, I would recommend using polyester. You have already gained some experience with it in making the mould. Go for epoxy when you graduate onto more exotic reinforcements like carbon or aramid materials, which perform better in an epoxy matrix. We will assume that in the interests of weight saving, you will not require a surface gelcoat, since the part will be painted eventually. However, applying the first layer of resin and fabric directly to the mold surface can result in surface irregularities, pinholes, and print-through of the fabric weave pattern. These blemishes can be corrected once the part is removed from the mold, but it will require tedious sanding and filling. Using a lightweight fabric, such as 76gm, as the first layer can minimize these problems. Sometimes, I make a ‘gelcoat’ by mixing micro spheres into some lay-up resin and paint that on first, allowing it tack off before continuing with fabric. When laying up the reinforcement, try to utilize a single, uncut piece of fabric for each layer. Unfortunately, this is not always possible. Sometimes a part is too large to be covered by a single piece of fabric, so two or more pieces must be used. When two separate pieces must be joined together in a mould, overlap the pieces by 15–20mm, instead of butting them together. The contours and shapes of a part may also make it difficult to get good adhesion using a single piece of fabric. Composites can be formed into many shapes, but it is very difficult to achieve sharp angles (90 degrees or sharper) with continuous pieces of fabric. The fabric will tend to lift in these areas, resulting in air bubbles and weak spots in the laminate. Typically, a sharp angle is formed at the return formed at the canopy opening. You can form a radius at these corners by filling with a mix of micro spheres and resin; or light fabric that is cut on the bias (at 45°) will fit more easily. With indentations such as wing roots, it’s better to cut a smaller piece of fabric to fit the indentation rather than trying to force a larger piece of fabric down into it. However you address it, these corners must be dealt with; otherwise air pockets will form, creating cavities on the part surface. As with mold construction, stipple the resin into the fabric with a brush to thoroughly saturate it. Work air pockets out of the laminate and compact the layers as much as possible. One layer of 200gm and one layer of 300gm fabric will produce a light part that should be strong enough for average use. You can provide local strengthening by applying another layer of 200gm where needed. Remember that the main strength is required in the triangle that links engine, landing gear and wing mount – the rest is there to keep the pointy end at the front during flight (and to look good)! Extra strength can also be gained by utilizing a sandwich core. The strength and stiffness of a part can be increased significantly, with very little extra weight added to the part. This sandwich can consist of thin polyurethane foam or thin slices of end–grain balsa, strategically placed. Or you can use purpose made ‘coremat,’ which comes in varying thicknesses, but 2mm or 3mm will be plenty. When using a core material, it is good practice to mix a slurry of resin and micro spheres to taper off the edges, so air bubbles do not form around the edge of the core. Allow the laminate to set–up sufficiently that the edges can be trimmed off cleanly with a sharp knife. Don’t leave it too long or you will have to saw or grind the edges. You will soon tell if it is too early to cut, the fabric will tend to fray. In that case leave it for a while longer and try again. If you are using split moulds, there is still to come what can be the most testing process, depending on the cross section and length of the part. Hold the moulds together, using the locating dimples to position the two halves, and bolt them around the flange. Cut some 300gm fabric into strips about 25mm wide and simply lay them along the inside of the join, saturating them with resin. Sometimes easier said than done, but patience and ingenuity will ensure success. Once the part has cured, after at least 48 hours at room temperature, remove it from the mould in much the same manner as the mould was removed from the plug. Any residue from the release agent can be rinsed off the part, and it can be finished in whatever manner is necessary. Inspect the mould for any damage or dulling of the surface. If everything is fine, reapply the release agent when you’re ready to build the next part. If repairs or buffing are necessary, carry out those operations as described earlier. The points outlined in this four-part treatise are undeniably sketchy (that’s why whole books are written on the subject). But hopefully they serve to show that you can produce molds and finished parts that meet or exceed your expectations. If these articles have raised questions, that is good; it means you understand the basics. Much can be learned by asking questions, or by just giving it a go. If something does go wrong, nearly any damage or problems can be repaired. Remember that working with composites is like any other new skill you learn, the more you work at it and practice honing your abilities, the better the results will be. Once you have mastered the basics, and then refined those skills, nearly anything is possible. We should finish on a final note about safety. The following information will help you to view the chemicals we use with due seriousness. Styrene, contained in polyester resin. If you breathe high levels of styrene (more than 1000 times higher than levels normally found in the environment), you may experience nervous system effects such as changes in color vision, tiredness, feeling drunk, slowed reaction time, concentration problems, or balance problems. The International Agency for Research on Cancer (IARC) has determined that styrene is a possible human carcinogen. Ensure there is adequate ventilation and wear a suitable mask. Acetone, as used for cleanup. Exposure to high levels of acetone can cause death, coma, unconsciousness, seizures, and respiratory distress. It can damage your kidneys and the skin in your mouth. Breathing moderate-to-high levels of acetone for short periods of time can cause nose, throat, lung, and eye irritation. It can also cause intoxication, headaches, fatigue, stupor, light-headedness, dizziness, confusion, nausea, vomiting, increased pulse rate, and shortening of the menstrual cycle in women. MEKP. Methyl Ethyl Ketone Peroxide is an organic peroxide. It is a severe skin irritant and can cause progressive corrosive damage or blindness, so wear gloves and goggles. It acts as an oxidizer. Oxidizers mixed with a simple solid, or liquid, fuel can launch a 1000 tonne space shuttle. They can add enormous power to the combustive process. There was a time when my mate Keith nearly launched his Holden utility into space. He and I used to make fiberglass swimming pools – formed on site in the ground. This particular day I was off site and did not witness the spectacle. It was lunchtime and Keith was in the ute munching on a sandwich. On the back was the usual paraphernalia: a 200 litre drum of resin, 20 litres of acetone, 10 litres of MEKP, plus boxes of glass roving, the chopper gun, etc. At some point Keith became aware of suspicious happenings in the back–a cloud of very black smoke was emerging. Just as he got out to investigate there was a huge explosion as the acetone ignited, and the newly reshaped 20 litre container rocketed into the air, fortunately coming down without harming anyone. The plume of black smoke could be seen for miles. By the time the fire brigade arrived all they could do was cool down the lump of molten metal that used to be a very nice ute. No one knows what really initiated the fire but whatever it was, the result was disastrous. Take care!

A few years ago I read an article in the Blackfeet Magazine, written by Mike Beckingham describing how he had recently painted a model using water based paint. It sounded interesting and I decided to have a go myself.
The paint used was 'Benjamin Moore' Impervex Acrylic Latex Enamel.
My 1/3 scale Fly Baby and Graeme Bradley's recently completed G.B. Sportster are both finished in 'Impervex'.
Now for the strange part - the paint is thinned with 20% - 30% Bars Bugs windscreen washer fluid. This fluid has detergent in it which thins the water (breaks its surface tension) and slightly slows the drying process. The alcohol in the fluid evaporates quickly and leaves the detergent, water and paint behind. The water evaporates next, leaving the detergent and paint. Slowing the drying time a bit gives the paint more time to flow out and cover the surface more evenly.
I used a touch up spray gun at between 20 and 30 P.S.I. As with any sort of spray painting one has to add thinners until the optimum amount is achieved. Some trials need to be done on scrap material (or the wall of the neighbours house). I found my heat gun quite useful as well, blowing hot air across the newly painted surface from a distance of about 600mm.
This paint has several advantages.

• Mike Beckingham quoted only adding 400grams (1.5oz) to his 1/3 scale Stearman. ( That was to an area of 14sq ft!)
  
• There are no nasty fumes to inhale
• Clean up is a joy. Just have a bucket of water on hand, or the hose on low pressure
• A huge saving on thinner costs.