There can be no denying that Expanded Poly Propylene (EPP) has completely transformed the slope soaring scene since it was introduced to the hobby by American Pat Bowman about twelve years ago. Gone now are the days when a week of repair work would follow a slight launch mishap, mid-air bump or heavy landing. The design, construction and finish of EPP models has developed along with the materials used to put them together and as the years have passed by, the quality, performance and styling has improved dramatically. So much so, in fact, its sometimes difficult to differentiate between a well-finished modern EPP slope racer and a fully moulded carbon thoroughbred, even when theyre sat on the deck alongside each other.
EPP is a rather versatile closed-cell bead foam that provides outstanding energy absorption, multiple impact resistance, thermal insulation, buoyancy, water and chemical resistance, and an exceptionally high strength to weight ratio. Moreover the stuff is 100% recyclable. The base beads of the foam can be made in a wide range of densities, from 15 to 200 grams per litre, which are then transformed by moulding into densities ranging from 18 to 260 grams per litre. Individual beads are fused into the final product form resulting in a strong, lightweight shape.
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The use of this new material has required the development of new building techniques and often the adoption of new-age products, adhesives and coverings to enable what is essentially a large piece of foam packaging material and sticky tape to cruise around the skies with an appearance and efficiency envied by many a traditionally built model.
Its been a good 10 years or more since the Avicraft Bullet soarer first appeared in the UK. Since then, however, modellers have developed new and improved ways of working with EPP as theyve gradually got to know it better. Gone are the days of using silicon sealant to glue spars and Copydex to hold covering and multicoloured vinyl tape finishes across the board. Now we use foaming polyurethane glues, household Goop, Drywall Spackle, 3M77 spray adhesive and Profilm!
If thats left you scratching your head, wondering just what Im talking about, then read on. Who knows, you too could end up building a model thats made of foam but looks like glass.
WHAT IS GOOP?
Now heres a question to be found on any slope soaring internet forum. Usually asked by non-U.S. flyers, the question will always arise after scrutiny of the instructions from one of the popular U.S. kits which will almost certainly advise using Goop for a particular joint. Well, Goop is a flexible contact adhesive and sealant made by Eclectic Products in the USA. It dries to a rubbery consistency not unlike hot glue gun adhesive (though more flexible). Advertised as a suitable adhesive for use with a host of products, it can be found in many forms such as ordinary household Goop, marine Goop and shoe Goo(p) etc. Mind you, it isnt particularly easy to find in the UK.
Our cousins across the pond have developed many uses for the glue on EPP models, from just about every part of the construction process through to finished coverings using thinned layers. Quite surprising, since the manufacturers advise against using Goop on polypropylene products! Do note that Goop is flammable and gives off harmful vapours. The label, in fact, contains warnings linking the product to birth defects and other reproductive harm. Start exposing yourself to the thinning down process with the industrial solvent Toluene and, believe me, your health will start to suffer.
One simile likens Goop to silicone bathroom sealant but in practice the glue couldnt really be much further from it. Ive always found Goop a tricky product to work with and nowadays restrict its use to holding things I dont want rattling around inside a model, but might like to remove later. A virtually identical product in every way which is readily available in the UK is Pacer Technologys strangely named Zap-A-Dap-A-Goo. If you really, really absolutely must have nothing but Goop and your build process is halted because you cant get it, then buy that instead.
If youre not too embarrassed to mix with our yoof, try a skateboard shop which might carry the variant Shoe Goo in small quantities. Another product more readily available in the UK is E6000; Goop but made for industrial purposes. Its basically the same stuff as ordinary household Goop except that its base solvent is perchloroethylene instead of Toluene. This is easier to get in the UK than Goop as it is widely used in sculpting or flexible jewellery making. Google it up to find a supplier near you.
WHAT IS GORILLA GLUE?
Put simply, Gorilla Glue is a brand name for a proprietary Polyurethane wood glue. Polyurethane glues are fairly new to EPP modelling and have characteristics that draw moisture from the atmosphere causing the glue to expand by foaming slightly over a period of time.
Not to be confused with the foaming qualities of aerosol based gap filling products, Polyurethane wood glue comes in many forms from fast-grab to hard-setting. Gorilla Glues are to Polyurethane glues what the iPod is to the Mp3 player. Its the most hyped because it has been aggressively marketed. Its tag as the strongest glue in the world no doubt helping tremendously here.
Polyurethane glue is ideal for installing wooden or carbon spars in EPP foam models and performs much better than any form of epoxy in the same scenario. The expansion of the glue itself sees it growing through the matrix of the foam like a tree sending out roots to grab at anything it can reach. Care must be taken when using the product because this expansion is unstoppable until the glue sets fully and a little practice is often best advised before you commit the stuff to your model.
It is not uncommon to read of a builder sticking a foam core to the bench, so use this glue in excess at your peril. The fast-grab, quick setting stuff is particularly unpredictable as the activity which makes it so popular in use, happens with so much more force.
When set, the foamed glue is easily sanded smooth to comply with the surface of the EPP. Some people advocate adding a little moisture to the mix by slipping a damp cloth up the spar slot or even mixing a little water into the glue before adding it to the joint. This will help promote the foaming but in practice weakens the bond and should not be used in countries like the UK with a high air moisture content. Be sure that the piece youre gluing in is intended never to come out, as it certainly wont once the glue has cured.
Many builders spread Polyurethane glue onto EPP model noses or wing tips and sand it back when cured to provide a hard shell to resist damage even further.
WHAT IS SPACKLE?
Spackle is a term popularised on US forums and relates to nothing more than interior wall filler or dry wall Spackle as it is correctly known. In modelling terms many builders are using it to provide a glassy surface finish, free of the bumps EPP foam can leave under the model covering.
The product I use for this task is Polycell quick drying filler from DIY outlets. If you pick up a large tub and think its empty, then you have the right stuff!
To proceed you must sand the EPP surface in its entirety and make sure that any servos, leads, receiver antenna and battery packs etc. are all in place and recessed into the foam as necessary. When youre happy that the model is sanded as smooth as you can achieve, liberally coat it in Spackle as if you were icing a cake. Go over everything except the receiver – that includes the servos which may only have their arms poking above the surface of the wing. Dont forget to spackle over any ballast tubes leaving easy access to the hole! A long steel ruler or smaller plastic credit card is ideal for the application of spackle, try to build it up around wing tips or leading edges leaving it quite thick to begin with.
Quick drying filler it may be but its not going to set hard in 15 minutes in your cold shed at the bottom of the garden. Get it in the house overnight and let it really bake dry. Now it can be cut back with glass paper to leave a very smooth surface. You will note that there may be some cracking and that when sanded this first coat has only really filled the patina of the foam. Thats why you must now Spackle the whole model once again. This coat however can be much thinner, paying particular attention to areas you might have inadvertently missed first time around. Again you will need to let this dry very hard overnight before cutting back to a fine, smooth finish. This is really the first step in achieving that moulded model look. A properly spackled and sanded EPP model really does perform much better than a rough finished equivalent.
THE ELUSIVE 3M77
The model is now at the stage where we need to use fibre reinforced tape. This has been the traditional strengthening material for EPP since it became popular. Incorporating fibreglass strands within a very tough strapping tape, its available in different sizes and you should find it in good model shops.
Now then, some form of additional adhesive must be applied to that spackled surface before the tape is added, and for this many flyers use foam-backed carpet spray adhesive. The kind that comes out of the can like silly string and produces lumps under the covering if you spray from too far away. A much better (but more expensive) product is 3M77 adhesive spray. Again, this is used more in the U.S. than here but its available in the UK from an increasing number of outlets. It always sprays in a controllable fine mist and dries much quicker than the carpet adhesive. Some modellers use permanent mount spray from art suppliers but these are quite weak compared to 3M77.
The tape can be laid down in accordance with the kit instructions when the sprayed model is dry. Taping on at 45 degrees will increase torsional rigidity on some models and others often require no tape outside of the wing-servo arms. Follow the manufacturers lead but do not overlap the tape if youre striving to achieve that beautiful finish. Instead, and for obvious reasons, the strips should be butt-joined. Of course, this does have the disadvantage of allowing slight movement between tape strips. Mind you, a 4 inch wide tape reduces this by 50% over the more common 2 version. Since the fibre tape has a release agent on the backing (how else would we ever get it off the roll), said release agent must be removed from the surface of the tape before you progress to the next stage of covering. This can be achieved by lightly sanding. Just enough to get the agent to spall off in little balls but not enough to sand through to the fibres. Go lightly until the entire surface has a matt finish. The model is now almost ready for the final stages of covering.
A GLASSY FINISH
I think weve established at this stage that, if you want it to look good, theres quite a bit of work involved in finishing a typical EPP model. This next bit, however, is perhaps the one that makes all the difference.
Just what is going on under the covering that permits such a wonderful glass-like finish on the outside? Well, this can be achieved in a variety of ways. It is possible to achieve a near perfect surface finish on an EPP model by simply applying heat-shrink film onto the top of the strapping tape. Usually a little more adhesive will be required, this through either watered down PVA, thinned Clearcote or even a light dusting of 3M77. The use of PVA glue soaked brown paper is becoming more common. Although illegal as a 60 class racing material (it is a crunchy, non deformable covering material) many pilots do use it for sporting or Dynamic Soaring model use. It has the added bonus of binding the edges of the strapping tape together and greatly increasing the torsional rigidity of the wing, whilst also hiding the lines that can be left by the fibres within the strapping tape itself and the joints between the strips.
Application is simple but messy. Soak pieces of brown paper (previously cut to shape) in watered down PVA wood glue and apply with more of the mixture using an old paint brush. It provides a hard shell structure when set, which is very resistant to dings and other minor damage. Sailing closer to the wind as far as racing class rules are concerned, is the use of a free flight tissue covering applied with 3M77 spray. This too binds the edges of the tape together but remains more flexible to achieve a similar effect before the final, outer layer is added – usually a heat-shrink film.
In my experience Profilm has produced the best results for a typical racing finish. It does not de-laminate like older Solarfilm and is generally tougher. Many pilots have tried products such as Glosstex or Solartex but whilst they do contribute more to the rigidity of the model, they seem take a little off the speed. I have no hard proof of this except the accounts of pilots flying models covered in both materials. I generally use Solartex on a fuselage of a model as it tends to wrap compound curves much better than film, finishing it off with a matt or gloss finish using Clearcote.
PVC tape is not a serious finish for a 60 racing model. Great for sport or combat, but not for speed or efficiency. In short, its significantly less durable and suffers greatly in the sunlight. Many pilots have started to achieve good results with sticky backed Fablon type material. This is available in many patterns and makes for some striking colour schemes on the slopes. Do note that since were applying the covering to a foam model, the iron temperatures must be lower with the effect that the adhesive may not grab as effectively as it should. The model can be spruced up with sticky trim later but the fewer edges capable of lifting away, the better. In practice, many pilots actually tape over the leading edges with clear Sellotape Diamond to alleviate this particular nuisance.
Take care to cover the undersides of the model first to ensure that leading edge overlaps end up the right way round – dont forget to consider how you are going to hinge your control surfaces, either. If your elevons are film covered then you can also film hinge them. If you carbon bagged your elevons then the best results can be achieved by hinging with fibretape or better still 3M Book Tape, which is generally obtainable online.
Carbon Bagging is a popular modification for elevons or other balsawood parts. It entails covering the surfaces with carbon fibre cloth or strands to improve stiffness and torsional rigidity, This, in turn, improves control response at high airspeeds. Note that some kits are supplied with the material to do this. Bagging is best achieved with the use of a vacuum pump, although good results can be achieved by pressing the carbon on against a plastic or glass surface. Carbon cloth can be obtained at a number of outlets in the UK but for best results the weave should be heavy (at least 200g) or sewn tows should be used instead. The latter is more difficult to source and has the appearance of straight, flat carbon tows sewn together by clear monofilament or fibre glass.
Well there you have it. A quality finish which will improve the flight envelope from your EPP model. True, the process might be a little onerous if you intend to combat to the death but if you want to go fast, stay fast and win races then it will undoubtedly help.
See you on the slopes.
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