Building with Depron Foam.

This was going to be an article in rc model flyer magazine, but as we all know the magazine folded.

Depron.
Have you ever used it.

Have you ever thought of using Depron foam for large sized models, by large sized I mean models of over 48 inch wingspan. The thought of using Depron as a building material had never crossed my mind until I was asked by a friend to look into the possibility of building an 80 inch wingspan Vulcan bomber for his model collection.

Depron is an extruded polystyrene insulation material suitable for use below wood, laminate or even carpets. It is a thin insulation sheet manufactured of fully recycled material. The sheet is produced of foamed polystyrene, a material that does not age or degrade. The cell structure in the sheet is built up of fine closed cells that give it its excellent physical and mechanical characteristics with the added bonus of being very light, ideal for model building in fact.

I have seen it used mainly on the smaller shock type models and indoor flyers, though there are quite a few talented builders who have built much larger models with it and very successfully too. So after some internet research and some conversations with other users of the foam material, who I found to be most helpful, I plucked up courage and decided to accept the challenge and have a bash with it myself.

Depron can be purchased online and is quite cheap to buy, it comes in grey or white sheets of 1250mm x 800mm sheets in various thickness's including 2, 3 and 6mm, I buy mine from here www.depron.co.uk as they deliver next day and are cheap enough for my wallet.

You will find that when you hold up a sheet that it has both a shiny and a flat side.
I have found that the flat side takes glue better than the shiny side, although the shiny side is that little bit harder to cover but in it’s raw state, it will take paint just fine .

Talking of glues, we better decide what glue to use to stick our model together with. Your best bet is to use a contact adhesive like uhu por, though you can also use gorilla glue, polyurethane wood glue, epoxy and foam safe cyano. Whatever glue you choose to use it would be best to test a small area of scrap foam first as I have found that some of the cheap epoxy's will dissolve Depron. Do remember that if you do use foam safe cyano it might be a good idea to make sure that the kicker is foam safe as well. Also, if you are using one of the foaming pu glues you will have to clamp the two pieces of foam together while gluing as the foaming action has been know to force the parts apart as it expands and cures.

Contrary to popular belief Depron does have a grain of sorts and will curve better in one direction more than the other. The best way I have found to find the grain is to flex the sheet to gently to check the direction that curves the easiest before you cut the pieces you need. I use the shiny side on the outside of the model as it is the slightly harder side.

If you warm the foam slightly as you curve it, it will bend smoothly and will hold it's shape quite well when released. I have found the easiest way to do this is to just rub your hand firmly along the foam a few times to build up some heat from friction and then bend it slowly over a former or some other curved object a few times until the Depron obtains the curve required for the job. You can also use steam or a warm covering iron if you place a piece of cloth between the iron and the Depron just in case it melts (it also saves the covering iron from getting ruined if the foam melts).

If you gently kneed the foam between your fingers and thumbs you can make some quite complicated shapes and compound curves, this is one of the areas where Depron is superior to balsa wood as it can save hours of planking when building curved and complicated sections. Of course you can cut planks from the Depron and build in exactly the same way as you would with Balsa wood, though this sort of goes against why we are using it in the first place. But as with all things, the choice is yours in the long run.
 
Now we have discussed what Depron is and how best to stick it together, we aught to think about the best ways to construct our model aircraft using the product.

Through trial and error, I have found two main ways of building with the material. The first is to just replicate the normal balsa construction method, replacing every wooden piece with a same sized piece but in foam instead. This is fairly strong and is lighter than using balsa wood, though you will have to use balsa for all the wing spars to give the wings the strength they need.  However, this method leads to too complex a structure and doesn’t best use the characteristics of the material.

My preferred method is to cut solid sheet cores for both the side and plan profiles of the intended model out of 6mm Depron sheet and then join them along the centre line, I like to cut slots along the centre line of the main core and then cut notches in the two half cores that will line up with the slots, this gives more gluing surface and produces a stronger join. I then fill the gaps with quarter formers spaced at regular intervals to make up the required profile shape ready for sheeting with the 3mm sheet, remembering to add ply where necessary for mounting points.

Try to sheet the surface of the model with as large a piece of 3mm as you can as this will give more strength to the construction than if you use lots of smaller pieces of foam .This can’t be helped in some areas but on the whole it is better to keep the use of small pieces to a minimum if at all possible. The main reason being is that the Depron sheet is forming a monocoque skin and this gives the construction its strength and rigidity, exactly the same way the aluminium skin gives the spitfire its structural strength.

You can lighten the construction even more if you are that way inclined, by using a sharpened tube to cut out lots and lots of holes between each former. Though I have not yet bothered with this myself, as it seems to be lots of work for little weight saving, on my next big ¼ scale model I am trying it out to save as much weight as possible.

Wings are done much in the same way as the fuselage, I cut the wing outline from a sheet of 6mm and then cut and glue half ribs at the required spacings to give the wing the shape you need for your specific aircraft. Spars are then added to give more strength to the wing and to help it keep its shape, though most of the strength comes from the stressed skin on the outside once the wing has been fully sheeted.

Once happy with the shape, you can turn the wing over and rib the underside to match the top.
If you want wings that can be split for transport, this is the best time to add the sliding aluminium or carbon tubes using an epoxy glue for strength. If you are building a one piece wing you will need to add the ply dihedral braces (if needed for your particular model), also any ply parts that are needed for the u/c mounts, wing bolts and servo mounts. Don't forget to cut holes in the ribs to take the servo wires, a job I always forget to do!

A balsa trailing edge will also need to be added to the wing to give the hinges for the control surfaces something solid to glue into. The balsa is there to enable you to cyano hinges into the wing without running the risk of melting the foam, plus it gives a little more strength to the construction.

You will find that to avoid oil canning of the leading edge ( where the sheet curves inwards between the ribs)  you will need to add more part ribs than would first appear necessary to give the 3mm covering  sheet the  support it requires along the leading edge curve.

Of course you will have to use 6mm ply for the motor mounts and also you  might  need to run some ply braces back along the fuselage or wing for a short distance  to keep them  from twisting under the load torque applied by the motor and propeller combination. Though if you are clever you could incorporate the lipo battery tray into the strengthening set up and save even more weight.

Undercarriage mounts will have to be made from a hard wood and a wood gussets added to the Depron formers to take the extra strain of landings. Hand launch and belly landings are another option of course and if you opt for this launch method an extra layer of foam on the underside of the fuselage will help to protect the model when landing.

Any joints that you have between sheets in the surface skin can be filled using a light weight filler and just sanded smooth. Depron sands well though it does produce some fine dust, so I would suggest wearing a dust mask and sanding outside as much as possible.

Once happy with the construction of your model we can then think about covering and painting. Depron does not need to be covered as it will take paint (non solvent based) quite happily in its bare state, but if you want to add a little more strength to protect the surface from the inevitable hanger rash there are many ways of doing it.

There are several options. Tissue and a water based sealer is light and simple, but not very ding proof. Brown paper and pva glue are slightly heavier but considerably stronger. Glass cloth and a water based sealer or epoxy would be the ultimate in ding proofiness, but it will add weight to the finished model.

When I covered the Vulcan bomber I chose to go the brown paper and pva route on the grounds that it was easier to apply than glass cloth and epoxy but not that much weaker in the long run. I also found it easier to use as there is no smell and everything can be cleaned with water when you have finished. This is a big bonus if you are working indoors as it will also keep the wife happy.

If you dilute the pva 50/50 with water and wet the brown paper first before gluing it onto the surface it sticks well and when dry it is quite strong and has the bonus of giving a great surface for painting on.

Well that's it really, there's not much more I can add to this basic guide to Depron. Your best bet now is to buy some and have a bash, making mistakes and sorting them as you go along, it's the best way to learn with any new material.

Now I am no expert and this is only my way of using the substance, I expect there are many more and many better ways than my method but the above seems to have worked ok for me so far.

In the next instalment we will start cutting and building the fuselage for a 48” wingspan sports model for a brushless electric motor set up running on a 3s lipo battery.



© Anthony Bennett.

2014

1995 words.

Great work Tony, good read