When I first started building model airplanes I was a typical beginner,wanting to fly fancy models like Spitfires or F-16's. Luckily it proved so difficult for me to build those complex shapes using the conventional materials such as balsa, so I flew kits for the first couple of years.

As I started to design my own PSS models I really felt the need for something that would help me to build those complex shapes more easily and lighter. The answer came to me when I saw how the british PSS'ers simply cut their model out of foam and covered it with glassfibre and dissolved the foam out.

Five years and some 16 models later I can say that I've developed my method into a succesful process that enables me to build a fuselage of any shape in a day's time. the method is easy, low cost (about £7 for an average fuselage), fast and light, I still work with balsa, it's such a nice material, but the lost mould technique saves such an amount of time and money that it cannot be ignored.

              STEP ONE                     Here's the recipe: First we'll cut the fuselage out of a foam block. The foam must be dense. I use white foam of 25 kg / cu.metre, about 2 to 3 lbs per cu.ft., or blue foam, the white one is less expensive.

Use Cardboard ( cornflakes boxes will do ) and cut out the fuselage side and top templates twice. The foam block is then made up and cut

                                        

 
to size. Use pins to fix the top and bottom templates onto the foam block and make sure that they are well aligned. Now the foam can be cut. I use the same hot wire that I use to cut my wing cores. Remove the templates and repeat the process for the side view templates

                 STEP TWO                   Our fuselage is now correctly shaped in two dimensions, the third one is added by sanding. Don't worry, because of the density of the foam it'll sand just like balsa, there is also the same amount of dust, but this is statically charged so wear an overall. Use a 150 grit paper for the shaping, and a 600 grit to smooth the fuselage when you're satisfied with the result. Most fuselages are round, and you can speed things up by cutting off corners with a sharp knife before sanding.

It's a good idea to have some photo's or better still a plastic model of the aircraft you're making. It helps you to get the shape right during the sanding. Remember that this is the stage at which the final shape of the fuselage is determined.

               STEP THREE

After smoothing the entire fuselage it has to be covered with plastic heat shrink film or brown plastic parcel tape. You can use an iron to smooth out the tape much as you would for Solarfilm. There are several reasons for this covering :

  1. It keeps the resin from penetrating into the foam, and thus keeps the final weight of the fuselage down.
  2. It gives the inside of the fuselage a smooth surface, so it's easy to install bulkheads and the radio gear.
  3. It prevents the foam mould from beïng dissolved while putting on the glassfibre if you're using polyester resin.

The choice between polyester and epoxy is upto you. My experience is that polyester dries faster than epoxy, is better for your health and smells worse.

  

         

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Lost mould methode
the fuselage is cut out of a foam block, sanded to the right shape, and covered with glassfiber. Later, the foam is dissolved.
                                                                                            

 

All foam methode
Foam is cut using templates, is then veneered and used to build up the airframe, small parts may be covered with glassfiber.

 

Foam-veneer-balsa methode
The quickest method to build a fuselage. The core is cut out of foam, and balsa sides are glued onto this core, later the fuselage is sanded to shape. The resulting airframe is light, very strong, and low cost.

 

                    Paul Janssens Lost Foam modelling techniques
                STEP FOUR                     The fuselage is now covered with a coat of de-moulding wax, any floor wax will do.

Now the fuselage is covered with two layers of 160 grams / sq.metre glassfibre cloth and resin folowed by a final layer of 80 gram cloth. An hour later apply a thick coat of thick microballoons and resin.

                STEP FIVE                   The next day you can cut away any excess glassfibre, and sand the fuselage to obtain a smooth surface, use a 150 grit to start with and smooth it with 600 grit and water. Be careful not to sand into the last layer of glasscloth or tissue, only the microballoons should give you a smooth surface.

Now you can remove the exposed tape and pour in some acetone to dissolve the foam. The rest of the tape can now be pulled out of the fuselage. That wasn't too difficult was it?                    Finishing is now just a matter of spraying up in the usual way.

                  STEP SIX                        Now you have to keep in mind that there may still be some wax inside your fuselage. So every time that you need to glue anything into it you have to roughen it up with sandpaper.

              VARIATIONS                   The mind of a modeller is ingeniouse so you may well be thinking about how to improve or further develop this technique. Firstly, you can use exotic materials in high stress areas to strengthen the fuselage. Kevlar put on as the second layer will help cloth or unidirectional carbon tapes.                  Another variation for airframes where weight is less important than strength is to epoxy the glass cloth directly over the foam and only axtract enough foam for the equipment installation. This would also allow one layer of 160 gram cloth to be deleted.                               You'll find that this method opens up a whole new world, there isn't an airframe that you cannot build. It might not be as much fun as working with balsa, but if you need to produce odd shapes at a good rate this is it !