Glass lay-up of the fuselage mould and fuselage

I’ve had a few questions on what glass fibre layup we use for our fuselages and the fuselage mould. Note that of course the choice of material s also depends on what is available and what quality the materials have. We source most of our materials, including glass and epoxy resin, from Suter Kunststoffe AG in Switzerland as well as from EMC Vega in Germany.

The fuselage mould

We’ve been experimenting a bit with the layup of the fuselage mould, and have reduced the number of glass sheets in our latests mould (Urupema). Note however that we only produce a small number of fuselages (usually 4), so our moulds don’t need to be that robust. If you want to produce more I would recommend using more sheets. Important is that the layers mirror eachother, so that the inner and outer layers are equal. Of course, before applying the first layer we first apply special mould resin in a thick layer on the plug and let this harden out somewhat. In our latest mould we used the following layup:

  • 160 g/m²
  • 280 g/m²
    • 280 g/m² reinforcement around the edges
  • 280 g/m²
    • 280 g/m² reinforcement around the edges
  • 280 g/m²
    • 280 g/m² reinforcement around the edges
  • 280 g/m²
  • 160 g/m²

The picture on the right is our “track sheet” that we use to keep track on where we are during the mould building process, we’re often so absorbed in building the mould or fuselage that we forget how many layers are in there already!

The fuselage

The below is our “standard” approach to building a fuselage. I know that some will think this is too much, others will think it’s not enough. We’ve built many fuselages this way, and also had quite a few crashes. In our experience, the below gives a perfect balance between weight and robustness of the fuselage. Experience shows that these fuselages are very robust, but also easy to repair after a crash.

We’re lucky to have a professional painter in our building team, who spray paints the fuselage moulds before we build them with a very thin layer of gelcoat. The fuselage is spray painted a second time once it’s fully built. On both occasions we try to minimise the amount of paint used. Paint is weight and doesn’t add anything to the rigidity of the fuselage. We often find that commercially available fuselages are covered with thick layers of paint, adding unnecessary weight.

Our JS3 fuselages, freshly out of the mould, weighed around 1200gr. Our Urupema fuselages weighed around 1400gr. Flying weight of our planes (with FES) is usually between 5.6 and 6.3kg.

The picture below (click to enlarge) shows the layup. Important is the direction of the fibers, and where/how you overlap the various sheets in the middle of the fuselage. Note that we use “UD band” (uni-directional glass fiber band, 220 g/m², around 5cm wide) to reinforce the cockpit area (“UD Glas”). We also use 24K glass rovings to reinforce the frame around the cockpit, the area where the wings join the fuselage as well as the front and to end of the tailfin. We join the two fuselages “wet in wet” (after allowing the two fuselage halves to harden out a bit) using the last layer of 280 g/m² glass.

The layup we use is as follows:

  1. First layer of 160 g/m² in the entire fuselage. Watch the overlap in the middle of the fuselage (blue vertical in the drawing);
  2. Then add the glass rovings around the cockpit area, around the area where the wings join the fuselage and in the tailfin (pink). Don’t save on thickened epoxy (using aerosil and/or cotton flakes) in the area where the wings join the fuselage;
  3. The second layer is 280 g/m², cut at a 45 degrees angle (so that the glass fibers are diagonal), throughout the entire fuselage (red in the drawing). We use this layer to join the fuselage halves later on for the rear end of the fuselage (rather than the 160 g/m² 3rd and final layer used for the tail end of the fuselage);
  4. In-between the second and third layer we insert the UD glass, first layer longer than the 2nd (green in the drawing);
  5. The third layer is 280 g/m² for the front half of the fuselage, and 160 g/m² for the rear half (blue again in the drawing). Note that it’s important to shift the “distribute” the overlaps in the middle of the fuselage so that you have extra rigidity in the part where the wings join the fuselage.
  6. Also note the overlaps of the various layers in the tail-end of the fuselage. Especially with T-tails the rigidity of the place where the tailfin joins the fuselage is very important (torsional strength).

Important to note:

  • in planes with a T-tail we usually insert a diagonal reinforcement (3-4mm Balsa with thin 45 degrees glass epoxied to both sides) to ensure rigidity of the tail (with a T-tail the weight of the elevator on top of the tailfin creates a lot of extra torsional pressure);
  • we close the space between the tailfin and the rudder with 3-4mm Balsa with thin 45 degrees glass epoxied to both sides. For our Urupema fuselage we used carbon fiber on light plywood, as a “slope racer” this one needs to be extra robust. Moreover, the Urupema has two servos in the tail (we usually place the rudder servo in the front part of the fuselage), which means that the extra weigh increases the forces on the tail end of the fuselage;
  • the above layout is based on a very light build of the elevator and rudder. If you have a heavier elevator and/or rudder you may want to consider increasing the strength of the tail end as the strain on the fuselage will be higher during “sub-optimal landings”;
  • we don’t use carbon fiber for building our fuselages. Some swear by this, because of its rigidity and light weight. We’ve found that a full glass fuselage gives the right combination between rigidity and flexibility, absorbing forces rather than breaking, where a carbon fuselage might break during a “sub-optimal landing”. Also, we’ve found that a breakage of the fuselage, absorbing forces of the crash, saves other, more difficult to repair, parts of the aircraft. Moreover, we have found a carbon fuselage to be more difficult to repair once it breaks.

As with so many things in model aircraft building, the above are personal preferences, based on many models built and many years of experience. But of course others may have other preferences.