Fold and Roll: Polystyrene foam sheets

5 December 2023

Whether it is a skeletal or a solid structure, the outerskin is the finish, adds integrity/stiffness and is necessary. The outerskin is usually coloured tissue further decorated with lines and symbols. Since it is used throughout, it cannot be omitted if any standard of visual representation is required and there is no different in weight except for the gluing area.

For flat flying surfaces, the structural weight of a foam sheet or balsa built up is about the same, but obviously, the foam sheet is easiest.

For solid surfaces, foam blocks, folded/rolled sheets is fastest. 

21 February 2022

Tissue-foam technique might just be ideal for (under-)cambered wings for light models. :

1. Cut a tissue sheet that is an inch wider all round, use the leading edge of the wing as the guideline;
2. Glue the tissue onto the leading edge of the wing blank;
3. Tape the excess tissue at the leading edge down on a large cylindrical former;
4. Apply glue stick to the other 3 borders of the wing blank;
5. Stretch the tissue to curl the foam around the former, start at the middle of the trailing edge and tape down, carefully pull/reposition and tape the tissue all around;
6. Cut an oversized piece of baking paper and use the household laundry iron to roll over this baking paper that is set over the tissued foam. This will hopefully set the glue and perhaps a bit of the foam curvature, apply alcohol and repeat the ironing process;
7. Release from former and trim the tissue.

The above is for top covered wings. Tissue is used primarily for aesthetic, it also adds a bit of strength. By skipping out covering the bottom of the wing, a little bit of weight is spared and the covering work is halved. If tissue shall cover the bottom of the wing, then the tissue shall be bonded to the underside of the wing first, and this has to be throughout the entire bottom side of the wing.

Notes:

1. The grain direction in tissue sheets means it is weak across the grain and it is easier to curl across the grain.
2. When the tissue is bonded to the underside of the foam sheet, curling the composite sheet to form an (under-)cambered airfoil will not tear the tissue because the force is compressive and it is more flexible than the foam; if tissue is also bonded to the upperside, then the tissue will probably tear along the grain.
3. The top tissue should be applied after the foam is curved. The bottom tissue can either be applied before or after the foam is curved. 

 3 January 2022

Balsa is nice but you need to buy it and the wood differs. Foam is relatively consistent, cheaper/free and flexible but does not come readily available in thin sheets.

1. Fix/screw/pin/tape/glue 2 dowels (bamboo, wire, anything that will not snag on the cutter wire) to my blue foam building board or any corrugated cardboard that is purpose-cut to width for the bow cutter.
2. Cut a polystyrene foam block so that it fits loosely between the 2 dowels.
3. Weight/press the polystyrene foam block.
4. Drag bow cutter along the dowels and the resultant bottom sliced sheet will have a thickness slightly less than the dowel's diameter.
5. Remove sliced sheet from the polystyrene foam block, repeat steps 2 and 3 for more sliced sheets.
6. After a few slices, clean the bow cutter's wire by swiping the wire with a damp sponge, the one that is used for cleaning soldering iron's tip.

 I think I can make a lot of model planes using thin polystyrene foam sheets.