Foam Shell Construction

13 January 2016

I am into small RC aircraft that are powered and controlled by WLToys' receiver board and geared motors; I don't see how it is possible to build and fly bigger models in the future when there is no field large enough to fly them in. I prefer metal fuselages like the P-26A to be rounded and not having the starved horse appearance that is common with stringered construction. It is possible to plank the fuselage, but I have noticed that all rubber scale models in this size uses stringers for lightness and planking seems too heavy. It seems that doing a scale model is already detracting flight performance and if the model is built heavy, it might never fly.

I think foam is light and less glue should be used. The most direct method I can think of is carving and hollowing two foam halves of a fuselage profile. This is definitely a messy method and not what I fancy doing. The possibility of a slip and destroying hours of hard work is a bit much. Another one is to make either a male or a female mould and do a casting or perhaps heat forming. It seems to involve a lot of things and anyone of them can go wrong.

NO! I don't intend to actually do this, but I am writing my thoughs of planking with foam:

Mould

1. Make two male moulds, one left, one right, together they form one fuselage. Alternately, top half and bottom half, or many sections.
2. Moulds can be carved and sanded from blue foam, a bit messy, but it is possible to make more than one model if you are careful and you keep the moulds.
3. The moulds may be replaced by framing, using a few foam formers and a keel to hold them upright and in the correct position.
4. Raised the left/right mould off the building board with some scrap foam.

 

Making a Shell

1. For the skin, cut a slightly over-long piece of 2mm foam sheet.
2. Curl the skin width-wise, as much as the smallest curved section, usually at the nose. If the tail end of the fuselage is flat, although it is smaller than the nose, still go for the nose.
3. Measure the width of foam that is required to cover the smallest curved section. Divide this measurement by 3 and that is the width of the curled strips that will be cut in the next step.
4. Strip lengthwise, 3 pieces of the curled skin to the width determined in the previous step. A straight edge and sharp cutter is more positive but it might be difficult to hold the straight edge steady against a curled sheet. Alternately, it might be possible to use a master airscrew balsa stripper.
5. Place the first strip onto the mould, right in the centre, leaving equal distance at the nose and tail. Trim to length and secure temporary with tape to the building board.
6. Place the second strip along either the top or the bottom of the fuselage half mould, choose the edge which is continuous, at the nose, it will meet the first strip and its other edge will cross the the mould's edge. If necessary, trim the rear end of this strip to match closely with the first strip. Trim the strip to length. Apply UHU POR to the surface in contact with the first strip, avoid any glue on the mould. Tape in position to the building board. If there shall be a gap at the tail, insert intermediary strip. The first and second strips must be in contact with each other. 
7. Place the third strip along the other edge of the mould, it will meet the first strip in the nose and its other edge will cross the the mould's edge. If there is a break, align the strip with the mould's edge, trim where necessary. Apply UHU POR to the surface in contact with the first strip; avoid any glue on the mould. Tape in position to the building board. If there shall be a gap at the tail, insert intermediary strip. The first and third strips must be in contact with each other. 
8. Remove the 3-strips frame from the mould and building board, place it outside (or inside if the curl has relaxed) against the curled sheet, trace the openings of the frame on the curled sheet.
9. Replace the frame onto mould and building board. Cut out the shaped pieces from the curled sheet. Trim to fit and apply UHU POR to the edges of the shaped pieces and lay them onto the frame.
10. Repeat the steps above for the other half and glue the two halves together to form a fuselage shell.

Keel, Spines and Formers Option

This is an option that incorporates the keel/spine and formers with the shell. Obviously, with this option, we won't need to create a moulds and the result should be even stronger and stiffer (and heavier).

1. Assemble a frame/sheet to the profile of the fuselage.
2. Add half formers to both sides of the profile.
3. Then the skinning can be done in the air.
4. If a single keel/spine is used, perhaps it will not be possible to trace the infil shapes. However, if two keels/spines are used, then it is still possible.