This project was an exercise to see if it was practical to create a custom designed fuselage for a small helicopter. As it turned out, it was. Lessons learned on this project later led to creating a fuselage for a larger helicopter.
This what I started with. It is a Twister Bell 47 Navy helicopter. The plane was to remove the Bell 47 frame and replace it with brand new design.
With the frame removed, AutoCAD drawings were made from measurements of the helicopter.
These drawings were printed out at a 1:1 scale. The new design was then drawn on these printouts over the top of these drawing views.
Photocopies of these drawings were made and the outline of the fuselage was cut out of the photocopies. These were then use to transfer the outline to each side of balsa wood blocks. The outlines were then used to shape the moulds using a saw, files and sandpaper. The moulds were covered with a thin layer of wood filler and sanded again to obtain a smooth surface. If balsa is moulded without a smooth coating on it, the wood grain shows through on the moulded part.
Note that holes were drilled in the opening of the fenestron at the tail. These holes went through to a cavity under the mould that allowed air to be evacuated from this part of the mould so that polystyrene sheet would be drawn down into the fenestron opening.
The material used was 0.373mm thick HIPS. The moulds were made so that the two halves joined together by overlapping by 4mm. One part was 4 mm wider than the other and fitted under the other so that the join was in the centre of the fuselage. The overlap was glued with Revell glue for plastics.
I concluded that overlapping two halves of a mould is not the best method, even though it was convenient for this small project. Next time I will butt the two halves together with a joining strip overlapping the two halves on the inside.
The decals were created in a computer and printed on transparent self adhesive ‘label’ making sheet using a bubble jet printer. This type of sheet is available from office stationery supply retailers.
A panel was cut out of the front of the cabin to allow good airflow over the motors and electronics. The gap was covered with plastic fly wire screen mesh and glued on the inside with epoxy. The polystyrene sheet was sanded before applying epoxy to allow a better surface to grip on.
The aircraft was balanced with the fuselage on so no weight was needed for balancing.
The weight of the Bell 47 frame taken off the helicopter weighed 19gm. The weight of the XB47A fuselage is 17gm! It is still strong and rigid enough to hold the helicopter horizontally by just the tail.
Why XB47A? Well, ‘X’ is for ‘It used to be’, B47 is for ‘Bell 47’, and ‘A’ is for version A. This project worked out so well that further versions were not required.