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VACUUM FORMING – Project 003

 

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Project 003 - Squirrel Fuselage For A Honey Bee King II Helicopter

This project was to create a fuselage for an Esky Honey Bee King II (HBKII) helicopter. The design is based on a Eurocopter Ecureuil (Squirrel).

The commercially available fibreglass fuselages I thought were too heavy which reduced the flight time and performance too much. Also, I wanted a squirrel and I could not find one. So I made one.

 

This is the original helicopter.

Parts identification drawing from the helicopter manual.

 

The drawing from the manual (above right) was used to create a full scale drawing in Autocad. It was printed out in 1:1 scale

 

An Autocad drawing of a Squirrel helicopter was then created from a photograph. The drawing was scaled so that when it was  plotted out and overlaid on the HBKII drawing, the HBKII frame and components fitted inside the body of the squirrel fuselage.

 

These 1:1 scale drawings were then used to make the moulds.

The moulds were cut from balsa wood. The outlines of the drawings were traced onto the side, top and end of the balsa wood and cut along the lines. I then basically “rounded the corners”. They were shaped using a saw, files and sand paper. A  thin coating of wood filler was then applied which served to protect the soft balsa wood and provide a surface that could be sanded smooth.

 

Here you can see the lines on the moulds used to cut the formed sheet after it is moulded. For example, there is a line around the base about 10mm from the edge. Each of the moulds is 10mm higher than the final trimmed formed sheet.

This was one of the first prototypes. This was used to work out where to cut the large cabin mould in half so that the front could be detached and allow access to the inside. This was made from PETG. Later it was decided that 0.5mm thick HIPS was the better material to use.

 

I made a couple of fuselages .. crashed a couple.

I tried some different graphics.

 

The head assembly was changed to a metal one so the design was modified to allow more clearance below the flybar disc.

A few more fuselage designs were considered …

 

… and I settled on this one. A big advantage of this one is in the three horizontal contrasting bands of colour. (The orange is fluorescent.) If the helicopter is side on to the pilot, it is often difficult to judge the roll attitude, particularly at a distance i.e. whether it is tilted towards you or away from you. The relative heights of the orange section and the red section provide excellent feedback to the pilot of the roll angle. It is much better than trying to judge if the nearest skid is above or below the furthest one.

 

The front of the cabin is held to the back of the cabin by four rare earth magnets glued to the inside of the joining flange.

There are also four rare earth magnets glued inside the joining flange of the front of the cabin.

 

Four locating pins were used to align the front of the cabin with the back. Carbon fibre rod 1.5mm diameter was used for the two upper pins and 5mm carbon fibre tube was used for the lower two. They fit into holes in the joining surface of the back of the cabin.

There are ventilation holes beneath the back of the cabin and in the top of the cabin. The ESC and motor are placed between the top ventilation holes and the lower ventilation holes and get good airflow flow from the rotor blades.

 

The ventilation holes in the top and bottom are covered with plastic fly screen mesh, glued on the inside with epoxy glue. To give the epoxy better grip, the surface of the polystyrene was sanded with 240 grade sandpaper before gluing.

 

 

 

 

 

 

The two formed halves are joined by a 6mm wide strip of polystyrene on the inside. The strip is first glued to one side and when the glue dried, the other side was butted against the first side and glued to the strip.

 

 

 

 

 

 

 

 

 

A hollow polystyrene beam is used to connect the tail to the boom. The beam is glued inside the boom and extends beyond it. The tail slides onto the boom and locating pins through the boom hold the tail rigidly in place.

 

 

The edges of the two parts are butted together but a tongue and groove on the inside ensure the fuselage skin stays butted together in line.

 

 

 

 

The edges of the vertical and horizontal stabilisers were joined using polystyrene tube. The tube was first glued to one side and when the glue had dried, the other side was glued.

 

 

 

 

 

 

To balance the weight of the tail, the ESC was mounted in front of the motor and below the ventilation holes in the roof of the cabin.

Also, the battery was placed in a battery tray (also vacuum formed for the purpose) that extended out and angled up to fit neatly into the nose of the fuselage.

 

The large removable front section of the cabin, when it is removed, allows good access to much of the mechanics for maintenance.

 

The weight of each section is as follows.

 

A - Cabin front half _____39 gm

B - Cabin back half _____32 gm

C - Boom _____________14 gm

D - Tail _______________11 gm

E - Horizontal stabiliser __ 6 gm

TOTAL _____________102 gm

 

In comparison, the weight of a commercially made fibreglass airwolf body for the HBKII was 160 gm!

 

 

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