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VACUUM FORMING – Project 004

 

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Project 004 – Pontoons for 400 / 450 Size Helicopters

The object of this project was to create pontoons to conveniently attach to, helicopters of sizes 400 and 450 so they could land on water. An important design feature was also to provide visual signals of the roll attitude of the helicopter.

 

The two helicopters intended for the pontoons are these.

 

Esky Honey Bee King II (HBK2) with Squirrel fuselage.

Weight = 670gm including 3S 1800mAh battery

CopterX 450SE.

Weight = 808gm with 3S 2200mAh battery.

 

Making the Moulds

To calculate the size of the pontoons, we use Archimedes principle, which states

 

“Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.”

- Archimedes of Syracuse

 

The dimensions of each pontoon I chose are as follows.

This is the volume calculation for one pontoon.

R = 2 cm

L = 40 cm

H = 0.6 cm

 

A + B + C + D = Volume of a sphere with R = 2 cm

Therefore A + B + C + D = 4/3 x pi x R3 = 33.5 cm3 

 

E + F = Volume of a cylinder with R = 2 cm, L = 36 cm.

Therefore E + F = pi x R2 x L = 452 cm3

 

G + H = Volume of a cylinder with R = 2cm, H = 0.6 cm.

Therefore G + H = pi x R2 x H = 7.54 cm3

 

J = Volume of a box with L = 36 cm, W = 4 cm, H = 0.6 cm

Therefore J = L x W x H = 86.4 cm3

 

The total volume is the sum of the partial volumes = 579 cm3.

Therefore both pontoons have a total volume of 1158 cm3.

 

 

Water has a density of about 1gm/cm3 so 1158 cm3 of water weighs 1158 gm.

The amount the pontoons will be submerged = (weight of helicopter + pontoons) / weight of water the pontoons displace.

The estimated weight of the pontoons is about 70gm.

 

Therefore, for the HBK2, the percentage the pontoons will be submerged = (670 + 70) / 1158 = 64%.

This means, therefore, that 36% of the pontoons will be above the water.

 

The pontoon assembly had to be positioned so that its centre of gravity coincided with the centre of gravity of the helicopter. Polystyrene foam blocks were positioned inside each pontoon to provide structural strength and to hold cross beams between the pontoons. The cross beams for the HBK2 helicopter were 3mm carbon fibre rod and 5mm O.D. carbon fibre tube for the CopterX helicopter. They were positioned just forward of the front helicopter legs and just behind the rear helicopter legs. This positioning was chosen so that the pontoons could be attached to the skids using only two rubber bands. The separation between the pontoons is greater for the CopterX 450SE than the HBK2 to suit the respective undercarriages.

 

  

 

Each pontoon was made from four identical parts glued together. Two identical moulds were made so that two parts could be made from the one sheet of HIPS. 0.5mm thick HIPS was used. The parts were made so that there was a straight section of 3mm on each part so that a joining strip 6mm wide could be glued along the inside of the join. Each mould had a thin coat of wood filler which was sanded smooth. The ends of each mould were cut at an angle so that the moulds could be separated easily from the moulded parts.

HBK2 Pontoon Assembly

 

The cross beams were glued into holes in the polystyrene foam blocks with epoxy glue. The ends of the beams were sanded with 240 grade sandpaper before gluing to provide a good gripping surface for the glue.

After assembly, the pontoons were painted so that the nearest pontoon to the pilot was always red and the furthest one is always white. This makes it much easier for the pilot to see the roll attitude of the helicopter.

 

 

The pontoon assembly was attached to the helicopter with two rubber bands, each one hooked over the ends of the skids and under the cross beams of the pontoon assembly. The distance between the pontoons is about 4mm wider than the skids.

 

 

A strip of black PVC tape was applied to the underneath of each pontoon as sacrificial protection against scraping on the ground.

The final weight of the pontoon assembly for the HBK2 is 64 gm.

CopterX 450SE Pontoon Assembly

 

The size of each pontoon for the CopterX 450SE is the same

(The pontoons on the CopterX 450SE have been patched up after a minor bingle.)

 

 

The cross beams are positioned to suit the helicopter legs with about 4mm clearance. The rubber bands are thicker to hold the pontoon assembly to the heavier helicopter.

The final weight of the pontoon assembly for the CopterX 450 is 74 gm.

Summary

The final submersion percentage of each helicopter pontoon is as follows.

The submersion formula of the pontoons is = ((weight of helicopter + pontoons) / weight of water the pontoons displace) x 100.

Therefore,

HBK2 submersion percentage = ((670 + 64) / 1158) x 100 = 63%.

CopterX 450SE submersion percentage = ((808 + 74) / 1158) x 100 = 76%.

 

Note that the lower the submersion percentage, the more stable the helicopter is on the water. Taxiing across the water with low throttle settings is not recommended because of the drag caused by the water. With the helicopter throttle just below the hover setting, however, the helicopter sits on the water without much submersion, so slow taxiing is possible. The helicopter is more stable in the pitch direction than the roll direction because the pontoons are longer than the separation between the pontoons. Increasing the separation increases stability and bigger pontoons also increase stability.

Bigger Pontoons

Making larger pontoons is possible with the same moulds. The moulds are made so each part can be 20mm longer and / or 5mm higher.

This means pontoons can be made 40mm longer and 10mm higher. With pontoons this big, the displacement of each pair of pontoons would be 1624 gm. The submersion percentages would be approximately ..

 

HBK2 submersion percentage = ((670 + 80) / 1624) x 100 = 46%.

CopterX 450SE submersion percentage = ((808 + 90) / 1624) x 100 = 55%.

 

These would sit on the water with much more stability.

Video

Watch a video of the CopterX 450SE test flight with pontoons fitted.

 

https://www.youtube.com/watch?v=7mflmMyJkWU&feature=youtu.be

 

 

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