Monday, 22 September 2014

The Parachute

When sewing my own, I found inspiration in existing commercially sold parachutes. I googled a couple of images and based my design on them. As the material, I used Skytex which is used by a local paraglide manufacturer.


I decided for a hexagonal shape with six cords going down to the four corners of the payload box. It didn't turn out to be a regular hexagon, nevertheless, it's area was 1.7m2. With the payload weight of 940g and the balloon weight of 800g I used the Drag equation to calculate the expected descent speed at ground.

v = sqrt( ( g * m ) / ( 1/2 * r * Cd * A ) )

g...the acceleration of gravity (9.8m/s2)
m...the mass of the payload and the balloon in kilograms (0.94+0.8kg)
r...the density of air at ground altitude (1.205kg/m3)
Cd...the drag coefficient of a flat sheet parachute (0.75) for a dome-shaped parachute use 1.5
A...the area of the parachute

With these values, the expected landing speed was 4.71m/s. That turned out not to be an accurate estimate, because the recorded speeds during the actual flight fell down to about 2m/s. Another thing to keep in mind is that there is a possibility the balloon debris and cords will tangle up the parachute after the balloon burst. That might significantly reduce the parachute's ability to slow down the fall. This is important for estimating the predicted landing site. In my case, fearing the possibility of the parachute tangling up, I used values rather too high in the trajectory predictor (link is in the Links section), thus expecting the payload to lend much closer than it actually did.




As cords, I used ordinary clothesline which I sewed into the parachute. In the middle of the chute, I made a hole and sewed a couple more cords. These were used for attaching another cord that led to the balloon.


The parachute was ready for testing. At one point, I attached a 1kg dumbbell to it and threw it of the roof. It did a decent job slowing down the fall, so I concluded the parachute would be sufficient.



The balloon cord was tied and sewn into the top of the parachute. I also used some electrical tape to cover the tattered cord tails. At the end of the cord, I prepared a loop so I can easily reeve the balloon's ending through.


To attach the parachute to the payload box, I sewed a few straps together to form a sort of a fastening suspension.


Initially, I sawed the cords straight to the straps, but that turned out to be highly impractical as the parachute cords would tangle up making it impossible to mount it on the payload box. As a result, I decided to attach to both the straps and the cords simple key rings. Thus, I could quickly join them when I needed.



And this is the finished parachute with the straps and the cords. With the balloon and its initial height, the whole set reached 9-10m in length.



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