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Very Affordable Plastic for Parachutes / How to dimension small 'chutes

I discovered a new source of colorful plastic for making your own parachutes.  Just go to the nearest store that sells party supplies.  There you will find plastic table covers in a variety of colors.  Do NOT get the cloth or felt backed ones, just plain-old plastic.  This stuff is just the right weight for a reliable small or mid-sized parachute. It is lightly textured which should help keep it from sticking to itself.  It stretches and deforms rather than rips too.  I found I can get a large sheet for a rectangular table for just $1.99 !  That is about 40 square feet of plastic, enough for several larger parachutes up to 54" diameter, dozens of smaller 18" 'chutes, and many dozens of even smaller 'chutes.  I suggest you pick a color that is easy to see in the sky, so blue, white and grey (silver) colors would not be too good.  I went nuts and got almost all the colors.  The single sheet of plastic is not quite as deeply colored as it appears folded up in the package though, but they certianly are colorful and virtually free!

 parachute material

PART 2: You have the plastic and some string for the shroud lines. Now what?

I found out the hard way that cutting hexagonal or octagonal parachutes is not as simple as it appears to be.  I ended up totally bewildered by the dimensioning of the plastic, so I figured it all out with the help of a CAD program and a spreadsheet and created an easy to use table - once you know what the columns are.  Let's say you start with a square 12" x 12", just nip off the corners and you have a hexagonal 'chute, right?  WRONG! You need to start with a slightly rectangular piece to have it come out perfectly symetrical!  I found that out the hard way by making a very lopsided chute.

* You may notice odd parachute diameters in the tables.  These are so you can make parachutes that are even divisions of the above brand of tablecloths, which is 54" wide. Cut it in half gives us two 27" pieces. Cut in 3rds gives us three 18" pieces, and so on for 13.5" ... 10.8" ...  So this chart can tell you what is the biggest chute you can make with a given piece of material.  Also, you will see that octagonal parachutes of even diameters have to be made with odd sized squares.  Don't let this bother you.  Just pick a diameter and go with it.

* If you are unsure which type to use, six or eight sides, lean towards the simpler six-sided chutes for smaller diameters.  After getting up to about 16-20 inches, it is probably better to consider an octoganal chute because the added lines can provide a little more strength and overall roundness to the chute.  Just remember that six lines are both easier to make and lighter than eight.

* Decide whether you want six or eight sides and shroud lines.  Use the corresponding table and ignore the other one.

* Parachute diameters for this tip is defined as the length between opposite shroud line tie points.  For this table, the shroud tie points are assumed to be at the exact tip/corner of the parachutes.  Obviously they are tied to a hole just a bit inside the corners, but this is usually just a fraction of an inch and can be ignored.  If you are interested in the actual circle diameter of the parachute, one could just assume the excess material at the corners are part of the shroud attachment.  This definition of the diameter would be the circle inscribed inside the hexagon or octagon, and that diameter is shown in the tables below as the 'SIDE DIA'.

* The decimal precision shown here is 1/100 of an inch.  You really don't need that level of precision.  If you build to just 1/10 of an inch, that would be fine.  Would you really notice a difference of 1/20 of an inch on a finished chute?  Just ignore or round off the last digit.  Those digits are here for those good at math, for example 6.75" would be measured on a ruler as 6 and 3/4".

* If you want to verify that you are working from a true square piece of material, here's a good way to check it: Measure from two opposite corners diagonally.  Then do that to the other two corners.  If you are working with an exact square, these two dimensions will exactly match.

THIS IS FOR 6-SIDED HEXAGONAL PARACHUTES

DEFINITIONS OF THE TABLE COLUMNS

HEX DIA - This is the measured string-to-string diameter between opposite shroud lines.

SIDE DIA - If for some reason, you want to measure parachutes from the inscribed circle, use this diameter. It is the distance between the center of opposite edges (between the shroud attachment points).

SIDE LEN - This is the lenght of the sides of the hexagon. All six sides should be the exact same length.

RAW SQ. - This is the size of the raw material (plastic, nylon etc.) you will need to make the corresponding diameter hex parachtue.  This is the longer of the two dimensions needed, so if for example you have a perfect square piece, you can actually cut a little off of one side.

 ACT WID - The actual width is the 'other' dimension of the square of material you need to make this size chute. It is the value of RAW SQ. minus SHORT.

 SHORT - This is how much material you need to cut away from one edge of a perfectly square material to obtain the rectangle that fits the diameter parachute you will make.  I recommend you cut this away NOW - BEFORE YOU MEASURE AND CUT YOUR HEXAGON!  Actually, if you want to just draw a line to define the end of the parachute material, that would work too.

RISE/FALL - This is important.  Imagine the hexagon you are about to cut out on the big square of plastic in front of you.  Imagine that hexagon with the two points directly centered on the edges of the square in the middle of the top and the bottom, or the 12:00 and 6:00 positions if you will.  Mark those points now by taking half the width.  (The other points will be at 2:00, 4:00, 8:00 and 10:00, but don't bother to mark them).  The straight sides of the plastic will be the sides of the finished chute.

 

Got that?  OK. So now to measure where you need to cut, just measure down from the top corners of your big square and up from the bottom corners of your big square and mark the sides of the big square.  That is where you will need to start cutting.  Go ahead and draw a line and then cut from that start point to the exact middle of the top and/or bottom edge you marked earlier with a ruler-straight cut.

So let's put it all together.

1) Obtain or cut a sheet that is RAW SQ. from top to bottom and ACT WID across.

2) Mark the exact center edges at 12:00 and 6:00

3) Measure down and mark from the top corners on both sides the length of RISE/FALL.  Also measure up from the bottom of both lower corners that same amount and mark them.

4) Draw a line & Cut the four corners away using those marks, you should end up with a nearly perfect hexagon.  So go finish your parachute!

 

Here is the table, pick your size from any single line:

HEX DIA        SIDE DIA        SIDE LEN        RAW SQ.        ACT WID        SHORT        RISE/FALL

8                   6.93                4                    8                     6.93                1.07             2

9                   7.79                4.5                 9                      7.79                1.21            2.25

10                 8.66                5                   10                     8.66                1.34             2.5

10.8              9.35                5.4                 10.8                  9.35                 1.45            2.7

12                10.39               6                    12                    10.39                1.61            3

13.5             11.69               6.75               13.5                   11.69               1.81             3.38

14                12.12               7                   14                      12.12               1.88            3.5

15               12.99                7.5                 15                      12.99              2.01             3.75

16               13.86                8                   16                      13.86               2.14             4

18               15.59                9                   18                      15.59               2.41             4.5

21               18.19               10.5                21                      18.19               2.81             5.25

24               20.78               12                  24                       20.78              3.22             6

27               23.38               13.5               27                       23.38               3.62             6.75

 

 

 

THIS IS FOR 8-SIDED OCTAGONAL PARACHUTES

 Believe it or not, the 8-sided beast is actually a little simpler in the math, because you start with a square. But: It turns out you can actually make a parachute with a diameter a little larger than the dimensions of the square! How's that for cool!

DEFINITIONS OF THE TABLE COLUMNS

OCT DIA - This is the measured string-to-string diameter between opposite shroud lines.

RAW SQ. - This is the size of the raw material (plastic, nylon etc.) you will need to make the corresponding diameter octagonal parachtue.  For this you need a square piece, you don't have to do any of that crazy rectangle stuff.

CORNER - This is important.  Imagine the octagon you are about to cut on the big square of plastic in front of you.  Imagine that the octagon sits on this square, essentially the same thing with the four corners clipped off.  Measure in from all four corners along the sides of the square and mark this dimension. When you are done you should have eight points marked, all of them on the edges of the square of material you have.  These marked points will be the eight points of the octagon.


SIDE LEN - This is the lenght of the sides of the octagon. All eight sides should be the exact same length.

SIDE DIA - If for some reason, you want to measure parachutes from the inscribed circle, use this diameter. It is the distance between the center of opposite edges (between the shroud attachment points).

 

Got that?  OK. Review the 'CORNER' description above and mark the material on the eight points.  That is where you will need to start cutting.  Go ahead and draw a line and then cut off the four corners of the original square with a ruler-straight cut.

So let's put it all together.

1) Obtain or cut an exact square sheet that is RAW SQ. from top to bottom and also across.

2) Mark the eight points of the octagon from the original corners using the CORNER dimension.

3) Draw a line & Cut the four corners away using those marks, you should end up with a nearly perfect octagon.  So go finish your parachute!

 

Here is the table, pick your size from any single line:

OCT DIA        RAW SQ.        CORNER        SIDE LEN        SIDE DIA

9.75                9.01                2.64               3.73                 9.01

11.7                10.81              3.17               4.48                 10.81

12                   11.09              3.25               4.59                 11.09

14                   12.94              3.79               5.36                 12.94

14.6                13.49              3.95               5.59                 13.49

16                   14.79              4.33               6.12                 14.79

18                   16.64              4.87               6.89                 16.64

19.5                18.02              5.28               7.46                 18.02

20                   18.48              5.42               7.65                 18.48

22                   20.33              5.96               8.42                 20.33

24                   22.18              6.50               9.18                 22.18

26                   24.03              7.04               9.95                 24.03

28                   25.88              7.58               10.71               25.88

29.2                 26.99             7.91               11.17               26.99

30                    27.73             8.12               11.48               27.73 

32                    29.57             8.67               12.24               29.57

34                    31.42             9.21               13.01               31.42

36                    33.27             9.75               13.77               33.27

That's all.  Happy Flying!

 

Contributed by Rich DeAngelis

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