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How to choose and 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!
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.
One of my home-made octal parachutes in use on the Stonebreaker.
Most manufacturers of parachutes define the diameter of the parachute from the corners of the parachute where the attachment points are (I call this the "Hex diameter" or "Octal diameter") . This dimension makes the parachute seem "bigger". Others measure the parachute from the straight sides of the parachute, which defines the actual circle inscribed inside the material, and consider the excess corners as just part of the attachment points (I call this the "side diameter"). Either way is valid, but for me I use the actual corner-to-corner dimension (Hex or Octal diameter) where the shroud lines attach.
* 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 (at points half-way 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.
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 (at points half-way 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
Other notes on parachute construction
You have likely read somewhere that the shroud length should be at least equal to the diameter of the parachute. I noticed that almost every parachute manufacturer supplies only enough string to make the minimum length of shroud lines. I have also found out that my parachutes work better when I increase the length to 1-1/2 times the length of the diameter, so for a 10" parachute, I use 15" of shroud lines. It allows the parachute to float further away from the disturbing effects of the payload's wake and holds the shape better. Give it a try!Typically, when the shroud lines are attached to the parachute material, binder re-inforcement rings are used. I have found that they are not very strong, and dry out after a few years. Thinner string often used for shroud lines can easily cut through the paper ring. Instead I use a small square of electrical tape (I have it in various colors to match the parachutes, but black works well enough). The vinyl tape is much more resistant to being cut through with a shroud line string.
The normal method of creating shroud lines is to measure twice the length of the desired shroud line, and form three loops (or four if making octagonal). This is a great way to save time and not have to attach all the shroud lines together. The loops are just passed through the fishing swivel clip (or nosecone attachment point). This is a great way to connect them, but I found out that sometimes individual loops can slip, making one shroud line too long and the other too short. This makes for a lop-sided canopy and a more rapid descent. Apply a dab of glue to the loop where the shroud lines meet, not a lot, and let it soak into the fibers. This will hold the loops of the shroud lines and prevent any loop from slipping.
What size parachute should I use?
Choosing the size seems to be more of an art than science. It all depends. First, determine how fast you want the rocket to return. Delicate rockets with thin antennas or small protruding fins should fall slow, maybe 6-7 mph. This also goes for rockets landing on hard rock or packed earth. Tougher rockets or rockets landing on tall grass can fall a bit faster, maybe 8-9 mph. Very high fliers, and rockets that fly high in windy conditions need to fall faster. I always believed it is better to get a rocket back with some damage then to have it completely disappear or watch it swaying from the top of a hundred foot tall tree.To get a good handle on the return speed and parachute size, you need to know the weight of the rocket. Postal scales might be good, but I found a very in-expensive digital jewler's scale that is very precise. You can use ounces for measurement, but I found grams to be more precise and doesn't require a lot of fractional numbers. One ounce is roughly 28 grams. To convert from oz. to grams, multiply the oz by the number 28.3495
The chart below can be used to determine the parachute size based on the weight of the rocket and the speed you want it to return. I got this info from a reputable website, but I don't recall where. It seems to be reasonably accurate. Rember these are only guidelines, you may need to adjust the size depending on the particular rocket and the field where you fly.
Slow Return (about 6-8 mph):
Grams Diameter (inches)10 7
20 10
30 12
40 14
50 15
60 16
70 17
80 19
90 20
100 22
110 23
120 24
130 25
140 26
150 26
160 27
170 28
180 29
190 30
200 31
250 34
300 37
350 41
400 43
450 46
500 48
600 53
700 57
800 61
900 65
1000 69
Medium Return (about 8-10 mph):
Grams Diameter (inches)10 5
20 7
30 9
40 10
50 11
60 13
70 14
80 15
90 15
100 16
110 17
120 18
130 19
140 19
150 20
160 20
170 21
180 22
190 22
200 23
250 26
300 28
350 30
400 32
450 34
500 36
600 40
700 43
800 46
900 49
1000 51
Fast Return (about 12-14 mph):
Grams Diameter (inches)10 4
20 5
30 6
40 7
50 7
60 8
70 9
80 10
90 10
100 11
110 11
120 12
130 12
140 13
150 13
160 14
170 14
180 15
190 15
200 16
250 17
300 19
350 20
400 22
450 23
500 24
600 26
700 29
800 31
900 32
1000 34
That's all. Happy Flying, and have a safe recovery!
Wow! That seems incredible complete! Who knew that a simple mylar or plastic chute could be so complicated!
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