How to make an octagon for a parachute.

[Sooner Boomer]

An ideal parachute would be a perfect hemisphere. This requires cutting and attaching a series of curved gores. It's a lot easier (and cheaper) to make a flat parachute. and just add more shroud lines onto a regular polygon. An octagon (8 points) is just slightly harder to make than a hexagon (6 points).

 

[Sooner Boomer]

Again, all you need is something for the form, a marker, a straight edge, and a compass. You need a square piece of material for the form. If your original material is not square, you will end up with an octagon with uneven sides. I drew this in MS Paint, and did not start with a square. You'll see the results...



Draw a line from corner to corner.

 

[Sooner Boomer]

Measure the distance from the center to one edge (X). Set your compass to this distance.

 

[Sooner Boomer]

With your compass at the center, mark each diagonal this distance.

 

[Sooner Boomer]

At each mark on the diagonal line, draw a line at 90 degrees. The closer you are to 90 degrees, the more even the octagon will be.

 

[Sooner Boomer]

Cut off the corners you've just marked. If you were careful, you should end up with a perfect octagon. Attach shroud lines at the corners. (if you were off, or not square, you'll end up with something like mine. It's not perfect, but it will still work).

 

[BABAR]

Alternative way of contructing perfect octagons.
Note you have to start with a perfect square
You do lose a bit of material compared with Sooner.Boomer's method, though

For smaller chutes, it may be easier to do this with a piece of paper and trace the design onto your chute material.

 

[shockie]

Using either method, could a 16 sided parachute be made? If you did the fold to 1/16?

 

[jazzviper1]

An ideal parachute would be a perfect hemisphere. This requires cutting and attaching a series of curved gores. It's a lot easier (and cheaper) to make a flat parachute. and just add more shroud lines onto a regular polygon. An octagon (8 points) is just slightly harder to make than a hexagon (6 points).

Hemispherical chutes are far from ideal. The decent rate of a hemispherical chute is considerably faster than a flat chute or conical chute using the same amount of material.

 

[BABAR]

Hemispherical chutes are far from ideal. The decent rate of a hemispherical chute is considerably faster than a flat chute or conical chute using the same amount of material.

Hmmm, I thought the opposite was true. But maybe I am misunderstanding your statement. Definitely takes more material to MAKE a spherical chute, and you have to cut the gores (segments) individually and so a certain amount of material is discarded.

from Richard Nakka (I don’t know him but for what I have heard about him he’s kind of a legend in Rocketry)

https://www.nakka-rocketry.net/paracon.html
The design that is presented here is of a true parachute, having a "shaped" canopy, as opposed to what is referred to as a parasheet. A parasheet has a canopy that is flat when not inflated, and may be cut from a single piece of fabric. When a parasheet inflates, the canopy material is "gathered" by the shroud lines, forming an approximately hemispherical shape. A parachute with a shaped canopy is more efficient than a parasheet, since less fabric is required to produce the inflated shape.

also from. Randy Culp (Tripoli #6926)

http://www.rocketmime.com/rockets/descent.html

• Cd is the drag coefficient of the chute, which is 0.75 for a parasheet (flat sheet used for a parachute, like Estes rockets), or 1.5 for a parachute (true dome-shaped chute).

rest of this is my own understanding.

downside of hemispherical chutes is they are more difficult to make than parasheets. For low power, parasheets are quite adequate. The more corners the more efficient, but the difference between 6 and 8 is not much. I just find using the fold and cut method for 8 sides easier than 6, and adding 2 more shroud lines isn’t much extra work but provides roughly 33% lower load per shroud line, which Can’t hurt.

I am curious if anyone knows the history of why a six sided chute became the standard For kits. I am guessing because it looks and works better than 4, and it uses fewer shroud lines than 8 and that cuts down cost, maybe @hcmbanjo or @jadebox can chime in here.

 

[Banzai88]

I suspect that the hex was standardized purely for manufacturing purposes, when you cut hexagons from an industrial sized roll of plastic sheet on an industrial level.....there is only wasted material on the very edges of the roll that they're cut from.

 

[jqavins]

Here's a pattern for what the math says will be the shape of each gore for an eight gore hemispherical. The numbers are percent of the finished diameter. Of course, material must be added on the right, top, and bottom for stitching, as this is just the flattened eighth of the hemisphere.

 

[jazzviper1]

Hmmm, I thought the opposite was true. But maybe I am misunderstanding your statement. Definitely takes more material to MAKE a spherical chute, and you have to cut the gores (segments) individually and so a certain amount of material is discarded.

from Richard Nakka (I don’t know him but for what I have heard about him he’s kind of a legend in Rocketry)

https://www.nakka-rocketry.net/paracon.html
The design that is presented here is of a true parachute, having a "shaped" canopy, as opposed to what is referred to as a parasheet. A parasheet has a canopy that is flat when not inflated, and may be cut from a single piece of fabric. When a parasheet inflates, the canopy material is "gathered" by the shroud lines, forming an approximately hemispherical shape. A parachute with a shaped canopy is more efficient than a parasheet, since less fabric is required to produce the inflated shape.

also from. Randy Culp (Tripoli #6926)

http://www.rocketmime.com/rockets/descent.html

• Cd is the drag coefficient of the chute, which is 0.75 for a parasheet (flat sheet used for a parachute, like Estes rockets), or 1.5 for a parachute (true dome-shaped chute).

rest of this is my own understanding.

downside of hemispherical chutes is they are more difficult to make than parasheets. For low power, parasheets are quite adequate. The more corners the more efficient, but the difference between 6 and 8 is not much. I just find using the fold and cut method for 8 sides easier than 6, and adding 2 more shroud lines isn’t much extra work but provides roughly 33% lower load per shroud line, which Can’t hurt.

I am curious if anyone knows the history of why a six sided chute became the standard For kits. I am guessing because it looks and works better than 4, and it uses fewer shroud lines than 8 and that cuts down cost, maybe @hcmbanjo or @jadebox can chime in here.

Hemispherical parachutes are more aerodynamic than flat of conical chutes (conicals having the best drag coefficient of non-lifting round chutes). There is a lot of confusion caused by people using two different measuring methods for the diameter of the chute. One method is to measure the flat diameter of the canopy. This means laying the chute out flat and measuring the diameter that way or measuring from skirt to apex and multiplying time two. Non flat chutes can be measured using the second method, this will give the equivalent flat diameter. Drag coefficients measured this way for non-lifting parachutes are always less than one. The other way is to use the flying diameter of the chute. This means inflating the chute and measuring across the open mouth of the canopy or using a mathematic formula to find the flying diameter. Drag coefficients calculated using the flying diameter are always over one. Whichever you use doesn't matter because a 3-foot flat parachute is going to have a slower decent rate than a 3-foot hemispherical parachute as long as you use the measuring system for both. There was an individual who was using misleading advertising by using one method of measuring for flat chutes which he did not sell and hemispherical chute which he did sell. That is where some of this confusion comes from. The idea that a "real parachute" cannot be laid out flat is a fallacy also. For over 60 years the most common type of round personnel parachutes were the 28-foot flat circular and the 24-foot flat circular. Both are still in use today. The term "parasheet" is made up, the work parachute is French Para (shield or protect), Chute (fall). The half sphere shape of the hemispherical chute allows air to flow around it more smoothly leaving a smaller low-pressure zone behind it resulting in less drag than a flat chute. I have a chart that I will try to post later from "The Parachute Manual" it may help to clarify what I am talking about. Also shaped parachutes of any type are weaker than flat parachutes, during opening shock the canopy tries to flatten out, shaped chutes can't and so tend to blow out or rip.

 

[jqavins]

Here's a pattern for what the math says will be the shape of each gore for an eight gore hemispherical. The numbers are percent of the finished diameter. Of course, material must be added on the right, top, and bottom for stitching, as this is just the flattened eighth of the hemisphere.
View attachment 533119

WAIT! I messed up. The X axis is in the wrong units. That's degrees of declination form the top of the hemisphere; the length along X should only be π/4, 78.5%. I'll fix it.

OK, it's fixed.