31.5" Semi-Ellipsoidal Parachute Build [Part IV]

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Dane Ronnow

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Thank you for your comments. Couldn’t have made it without your guidance. I have made about 12 more of different sizes and each one has a spill hole and each one comes straight down, no circling.
That spill hole makes all the difference in the world. I'd never make another chute without one. And you made 12 more? You're a lot more ambitious than I am!
 

Dane Ronnow

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Interesting question. As there are more launches at your end of the world, it would be easier to find out there.
I'm curious now. I'll start paying attention at launches, also going back through videos I've seen. It should be easy enough to see once it get closer to the ground (and I agree that it won't change direction short of being hit by a strong gust of wind).
 

Scrapmaster87

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That spill hole makes all the difference in the world. I'd never make another chute without one. And you made 12 more? You're a lot more ambitious than I am!
On my test setup I tested a few parasheets and found them all to severely oscillate. All of my eliptical and torridal chutes were very stable on the rig.
 

Andrew Brown

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Dan Ronnow, I really wanted to thank you for putting this together. After having tried in the past, and failed, I was easily able to put together some quality parachutes in the last week with your instructions and some practice. After buying supplies, I'm not sure I'm saving any money with the qty that I'm actually making (I'm scrapping a lot... I'm picky!), but the sense of satisfaction makes it worth it all. My stitching skills still stink, but getting better.

Regarding the rolled hem on the skirt, my preference would be to do that after stitching the gores together to have a continuous hem. Do you see any issues with that? Ideally, I'd like to embed a thin Kevlar line inside the rolled hem, but have not figured out how. My fingers are large and clumsy.
 

Dane Ronnow

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Dan Ronnow, I really wanted to thank you for putting this together. After having tried in the past, and failed, I was easily able to put together some quality parachutes in the last week with your instructions and some practice. After buying supplies, I'm not sure I'm saving any money with the qty that I'm actually making (I'm scrapping a lot... I'm picky!), but the sense of satisfaction makes it worth it all. My stitching skills still stink, but getting better.

Regarding the rolled hem on the skirt, my preference would be to do that after stitching the gores together to have a continuous hem. Do you see any issues with that? Ideally, I'd like to embed a thin Kevlar line inside the rolled hem, but have not figured out how. My fingers are large and clumsy.
First, you're very welcome. I'm glad this thread helped you. And you're right about the satisfaction outweighing the costs and time involved. It's really pretty cool watching your rocket come down underneath a parachute you made.

To answer your question, yes, you can roll the hem on the skirt after the gores are assembled. There's no disadvantage, at least that I'm aware of.

Regarding sewing a length of Kevlar inside that rolled hem, it isn't as hard as you might think. In fact, explaining it in text would be harder. So I broke out the sewing machine and some scrap nylon, and took pictures.

This is how I would do it [Edit] if I were doing this on my parachute, a 36-inch semi ellipsoidal. If you're using flat Kevlar, just make sure the seam is wide enough (width of the Kevlar, plus 1/8 inch on each side for the zigzag and final stitching). [/Edit]

I'm sure there are other methods, but this works, and, as I've said, it isn't that difficult.

Note that I'm using a scrap to demonstrate this, so you don't see any radial seams.

1. Start by cleaning up the raw edges of the skirt of loose threads. Ripstop frays pretty easily, and those threads can get in the way of what you're doing.

2. Flip the canopy inside out. You'll be working on the 'wrong' side (the shiny side, if it's calendered ripstop). Measure up from the raw edge the width of your finished seam. (In these pictures, it's 3/8 inch.) Using a straightedge, mark each individual gore. Don't cross the radial seam with the line.

I don't know if you've got a pen that uses disappearing ink, but they're the best thing for marking up a parachute. The ink disappears after 48 hrs. https://www.amazon.com/Dritz-Quilting-Mark-B-Gone-Marking-Purple/dp/B001UAKKYU/

IMG_0715.jpg

You're using this line for a reference while anchoring the Kevlar. It helps keep the Kevlar, and your stitch, as close to the center of the hem as possible.

This is a piece of 200 lb. Kevlar (about 1.2mm) in position.

IMG_0716.jpg

If you've got a rolled hem foot, it works well for feeding the cord as you sew—exactly like anchoring shroud lines. If not, pin the Kevlar to hold it in place.

3. Before you start stitching, grab a piece of scrap and set up a zigzag stitch that has about 8 stitches per inch, with the stitch width barely wider than your Kevlar.

Important! You're anchoring the Kevlar with a zigzag stitch that doesn't pierce the Kevlar, but locks it in place with stitches alongside. (You'll tear the nylon before you pull that Kevlar loose from the zigzag stitch.)

Piercing the Kevlar, now or when you finish the seam, will dull the needle in the space of about 5 stitches.

IMG_0717.jpg

4. Start the zigzag stitch in the center of a radial seam. Don't worry about back stitching—when you get back to this point, you'll just continue your zigzag past your starting point by about an inch.

Move slowly, watching the edge of the fabric underneath the foot, keeping it aligned as closely as possible. If you're using pins, pull them out the side right before they reach the foot.

When you reach the next radial seam, stop with the needle down, in the center of the seam, then lift the foot and turn the nylon slightly to sew the next gore. Don't try and make the turn while you're sewing.

When you reach your starting point, keep stitching for an inch past it to lock the stitch. Then lift the foot and cut the threads.

IMG_0718.jpg

[Edit] Depending on the size of your parachute, the following may be easier to do in sections (after you've laid the entire run of Kevlar). Managing a lap full of fabric with straight pins every couple of inches might get tedious. [/Edit]

5. You're going to fold and pin the hem, one gore at a time. Fold the edge over once, the width of your finished seam, and press along the fold with your finger to crease the fabric.

IMG_0719.jpg

Then fold it over again, crease it, then pin it.

IMG_0720.jpg

Follow the same process for the remaining gores.

6. If you changed out your presser foot for the rolled hem foot, put the presser foot back on, and set the machine for straight stitch, 10-12 per inch, zero width.

Now sew the upper stitch, keeping the stitch about 1/16 inch from the edge of the fold. Again, don't pierce the Kevlar!

When you reach your starting point, go past it about an inch, then back stitch to the beginning.

Note: I never had much luck back stitching ripstop. It always bunched up, even when pulling the fabric through. I stop the machine with the needle down, then lift the foot and turn the fabric 180 degrees, then stitch back.

IMG_0722.jpg

7. Now do the lower stitch, 1/16 inch from the fold, all the way around, then back stitch.

IMG_0723.jpg

And that's about it. If you iron the skirt, make sure the iron is on a nylon/silk setting. If the plate sizzles when you touch it with a wet finger, it's too hot.

I hope this helps. If you have questions, ask away!
 
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Andrew Brown

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Dane, way above and beyond for answering! Thank you so much. I think this will actually make the rolled hem a bit easier for me as that thin ripstop seems to have a mind of its own when trying to roll it.
 

Dane Ronnow

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[...] that thin ripstop seems to have a mind of its own when trying to roll it.
It does. When I made my parachutes, I did all of the rolling—folding, actually—and pinning on an ironing board. I'd use the tip of the iron to flatten the first and second folds on the hem. Creasing it with a finger is okay, but an iron is better. And even then, the fold doesn't really stay put. I used a lot of pins.

I just read through my reply above and realize I didn't address the issue of joining the ends of the Kevlar—closing the loop. I think the best way might be to run the end of the Kevlar past the beginning (alongside) for about an inch with the zigzag stitch, then widen the stitch to capture both strands together. But you'd want to keep those two side-by-side ends as close to each other as possible so you don't exceed the width of the seam. Otherwise, you're not going to have room for the closing top and bottom stitching along that short section where the Kevlar is doubled.

Also, I wouldn't cut the Kevlar to length before stitching it into the hem. I'd cut it with a foot or so extra, then trim it when you get back to the starting point.

And as you're sewing the Kevlar in place, pull the loose portion of it through your fingers every 6 inches or so, just to make sure it doesn't get anchored with a twist in it.
 

Andrew Brown

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Ok... back for more questions.... Having made a few and flown one, I'm pretty sure the kevlar thread around the skirt is not necessary, at least on smaller parachutes. Next round or questions:

  • Ripstop by the roll seems to be out of strategic colors for me in 1.1 mil calendared ripstop nylon (I coach a school aerospace club and working with kids to make parachutes. They have some color requirements). I'm really at a loss as to the other materials they offer. Too many choices can often be a hinderance. Are some of the other materials choices, like 1.1 Oz Silnylon or 1.0 Oz HyperD suitable? Most other materials appear to be either too heavy or too light.
  • This is more of a characterization question, but how do you characterize the Cd? For the last launch, I pretty much just fit OpenRocket data to measured data and came up with 1.6, ignoring the effect of the spill hole.
  • Number of gores? I've been making 24-30" parachutes and 8 gores seems to be a pretty nice balance of aesthetics, not having too many seams and lines, and approximating the ellipsoidal shape. The next step is probably a 72" parachute. 12 or 16 gores? Is there a rational method of determining, such as the amount of weight distributed on the shroud lines, or is it based mainly on aesthetics?
Sorry for all the questions, but I really am excited that I'm finally making my own. Thanks!
 

Dane Ronnow

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Ok... back for more questions.... Having made a few and flown one, I'm pretty sure the kevlar thread around the skirt is not necessary, at least on smaller parachutes.
I agree. I'm not sure it's necessary on larger parachutes either. A rolled hem effectively strengthens the skirt.

When it comes to the shock of deployment on the canopy, the weakest point in the skirt are the radial seams—specifically, the final stitching that locks the seam flat on the canopy. That stitch creates a perforation in the single layer of nylon adjacent to the seam. If the nylon is going to rip, that's where it will happen, most likely right up the edge of the seam. That's why I never use a stitch longer than 10 per inch. Anything tighter is going to aggravate the perforation problem.

[Edit] I'm striking the following after a post by @Kelly (#47 below), in which he pointed out the mistake in my reasoning of load stress on the canopy. I thank him.

The idea of a single length of Kevlar running around the circumference of the skirt to strengthen the radial seams is good in theory. But in reality, the strength in the skirt comes from adding gores, thereby spreading out the shock.

As an example of this, imagine a canopy with just two gores, under a deployment shock of 5 lbs. You've got two seams, each of which has two rows of stitching—one on each side. So, you end up with 5 lbs spread over four rows—1.25 lbs per row of stitching.

The same canopy with 12 gores has 24 rows of stitching, each of which has just .21 lbs of force.

That's what you want to keep in mind when you're deciding how many gores you want making up the canopy.


[/Edit]

Ripstop by the roll seems to be out of strategic colors for me in 1.1 mil calendared ripstop nylon (I coach a school aerospace club and working with kids to make parachutes. They have some color requirements). I'm really at a loss as to the other materials they offer. Too many choices can often be a hinderance. Are some of the other materials choices, like 1.1 Oz Silnylon or 1.0 Oz HyperD suitable? Most other materials appear to be either too heavy or too light.
I went on RBTR's web site this morning to see what's in stock, and found that blaze orange—which is shown to be out of stock in the popup menu (has an X through the color swatch)—is listed as in stock if you select it. I emailed them to ask if it's in or out. They replied that it is IN stock.

However, the other colors that are listed as out of stock in the popup menu are, in fact, out of stock.

Regarding the substitutes you mention (Silnylon, HyperD, etc.), Silnylon is actually 1.1 oz ripstop, coated on both sides with silicon/polyurethane, which makes it water resistant (non-breathable).

I've never used Silnylon, but from what I read on RBTR's web site, and four other web sites that deal with utility nylons, I'd say it is as good as ripstop. It's the same weight, durable, and comes in the same colors.

If it were my parachute, I would not be afraid to use it.

You might order just enough to make one parachute and see what you think. If you like it, order enough for the school club.

This is more of a characterization question, but how do you characterize the Cd? For the last launch, I pretty much just fit OpenRocket data to measured data and came up with 1.6, ignoring the effect of the spill hole.
In OpenRocket, I enter the parachute diameter (36" for mine), then Cd (1.50). A spill hole is not part of OR's calculation, so I ignore it (make no changes to the actual diameter to account for a spill hole).

The Cd for a flat-sheet (or parasheet) parachute is ~.75, regardless of size, or the presence of a spill hole (unless the spill hole is huge). The Cd for a hemispherical or semi-ellipsoidal (again, regardless etc.) is ~1.5.

In my first flights using this parachute on a 21 oz rocket (with motor), the FlightSketch altimeter recorded an average descent speed of 10 fps. OR's calculation had the descent speed at 9.7 fps.

That told me that OR is accurate in calculating parachute drag without any specs for a spill hole.

Number of gores? I've been making 24-30" parachutes and 8 gores seems to be a pretty nice balance of aesthetics, not having too many seams and lines, and approximating the ellipsoidal shape. The next step is probably a 72" parachute. 12 or 16 gores? Is there a rational method of determining, such as the amount of weight distributed on the shroud lines, or is it based mainly on aesthetics?
Aesthetics play only a small part in it. I made my 36" parachute with 12 gores because I liked the way it looked. But I could have made it with eight and it would work just as well.

More important than aesthetics, though, is the load distribution on the parachute at deployment—the more gores, the more seams; the more seams, the less load on each seam.

I haven't found a definitive source on the relationship between parachute size and the numbers of gores. And if you ask around here, you'll likely get a variety of responses.

However, if you were to shop the online suppliers of hemispherical or semi-ellipsoidal parachutes, and look at the specs (where they are available), you would get a pretty good idea of what would work.

For example, Fruity Chutes (a popular reference when it comes to parachute making among TRF members) uses eight gores on their ellipsoidal parachutes between 12" and 30"; 12 gores on ellipsoidals between 36" and 72"; 14 gores on their 84"; and 16 gores on their 96" and 120".

If I were building a 72" hemispherical or ellipsoidal parachute, I'd use 14 gores, not 12. But that's just me.

Other considerations are the weight and length of the shroud lines. I can't advise you on the weight, but as far as length is concerned, I use 1.5 x diameter for mine (I like them long). Fruity Chutes uses 1.15 x diameter.

I hope this helps. And don't worry about asking a lot of questions. I've got a lot of time on my hands. (And I used to be a teacher :) )
 
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Dugway

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Ripstop by the roll seems to be out of strategic colors for me in 1.1 mil calendared ripstop nylon (I coach a school aerospace club and working with kids to make parachutes. They have some color requirements). I'm really at a loss as to the other materials they offer. Too many choices can often be a hinderance. Are some of the other materials choices, like 1.1 Oz Silnylon or 1.0 Oz HyperD suitable? Most other materials appear to be either too heavy or too light.
I use their 1.1 oz Silnylon for all my parachutes, and it works great.
 

Andrew Brown

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Dugway, thanks. I did order some and tried it out. I agree it works well. I do find it a little more difficult to handle than standard calendared ripstop as it's so slick, but otherwise, it's good stuff.
 

Kelly

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As an example of this, imagine a canopy with just two gores, under a deployment shock of 5 lbs. You've got two seams, each of which has two rows of stitching—one on each side. So, you end up with 5 lbs spread over four rows—1.25 lbs per row of stitching.

The same canopy with 12 gores has 24 rows of stitching, each of which has just .21 lbs of force.
I'm having a hard time visualizing this. If we're talking about circumferential stress, then the gores are all in series. Each one experiences the total circumferential load - 5 lbs in this example. It's like forming a long rope by tying 12 short pieces together: Each knot is going to experience the same, total load on the rope, not 1/12 of the load.
Am I misunderstanding the situation?
 
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