# Parachute Question

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#### Dane Ronnow

##### Well-Known Member
I can light it up just setting it on an armrest if I’m not careful!
I've got a lot of that now. I appreciate your input. I've been talking to my doctor about this for a couple weeks now. Sounds like I should take this more seriously than I have been. Thanks.

#### Blast it Tom!

##### Well-Known Member
TRF Supporter
Fascinating thread, as I am just venturing into making a nylon elliptical chute after having the flat plastic chutes not open well last winter in the cold. Fired up Solidworks, made a simple parameteric model per Nakka's b/a ratio and had a pattern in just a bit. I sure hope I can retain access to the company's software after I retire!

I was wondering, for you 'chute experts here, has anyone considered a spool or spreader disk fixed to the shroud lines near their anchor? It could be one more thing to get tangled up, but done right I could see it keeping the lines far enough apart that whatever twisting might have occurred would not recur once the chute opened and untwisted.

#### shockie

##### High Plains Drifter
I'm just asking because I don't know but somebody explain to me why it's not the opposite:

"air passes through the weave, reducing the drag coefficient." wouldn't this create turbulence increasing the drag coefficient? I mean for example lets say I have 2 identical plastic parachutes:

1. typical plastic parachute

2. typical plastic parachute with dozens of pinholes across its surface area ( take a t-pin and just barely poke a hole with its tip, don't push all the way through)

wouldn't #2 have a higher drag coefficient than #1

if not explain why.

here's my reasoning.....smooth surfaces versus rough surfaces......we all know that rough surfaces have a higher drag coefficient than smooth but on the opposite side is porosity......

now lets go back to nylon ripstop. Nylon ripstop has a weave and there is porosity between the fibers in the weave. Calendared nylon is made by basically heat pressing or squishing the weave together so there is little to no porosity at all. Since there is no porosity no air can pass thru the weave. so it figures that calendared nylon has a lower drag coefficient than non-calendared nylon .

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#### Blast it Tom!

##### Well-Known Member
TRF Supporter
That gets into an interesting area, maybe more fluids and aerodynamics oriented folks might show up here. Air passing through the weave is acting like true holes, remember to most molecules any hole you'd make and could see would be like a gnat going through the Chunnel. As you increase the size of the hole, yeah, you get exit turbulence and all that, but a hole is a hole and the drag of the chute begins to decrease.

One of the odd fluids experiments they show you in school is where they used a rough patch on the leading face of a bowling ball falling through water. The rough patch actually decreased drag by forcing a transition from laminar to turbulent flow, which seems counter-intuitive. I've thought about that with regards to our rockets but probably the surface roughness needed to induce turbulence and reduced drag in air is probably so small as to be meaningless. But it's things like this that spark my curiosity.

#### Dane Ronnow

##### Well-Known Member
"air passes through the weave, reducing the drag coefficient." wouldn't this create turbulence increasing the drag coefficient?
It could very well create turbulence that might affect drag. But my point was toward a more fundamental aspect of fabric porosity—using two extremes as an example, plastic vs netting. The more porous the material, the faster the object suspended beneath the parachute will fall. Whether or not the drag created by turbulence of air passing through the fabric would be greater than, equal to, or less than drag created by reducing porosity is a question I cannot answer. My uninformed opinion based on the plastic vs netting example is that plastic would make a better parachute than netting would. Calendered ripstop is more like plastic; uncalendered ripstop is more like netting.

Not an expert here. Just trying to use a little logic.

#### shockie

##### High Plains Drifter

Dane do you know what Calendared-DWR means ? I found a document that says there is 3 types of 1.1 oz ripstop nylon, and that they each have x amount of Air permeability. It also states that all 3 are 0.003" thick which is 3 mil.......I guess that's why they call it "Thin-Mil"

silktique....that's a snazzy name

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#### Blast it Tom!

##### Well-Known Member
TRF Supporter
It's a good question. Did you ever see the one where the candle flame won't go through the screen?

#### Dane Ronnow

##### Well-Known Member
Dane do you know what Calendared-DWR means ?
Yes.
I found a document that says there is 3 types of 1.1 oz ripstop nylon, and that they each have x amount of Air permeability. It also states that all 3 are 0.003" thick which is 3 mil...

#### shockie

Yes.

Interesting. But for my purpose—which is simply sewing a parachute (actually not so simple as it turns out)—the only thing that concerns me is if the fabric I'm buying is 1.1 oz, and calendered. Now that I have the fabric, my concern is finding Gutermann Tera 80 thread that is white, not colored, and doesn't cost $18 to ship one spool. what does Calendared-DWR mean? I don't know. #### Dane Ronnow ##### Well-Known Member do you know if ripstopbytheroll.com has a minimum or requires you to sign up for an account? I ordered two colors—white and blaze orange—1 yard each. I signed up for an account only because I planned on shopping there in the future. Shipping was reasonable, about$5 IIRC, and arrived 8 days later (they're on the east coast; I'm in Las Vegas).

But it's Tera 60. Still, it's a good thread for a 70/10 needle. Oh, and it's white (Yay!).

#### Dane Ronnow

##### Well-Known Member
what does Calendared-DWR mean? I don't know.
Sorry, shockie. DWR is a coating of some type. I don't know which. All of the 1.1 oz. nylon at RBTR is DWR.

Regarding calendering, this is from RBTR: "Calendering is a process by which fabric is run through hot, heavy rollers at the mill. This flattens the fabric, seals the weave, and leaves one side of the fabric shiny while the other is matte/dull in appearance. This process is done to make the fabric downproof, increase wind resistance, and improve hand feel."

"Seals the weave" is the operative phrase here. After calendering, the fabric is not porous.

#### Dane Ronnow

##### Well-Known Member
My stepdaughter, who sews a lot, is visiting from Seattle. She tells me the best needles for ripstop are Microtex, which means they're ultra-sharp. So, if you use Singer needles, look for Singer Microtex. Schmetz would be Schmetz Microtex.

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#### Tractionengines

##### Well-Known Member
TRF Supporter
Ok here it is... Let me start off by saying I an NOT a parachute expert. So I am sure some will take issue with my terminology, and / or explanations... Please feel free to correct me, but do not attack the general nature of this post. This is to be very simple diagrams, with descriptions which are to help with the "BASIC" understandings of parachute recovery of Model Rockets. It is HIGHLY idealized. ( I am also TERRIBLE at spelling, so sorry for any errors. )

In the Sketches I have the letters "H" and "L" for high and low pressure areas. The bigger or smaller the letter is corresponds to amount of pressure.

Sketch_1 shows a ideal case. This is the a typical "flat" parachute. It also mostly applies to Parabolic or elliptical. etc. Any parachute with a large number of shroud lines connected to a circular domed parachute.

Sketch_2 shows the case where "something" has upset the parachute shown in Sketch_1. This shows how the "higher" side looses air around the bottom of the skirt. This causes loss of pressure differential across that side, and shifts the drag force offset from the applied weight. To get back to equilibrium (ie sketch_1) the parachute rotates clockwise from the moment due to the drag force versus weight force. Since the parachute is now rotating clockwise to balance the forces on it, and the mass (rocket) is moving left to get under the drag force; it "overshoots" equilibrium, and will continue until you have a mirror image of what is shown. With all the forces opposite, it swings back and the whole system cycles. The parachute tipping and bleeding air out on the high side, and the rocket becoming a pendulum.

Note the length of the shock cord has an effect on the pendulum cycles per minute... If you have a rocket that wants to swing wildly try changing the length of the shock cords. If you are at a resonant frequency (the parachute want to oscillate at the same rate as the pendulum want to swing) the oscillations can really build up. Likewise if you get them out of phase the pendulum motion can dampen the parachute and vise-versa.

Sketch_3 shows the parachute from Sketch_1 with a "spill hole" added. This allows some air to "bleed off" from inside the dome. It also sets up a flow of air along the inside surface of the parachute. This keeps the air from being lost from the edge as in Sketch_2. This helps the parachute descend in a more even consistent manner. Note that if the diameter of the spill hole is 20% the outside diameter of the parachute. The area of the hole is only 4% the area of the parachute. NOT much loss of drag for the gain in stability.

Sketch_4 shows a 4-line high ratio parachute. It is shown from a front and side view. The front view shows how the wedges of material hanging down to the shroud attachments help with force/velocity control. If the rate of descent is faster than equilibrium, the effective area of the parachute will increase from the force pushing OUT on the wedge of material; opening the parachute wider. This will help slow the descent. Then the side view shows how the tops of the scallops are significantly higher than the shroud attachment points. This allows the parachute to "bleed air" a the top of the scallops without rocking too far from vertical. Any small rock greatly increasing the spill area, causing lost lift on that side, and stabilizing the system.

Sketch_5 shows how the wind turns the flat sheet parachute into a sail... This causes the drift and long walks for recovery. A 4 line parachute will not tip as much in the same amount of wind. While it has more side area for the wind to push on... it has almost no area working as a sail "Catching" wind under the parachute. So the 4 line parachute will drift less than the flat parachute.

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#### Dane Ronnow

##### Well-Known Member
Ok here it is...
I don't know how long it took you to put all that together, but it is GREATLY appreciated. Clearest explanation I've ever read. And the illustrations nailed it. Thank you, sir!

#### rdrown

##### Retired and loving it.
TRF Supporter
The following has been my experience with making a 30" elliptical parachute, from finding fabric, to choosing a machine needle and thread, adjusting tension, etc. YMMV.

The material at my local Walmart is 1.7 oz ripstop, uncalendered. This is too heavy, and because it is not calendered, air passes through the weave, reducing the drag coefficient.

I get my nylon here:

Wide range of colors, 1.1 oz. Be sure and select 'calendered'. And unless you're buying a roll, select 'cut and folded' for shipping. Way cheaper.

Ripstop nylon is difficult to work with because it doesn't stay folded. Hemming an end, for example, you'll fold it over a quarter-inch, press it with your finger, then fold it over another quarter-inch and press it again. Then run a stitch down the center of the fold. Ironing the fold (mind the heat) isn't that much better.

I use a fabric glue pen (like a glue stick but narrow, and water soluble)—glue an edge, fold and press, glue it again, fold and press, then stitch. The glue washes out of the fabric. Just soak the finished parachute in lukewarm water for 10 or 15 minutes, then hang it up to dry.

Glue pen:

FWIW, I have two different presser feet for seams—a rolled-hem presser foot and a felling foot (for the flat felled seams that join the gores)—and neither of them work very well with 1.1 oz. ripstop. They're great for denim, cotton, etc., but not nylon. So I fold and glue, then stitch with the standard foot.

Sewing a flat feeled seam with a standard presser foot:

Next comes the needle and thread. Everyone in the sport-utility fabric-sewing universe says to use a 70/10 needle for sewing ripstop. That's a very thin needle, and works well for ripstop. The problem is that they also say to use a heavy nylon thread (upholstery thread). And while it's possible to thread a 70/10 needle with upholstery thread, it will NOT feed properly. You'll tear your hair out adjusting thread tension, trying to eliminate the inevitable looping bottom stitch, when all along it's not the tension at the top rollers or the bobbin that's causing the problem. It's the thread being too wide for the eyelet in the needle.

If you're going to use heavy nylon thread, use an 80/12 needle or possibly a 90/14. But be prepared for overly large holes in the fabric. If you're going to use a 70/10 needle, use a lighter weight thread.

Also, use a 'universal' needle, not a 'ball point'.

The spill hole (apex vent, according to the purists on these boards) keeps the parachute from oscillating on descent. If you've ever wondered why the rocket circles underneath the parachute, it's because the parachute wants to 'dump' air from beneath the canopy. So it tilts to release pressure, first to one side, then the other. This swinging motion causes everything that's suspended from the parachute to circle. The longer the shroud lines and shock cord are, the wider the circle will be.

Typically, the diameter of the spill hole is 20 percent of the total parachute diameter. This seems like a very large hole in the canopy, but it's just 3 percent of the total area.

Shroud line material is a matter of preference. Some use Kevlar. That's a little stiff for me, and because I'm wrapping the parachute in a Nomex blanket, I don't need the heatproof properties. I'm using this for a 30" parachute:

It's braided, pliable, but strong.

The shroud line attachment to the canopy is the most critical part of sewing a parachute. If these pull loose, you're toast. The following product page from Apogee has pictures of a 36" parachute. The fourth picture shows how the shroud line is attached. Note the loop, and the stitch going through the cord, not zigzagged along the sides.

Here are a couple of more sites with info on making a parachute:

Basics of making an elliptical parachute (from Fruity):

More detailed info on elliptical chute design (plus excellent diagrams on how the fabric folds for flat felled seams):

FWIW, Nakka uses seam-reinforcing tape on all of his seams. This would be important on a heavy rocket (3+ pounds) with a large diameter chute (maybe 40" and up). My 2.6" inch scratch-built, 40" long, weighs barely a pound. I don't need the extra weight from seam ribbon.

Finally, and quite apart from making a parachute is this:

I have a similar problem with my left hand. If it's numbness (pins-and-needles feeling) in the pinky and ring finger only, it's a condition called 'ulnar compression', in which the nerve that runs past the elbow comes out of the channel that is supposed to keep it in place. The result is a constant feeling of pins and needles in those two fingers, extending down across the palm. In my case, it also leads to cramping of the hand.

There's a sleeve you can wear that restricts movement of the elbow. But it's really hard to glue rocket parts together if you can't bend that elbow. And surgery has mixed results.

I just live with it.

All of the foregoing (both parachute making and hand numbness) is my experience only. YMMV.
I want to thank you for posting all this information. This is exactly what I have been looking for to make my own chutes.
This is a wealth of information. Thank you again.

#### Dane Ronnow

##### Well-Known Member
This is a wealth of information. Thank you again.
You're welcome. Here's another tip:

Because the calendaring/DWR process can give ripstop a slightly rubbery feel on the back side, it can drag under the sewing machine presser foot. They make teflon feet, but they're expensive. As luck would have it, one of the sales people at the Hobby Lobby on Fort Apache in Las Vegas is a seamstress. She told me that covering the bottom of the foot with a single piece of Scotch tape (the dull finish tape, not the shiney) will let the ripstop slide easily. Just make sure the tape is flat, with no rough edges. (Hat tip to Glenna.)

I put the tape on so it extends beyond the edges of the foot, press it tight, then trim the edges and all the little openings around the foot with the point of an X-Acto blade.

It works great.

#### Dane Ronnow

##### Well-Known Member
Also, I abandoned the washable fabric glue for folding seams. It was okay, but not great. Because parachutes don't need to be water tight like tents do, I'm using pins. But they have to be ultra thin and super sharp.

These are called 'dressmaker' pins:

#### Dane Ronnow

##### Well-Known Member
Regarding shroud lines, a lot of the lines I've seen that are between 3/32" and 1/8" wide are stiff. I found this stuff on Amazon. It's braided, smooth and very flexible. The 1.8mm is the perfect width for a 30" parachute on a light rocket (mine is 15.5 oz. loaded and ready to fly), and can be stitched with no problems. For a heavier rocket, go with 2mm.

#### mooffle

##### Well-Known Member
TRF Supporter
Any specific tips on how to attach the shroud lines to the nylon? Or pictures of what you've done would be helpful.
I am able to attach lines just fine but its all trial and error on methods. And they end up really ugly sometimes.

#### rdrown

##### Retired and loving it.
TRF Supporter
Any specific tips on how to attach the shroud lines to the nylon? Or pictures of what you've done would be helpful.
I am able to attach lines just fine but its all trial and error on methods. And they end up really ugly sometimes.

He has excellent photos of how to sew the shroud lines onto the seams.

#### shockie

##### High Plains Drifter
Regarding shroud lines, a lot of the lines I've seen that are between 3/32" and 1/8" wide are stiff. I found this stuff on Amazon. It's braided, smooth and very flexible. The 1.8mm is the perfect width for a 30" parachute on a light rocket (mine is 15.5 oz. loaded and ready to fly), and can be stitched with no problems. For a heavier rocket, go with 2mm.

Hey Dane. You have any info or resources that have to with printing ripstop nylon?

#### Sluggo

##### Well-Known Member
Hey Dane. You have any info or resources that have to with printing ripstop nylon?
Seek out a screen printer.

#### shockie

##### High Plains Drifter
Seek out a screen printer.
But I understand there are some specifics to printing on ripstop nylon. I've seen YT videos were they kinda heat press it.

#### Dane Ronnow

##### Well-Known Member
Hey Dane. You have any info or resources that have to with printing ripstop nylon?
I don't, other than to say look for screen printer in your area. I've worked with them before and if you can give them a design, they screen it on your ripstop. I'd ask them how heavy the ink will be when it's dry. Also, how flexible.