Spill hole

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Well-Known Member
May 31, 2017
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So I have searched on this, but it mostly came back with LPR stuff. I have an EZI w/ the LOC 36" 'chute - empty weight 50 oz. It comes down about 16 ft/sec per calculations. That is too slow to me and I'd like to speed it up to 20-22 ft/sec. I've never cut a spill hole in a nylon chute - so I'm not sure how big to go and also if I need to sew it up around the cut or being rip stop nylon it will be ok as is. From calculation a 30" would get me where I want to be, but I know a 6" spill hole is not the same as a 30" chute. Also I don't want to go so big it causes opening issues. Thanks for any guidance!
I have herd of cutting a hole in a nylon chute using a soldering iron to stop the edges fraying. Never done it my self. Probably sew as well.

A word of caution. I am not an expert in this area!
50 ounces with a 36 inch chute, I get a sim rate of 22 ft/s. What Cd are you using for the chute? Parasheets only have a Cd of .75 or so.
All good questions... Especially the one asking how you got the 16 ft/sec number? If it was in a sim, then I would be very suspicious of the number unless you have a very accurate Cd for the chute.

With that aside, in actual flight does it drop too slow for you? If it does, I would consider a smaller chute rather than cutting up an existing chute.

Ripstop nylon will fray and tear. If you do want to cut a hole, I would do three things; use a hot knife to cut the nylon, use a Kevlar loop to size the spill hole, and sew a hem over the Kevlar. The problem with creating the spill hole after the fact is that you can't get enough material folded under to make a full hem. You have to split the material to get it to fold under. Those split points are week points. Stitching over helps, but a non-stretching Kevlar ring sown in the hem to take stress at the split points of your hem will make it much stronger.

As for sizing the spill hole, that's difficult because spill holes aren't really used to adjust decent rates. Most chute's Cd is calculate by using the inflated area of the chute and adding in some factors for shape and configuration. If your 36" chute is a flat octagon, the 36" may be measured from flat to flat, or point to point depending on manufacture. If it is a multi-gore chute, it is could be sewn with an inflated diameter of 36" or a flat (uninflated) diameter of 36", again depending on manufacture. With all of those variable, it's really hard to get a calculated Cd and a sim of chute decent speed. There are chute decent calculators out there, but most still require you to put in the Cd of your chute. Your best method for getting a decent speed is to measure it with a recording altimeter.

BTW, most spill holes are put in chutes to stop oscillation. It allows air escape from the top instead of under the edges which can induce oscillation and cause the rocket to swing back and forth as the air spills from one lower edge and then the other. This tends to make the rocket come down much faster then if there was no oscillation. Also, if the rocket happens to be swinging in the direction of travel when it hits the ground, the force of impact is much higher because the speed of the swing is added to the speed of the decent/wind. Spill holes in chutes are usually a good thing for stability, they just aren't really used to adjust decent rates.
Good points - the sim was probably not accurate as I used a preloaded cd and it was likely way too high. I feel like it drops a little slow but a chute release might be a better option because we do have concrete and grass at our site and don't want it to hit the concrete too hard.
Yes, the chute release works great and lets you use larger chutes. I've been using DD for a long time but when I got my first JLCR, it made flying my older motor eject rockets a lot more fun (bigger motors!).

Good luck.