Parachute spill holes?

[eggplant]

Simply put, what are the pros and cons of parachute spill holes for a MPR?

 

[Commonwealth.Net]

A spill hole will reduce the effective size of the chute.
The addition of the hole does reduce "cupping".
You may have noticed this when your chute tips up on one
end, the air captured spills out and the rocket drops faster for
a second or two until the cute fill back up. This happens
often and depends on the construction of the chute (shroud lines)
and other factors.

In general a spill hole is a plus, our 66 inch chutes have a 4 inch spill
hole. On smaller chutes you may have to create the hole yourself.

If you have a lot of cupping action the spill hole may actually increase
performance and provide a longer decient time.

 

[jef955]

For the most part they come down a little straighter and slightly faster without drifting quite as much. Biggest I have flown is an E-20. I use them on basically everything I have that uses a parachute.

 

[georgegassaway]

For the most part they come down a little straighter and slightly faster without drifting quite as much.

I think it should be pointed out that the not drifting as much is totally proportional to how much faster it comes down (since putting a spill hole in the chute reduces the chute area and makes the chute descend faster). If it is in the air for 75% as long as it would take to come down without a spill hole, then it will drift 75% as far.

That is probably along the lines of what you meant, but some may take it to mean something different.

Some think there is such a thing as certain chute shapes (like "X" parachutes), or chute mods like spill holes, that "reduce how the wind grabs the chute", which is not true.

If a model is in the air for 60 seconds, it is going to drift the same distance downwind as it would regardless of whatever recovery system it is using (the only exception being if it glides).

This assumes the wind is at a perfectly constant speed, and no updrafts or downdrafts (all of which are a bad assumption). So, to drift less than it would travel downwind in 60 seconds in the air, make it land sooner. And/or wait for the wind to drop some (except that sometimes the wind drops because a thermal is on the way, and if the model gets caught in a thermal it can drift a lot farther, or... away).

- George Gassaway

 

[cjl]

Yep. It is true however that a spill hole dramatically reduces rocking while under chute, and in general makes a model descend in a more stable fashion. I pretty much always use one.

 

[Micromeister]

Ditto Cjl:
With the exception of Competition chutes as well as 3" & 4" micro chutes I use a spill hole of at least 3/4" in all Plastic and RS nylon chutes including X-forms, I experiments with x-forms see if adding a spill hole would help stablize spinning X-forms during decent, at least on the one's I've made it certainly did.

 

[o1d_dude]

Thanks for posting those pictures, Micro.

I'd been wondering about stitching around the spill hole as all my chutes are of the Topflite/LOC/Madcow variety. Now I know.

 

[Rocket_Man]

for us guys lanching paper rockets (estes) you need a big spill hole. launching E engines iam using no larger than 12 inch chutes with 8 inch holes in them. course i live in texas and the wind will actually pick the rocket up and sail it if a large spill hols isnt cut in it.

dont go looking for that authentic parachute landing, i just use what is necessary to get the rocket down without damage.

 

[vdotmatrix]

the chute(s) I made with the spill holes performed better than expected. Beautiful recoveries using this program to make these chutes.



I bult a 24" chute with a 2" spill hole and I haven't even used it yet and I am nervous. IS there any rule of thumb for the size spill hole I make?

https://www.rocketryforum.com/showpost.php?p=182392&postcount=1

 

[cjl]

That one is on the smaller side, so my guess is that it won't impact the descent rate much compared to a solid 24 incher. It will make the descent more stable though.

 

[jazzviper1]

The apex vents in round parachutes are generally there to reduce oscilation ( the pendulem or circular swinging of the load). These oscilations cause the chute to drop faster durring the more extreem angles. This is caused by air randomly spilling out from under the skirt, or random glide ( the tendancy of the chute wich is a crude airfoil to develop lift in random directions). The first solution was to cut a vent in the apex to allow excess air to escape thus "reducing" oscilations. In the early days of skydiving jumpers took advantage of the random glide characteristics of round chutes by cutting vents in the back. This gave the chute direction and some propultion and stearability. Using vents to increase decent rates is not the most efficient method. Using a smaller chute is the best way, "reefing" (temporrarily restricting the open diamiter of the can opy be tying off the lines) the chute is the next best way. Oversize vents reduce the canopy structural integrity and and the chutes reliability.

I live in Texas and I can't immagine a 12 inch chute with a 8 inch vent that leaves only a 2 inch wide ring of canopy?

 

[Rocket_Man]

here it is in action:

[YOUTUBE]btWybsC9QbU[/YOUTUBE]

 

[vdotmatrix]

here it is in action:

[YOUTUBE]btWybsC9QbU[/YOUTUBE]

Very nice...no oscillations....

 

[Rocket_Man]

Well I lost every 2 stage CC Express I built because they literally drifted away
by the wind. I was going to go the streamer route but couldnt settle on the material / length of the streamer. Plus it was hard to pack the streamer length into the rocket. Thats when I thought of the "drouge chute" concept. Its easy to pack and if you live in windy area like I do I think its the only way to go. Iam modifying these paper rockets for E9 engines and as they go a long way up they have a long way to come down. I havent lost a rocket since using chutes like these. Its nice to bring em home time after time. Plus the 12 inch chutes i get at Hobby Lobby have that cool checker board pattern on them.

 

[Micromeister]

Well I lost every 2 stage CC Express I built because they literally drifted away
by the wind. I was going to go the streamer route but couldnt settle on the material / length of the streamer. Plus it was hard to pack the streamer length into the rocket. Thats when I thought of the "drouge chute" concept. Its easy to pack and if you live in windy area like I do I think its the only way to go. Iam modifying these paper rockets for E9 engines and as they go a long way up they have a long way to come down. I havent lost a rocket since using chutes like these. Its nice to bring em home time after time. Plus the 12 inch chutes i get at Hobby Lobby have that cool checker board pattern on them.

While it is quite true spill holes can vary greatly depending on the mass of the model it is lowering.
Once you've cut that hole there is no going back. What I've done over the years is to collect a number of extra estes plastic chutes in various sizes. These have been altered with different size spill holes for different purposes. I have a 12" chute with a 10" hole used in parachute spot landing. Works more like a drag racer chute then a parachute but it works well for spot landing with light models. I keep the baggie of different spill hole chutes in my range box, each in its marked bag so I can quickly see what size hole to choose for the job.
I also have another baggie of various length, width & material streamers pre-made with snap swivel attachment leads that can be swapped out in an instant. Sure makes it easy to fly the field and weather conditions.

 

[Kinesthesia]

I've been thinking about this quite a bit, so I downloaded the free trial of RockSim and grabbed a sim file for my Big Bertha. You can edit the parachute and add a spill hole, adjust the size and watch the descent rate change. I have to admit the change is less significant than I expected. In the RockSim file that I have, it shows the descent rate of a stock Big Bertha to be about 11 ft/sec. It takes a 6.5 inch hole to increase that to 12 ft/sec.

I can definitely see how the key is the weight of the rocket more than anything else. Though I'm sure a physicist would point out the additional drag of a longer or fatter rocket and other factors ...

Anyone know roughly ... how much descent rate is too much for a paper tube / balsa finned rocket to survive ? Again, I realize there are a million variables ... the surface you land on, thickness of fins, etc ... I'm just saying ballpark ...

 

[vdotmatrix]

2" hole made me nervous so I make all my 8 gore chutes with a 1" hole that in the end is a little better and performs very well

 

[AlfaBrewer]

on a related note, Openrocket doesn't recognize spill holes. How would I adjust the size of my parachute in a sim to account for a spillhole?

Example - 10" parachute with a 2" spillhole ~ 8" chute???

or does it not translate?

 

[cjl]

No - it's much less significant than that. 10 inch chute with a 2 inch spillhole will probably descend about the same as a ten inch chute alone as far as speed is concerned, it'll just come down straighter and more stably.

 

[bobkrech]

Attached is a plot on how the parachute area is reduced by a spill hole as a function of the ratio of Spill Hole I.D. versus Parachute O.D.

It's not very large even for what seems to be a big hole.

Bob

 

[scsager]

on a related note, Openrocket doesn't recognize spill holes. How would I adjust the size of my parachute in a sim to account for a spillhole?

Example - 10" parachute with a 2" spillhole ~ 8" chute???

or does it not translate?

Here's what I did...

I have a PLM Explorer with the stock 30" chute w/spill hole. I have on-board video of a flight which I used as a "real-life" example.

I adjusted the CD to 0.55. and left the chute size at 30" in OpenRocket. The Idea is to make the sim decent time match the real (observed) decent time.

Changing the CD from 0.75 to 0.55 shaved about 3 seconds off the decent from 150 meters (a low/slow flight) and the sim seems to accurately reflect what happens in the video.

 

[AlfaBrewer]

Bob, Scott, CJL - Thanks. Fluid Dynamics wasn't my best subject in school, and pictures always help.

 

[luke strawwalker]

on a related note, Openrocket doesn't recognize spill holes. How would I adjust the size of my parachute in a sim to account for a spillhole?

Example - 10" parachute with a 2" spillhole ~ 8" chute???

or does it not translate?

Subtract the area of the spill hole from the total area of the parachute, then figure out what diameter chute gives you that area...

For instance, for a circular chute, Area=Pi times Radius squared. Say you have a spill hole 2 inches in diameter (1 inch radius), which gives you 3.14 times 1 times 1, which is 3.14 square inches. Say your parachute is 12 inches in diameter, (6 inch radius), which gives you 3.14 times 6 (squared) which is 113.4 square inches of surface area of the chute. Subtract the 3.14 inch spill hole area from the 113.4 inch total area of the chute, gives you 110.26 inches of area for the chute with the spill hole. Reversing the equation to get back to the chute radius, we divide the 110.26 square inches of surface area by Pi (3.14) to get radius SQUARED, which is 35.115 (rounded off) and then we take the square root of that to get back to the radius, which is 5.926 inches (rounded off).

Checking the math, 5.926 (squared) times 3.14 (Pi) equals 110.268 square inches of area.

SO, a chute 11.852 inches in diameter with NO spill hole has the same surface area as a chute 12 inches in diameter with a 2 inch spill hole. Make sense??

OF course, this is ignoring aero effects, which is MUCH more complicated and above my pay grade... LOL Basically, a spill hole is venting into the low pressure area behind the chute, which increases the descent rate by lowering drag. There are aerodynamic laws which define the airflow, turbulence, drag, etc. which are very complicated, and create intricate differences in the flow characteristics of the fluid the chute is moving through (the atmosphere) compared to a chute with NO spill hole, which is NOT addressed by merely adapting the surface area of the chute without a spill hole to match that of a chute with a spill hole... but of course, a lot of computer models don't really account for all these complex and intricate differences in aerodynamics anyway; at least adjusting the surface area of the chute downward by the amount of the spillhole area will get you a CLOSER estimate of performance out of these simplistic computer models than simply ignoring the area of the spill hole altogether...

Hope this helps! OL JR

 

[luke strawwalker]

Attached is a plot on how the parachute area is reduced by a spill hole as a function of the ratio of Spill Hole I.D. versus Parachute O.D.

It's not very large even for what seems to be a big hole.

Bob

VERY handy... that should be a "sticky"...

Later! OL JR

 

[Kinesthesia]

I took the 18" chute that came with my Big Bertha and snipped a bit of the tip off, leaving a very unscientific hole about 3" in diameter. As RockSim predicted I didn't see a big difference in descent rate. However, as has been stated here, it is a much smoother descent even in relatively high winds (within the range of what I would launch in). I will probably continue to increase the size of the hole and see how it goes.

 

[Brodon]

I've been thinking about this quite a bit, so I downloaded the free trial of RockSim and grabbed a sim file for my Big Bertha. You can edit the parachute and add a spill hole, adjust the size and watch the descent rate change. I have to admit the change is less significant than I expected. In the RockSim file that I have, it shows the descent rate of a stock Big Bertha to be about 11 ft/sec. It takes a 6.5 inch hole to increase that to 12 ft/sec.


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