parachutes: Size v. Spill hole

[graylensman]

Having had one too many rockets drift off into the rocket-eating forest that borders our flying field, a couple of questions comes to mind:

Is it better to use a larger chute with a spill hole in the middle, or just a smaller chute in order to reduce wind drift? Would the descent rate of a spill hole chute for a heavy rocket be high enough to cause concern for impact damage (snapped fins)?

 

[FredA]

Assuming net equal canopy area, I would opt for the smaller chute due the smaller cross-section it presents to the sideways push of the wind.

 

[heada]

There is an effective parachute size. If you have a parachute that is 18 inches with no spill hole, the effective size is about 18 inches. If you have a 24 inch parachute with a spill hole of 6 inches, the effective parachute size is about 18 inches (I don't know that for fact, just giving an example) The biggest benefit of a spill hole is a more controlled descent. (should sway much less)

-Aaron

 

[FredA]

Bzzz. wrong.....

Chute drag is area based.....(first order)

A 18" round chute has 255 square inches of canopy.
A 24" round chute has 452 square inches of canopy.
A 6" hole is only 28 square inches.
A 24" chute with a 6" hole has 424 square inches of canopy.

Area = pi*r*r

Choose the size of chute (area) needed for a safe, slow decent.
Minimize the cross section in the wind.
Skip the blow hole.....

 

[heada]

Bzzz. wrong.....

Chute drag is area based.....

A 18" round chute has 255 square inches of canopy.
A 24" round chute has 452 square inches of canopy.
A 6" hole is only 28 square inches.
A 24" chute with a 6" hole has 424 square inches of canopy.

Area = pi*r*r

Choose the size of chute (area) needed for a safe, slow decent.
Minimize the cross section in the wind.
Skip the blow hole.....

Yes, but a parachute with a spill hole does not have the same amount of drag as one without a spill hole as some of the air is escaping through the spill hole so you can't compare them directly on area. You have to compare them on effective drag. I'm sure there is a formula out there that will calculate the Cd of a parachute of various sizes, with and without a spill hole.

There is a measurable benefit to having a spill hole. The rocket will not swing back and forth as much as one without the spill hole. This swinging can cause damage on landing as well. Rockets that I've launched that have a spill hole have all come down straighter than those with a solid parachute.

-Aaron

 

[FredA]

Yes, but a parachute with a spill hole does not have the same amount of drag as one without a spill hole as some of the air is escaping through the spill hole so you can't compare them directly on area. You have to compare them on effective drag. I'm sure there is a formula out there that will calculate the Cd of a parachute of various sizes, with and without a spill hole.

There is a measurable benefit to having a spill hole. The rocket will not swing back and forth as much as one without the spill hole. This swinging can cause damage on landing as well. Rockets that I've launched that have a spill hole have all come down straighter than those with a solid parachute.

-Aaron

I thought the question was how to keep the rocket in the park....not swing control.
Again - the smaller side cross section the less wind driven drift.

 

[Larry_Pollack]

Do you use dual deploy? If you do already, and are getting too much drift under the drogue, you can try a streamer for a drogue.

X-form chutes are another possibility.

 

[jadebox]

Again - the smaller side cross section the less wind driven drift.

A parachute tends to move along with the wind. So, the only thing that really affects how much a parachute drifts is how long it's in the air. So, two 'chutes that keep the rocket in the air the same amount of time will drift the same distance (regardless of the side cross-sections of the 'chutes).

-- Roger

 

[Nathaniel]

A parachute tends to move along with the wind. So, the only thing that really affects how much a parachute drifts is how long it's in the air. So, two 'chutes that keep the rocket in the air the same amount of time will drift the same distance (regardless of the side cross-sections of the 'chutes).

-- Roger

Correct me if I'm wrong but... I thought the issue was that parachutes w/out spill holes "overflow" with air and when the air escapes out one side it causes the rocket to drift in the other direction. I thought the purpose of spill holes was to prevent the "overflowing."

I don't remember where I heard that, so it could easily be completely wrong...

 

[jadebox]

Correct me if I'm wrong but... I thought the issue was that parachutes w/out spill holes "overflow" with air and when the air escapes out one side it causes the rocket to drift in the other direction. I thought the purpose of spill holes was to prevent the "overflowing."

That's true, but the 'chute will rock back and forth and around - not just in one direction, so that alone won't affect how far it drifts. A spill hole does help stop the parachute from rocking and can be useful when you do want the rocket to come down faster. You can think of it like a paper boat floating down stream. It may rock back and forth, but it won't move against the current.

-- Roger

 

[Nathaniel]

That's true, but the 'chute will rock back and forth and around - not just in one direction, so that alone won't affect how far it drifts. A spill hole does help stop the parachute from rocking and can be useful when you do want the rocket to come down faster. You can think of it like a paper boat floating down stream. It may rock back and forth, but it won't move against the current.

-- Roger

OK. That makes sense. Now, why do X-Forms not drift as much?

 

[shockwaveriderz]

I think chutes vs chutes with spill holes (of varying sizes) have differing "drag forms"........

I would like to see some air and wind circulation diagrams around and within the chute vs chute w spill hole.. a chute with a spill hole might be more "drag-gy" than a chute

Just because the chute vs chute w spill hole may be equal in area, they have differing planforms... there is probably a "sweet spot" for spillhole sizes vs chute diameters.

all of this is pure speculation on my part.

terry dean
nar 16158

 

[jadebox]

OK. That makes sense. Now, why do X-Forms not drift as much?

Because they offer less drag and therfore the rocket comes down faster.

There has been a lot of discussion about how different shapes of 'chutes change the drift rate, but I seriously doubt that the shape makes any difference. I'm sure other's know more about the subject than I do, but I can't imagine any way for any non-controlled parachute to "swim upstream" which is what would be required for it avoid moving along at the same speed as the wind.

-- Roger

 

[Larry_Pollack]

I found some articles on the subject of parachutes.

see the last paragraph of the first link about spill holes and why you want them if your parachute oscillates.

Some "round" parachutes produce lift, and if made for it, forward speed. Skydivers used to use the Paracommander type of parachute, before ram air type parachutes were invented. A paracommander is pictured in the first link, it is the bottom right image.


https://ditc.missouri.edu/designTasks/parachute/howParaWork/howParWork2.html


https://ditc.missouri.edu/designTasks/parachute/howParaWork/howParWork3.html

 

[graylensman]

Thanks everyone for the insightful and stimulating discussion. Seems like some field trials are in order.

 

[tbzep]

Skip the blow hole.....

As you said, Bzzzz....wrong!

The spill hole has definite advantages for descent stability as others mentioned. I just wanted to do the bzzz thing.

Go for the smallest chute, like you mentioned, but go ahead and cut a spill hole. Like you said, it isn't going to affect decent rate much anyway so go ahead and use that small chute with the hole.

 

[polaris]

The short answerr is - the recommended size of the apex vent is 10% of the ideal surface.

For little rockets that use parasheets - 10% of the total surface area of the sheet.

For bigger rockets that use parachutes - it depends.

 

[jadebox]

Some "round" parachutes produce lift, and if made for it, forward speed. Skydivers used to use the Paracommander type of parachute, before ram air type parachutes were invented.

We can't really compare our parachutes to controlled ones. A person can figure out how to turn the parachute into the wind. Our parachutes aren't going to be able to do that.

Even if you used remote or computer control, I'm not sure how much you could translate a parachute's lift into motion against the wind to reduce drift.

-- Roger

 

[Nathaniel]

Because they offer less drag and therfore the rocket comes down faster.

-- Roger

So than why not just use a smaller normal parachute?

 

[cjl]

Because some people still think they have magical, drift-killing properties?

Actually, X-forms are easy to make, and they do have a different appearance when descending. Some people do like that about them. Don't select one thinking it will reduce drift without increasing landing speed though.

 

[jflis]

Jumpin' in with my 2 cents...

Go with the smallest parachute for *one* main reason only...

...'tis always easier to repair than to replace...

 

[Nathaniel]

Because some people still think they have magical, drift-killing properties?

Actually, X-forms are easy to make, and they do have a different appearance when descending. Some people do like that about them. Don't select one thinking it will reduce drift without increasing landing speed though.

I know the military uses X-forms for various things. Why is that? Is it because they're easy to make or something else?

 

[cjl]

I would bet that it's as simple as that - they are easy, cheap and simple. Note that the military tends to use hemis where it really counts and where the passenger can't steer

 

[Rocketeer]

I know the military uses X-forms for various things. Why is that? Is it because they're easy to make or something else?

The primary advantage to a cross chute is reduced opening shock, which is why they are often used for applications that involve opening at high speed.
In rocketry, if deployment is at apogee, then there isn’t any advantage to using a cross chute.

 

[RoyGreen]

Having had one too many rockets drift off into the rocket-eating forest that borders our flying field, a couple of questions comes to mind:

Is it better to use a larger chute with a spill hole in the middle, or just a smaller chute in order to reduce wind drift? Would the descent rate of a spill hole chute for a heavy rocket be high enough to cause concern for impact damage (snapped fins)?

Smaller chute.

A spill hole reduces the frontal area by only a tiny amount. It DOES make the chute more stable, and therefore may reduce the drift a slight amount, but nothing like substituting a 12" chute for an 18" chute!

 

[RoyGreen]

Bzzz. wrong.....

Chute drag is area based.....(first order)

A 18" round chute has 255 square inches of canopy.
A 24" round chute has 452 square inches of canopy.
A 6" hole is only 28 square inches.
A 24" chute with a 6" hole has 424 square inches of canopy.

Area = pi*r*r

Choose the size of chute (area) needed for a safe, slow decent.
Minimize the cross section in the wind.
Skip the blow hole.....

While you are correct, I wouldn't "skip the blow hole." Just recognize that it has a different purpose.

 

[MarkH]

Be a man. Use a streamer!

 

[Micromeister]

Heres a few Sq inch comparsions that should help us understanding why variuos Chute canopy forms drift as they do.

24" true Hemi = 904.78 Sq in.
24" Flat circular= 452.39 Sq in.
24" X-form = 320.00 Sq in.

Spill holes are, as someone already mentioned used to stabilize non-porous canopy fabrics like Plastics, Rip-stop and such. However Oversized spill holes do certainly increase decent rate. Tho I don't recall exactly why I settled on 3/4" spill holes for 10 to 24" and 1" for 26" to 48" chutes, I can tell you for sure they don't swing anymore regardless of the size or weight model that's under it.
X-forms by there nature tend to spin quickly during decent. While they don't swing as much that high rate of spin can sometimes be transmitted to the model, causing almost as much damage on landing as swinging.
I tried a center spill hole in an X-form. didn't really help much, but does get a few Odd looks LOL!