# How to calculate the size of the spin hole?

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#### Manoel Victor

##### Active Member
How to calculate the size of the spin hole?

Spin hole?

Apex hole

#### Manoel Victor

##### Active Member
apex vent sorryyy

#### neil_w

##### Hunkered down and slowly going crazy
TRF Supporter
"Spill hole"

I'm not sure what the formula is; there's certainly a lot of flexibility in how big it can be. Looking forward to see if someone has an analytical solution.

#### tfish

##### Well-Known Member
Manoel,

First, welcome to this site second, and to Rocketry

A lot of rocketry is based on researching, like you are doing now. I still spend hours a day researching various subject matters.

a great way to search this site is to do the following
in your search engine type in your question...spill hole sizing (followed by a {space} and add the following) site:www.rocketryforum.com

which will look like this spill hole sizing site:www.rocketryforum.com

which will bring up this https://www.google.com/search?q=spi...n0KHXvQDZwQrQIoBDAAegQIARAN&biw=2377&bih=1171

if you still have questions by all means start a new thread and ask questions.

Tony

#### Manoel Victor

##### Active Member
thanks so much tony! I'm reading a lot of about recovery, is very nice!

#### Voyager1

##### Well-Known Member
TRF Supporter
I'm definitely no authority on this subject, but my understanding is that there is probably no simple calculation for determining the optimal size of a spill hole. Changing the size of the spill hole changes the drag coefficient (increase the hole size; decrease the Cd), and would be a trade off between the descent velocity and stability. Typically, the spill hole would only be 1-10% of the total flat area of the parachute.

To determine the effect of spill hole size on Cd, you could perform drop tests on equivalent chutes with different hole sizes.

Hopefully, there will be someone on the forum that can give you a more definitive answer or, at least, point you in the right direction.

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#### mpitfield

##### Moderator
Staff member
Global Mod
From Fruity Chute's site "Our standard design comes with a spill hole that is 20% of the diameter of the canopy. It is just 3% of the area."

thanks!

#### Buddy Michaelson

##### Well-Known Member
Typically a spill hole is 10% the diameter or 1% the surface area. This number is tricky, too big and you loose drag, too small and the parachute will be unstable.

#### jrkennedy2

##### Well-Known Member
Typically a spill hole is 10% the diameter or 1% the surface area. This number is tricky, too big and you loose drag, too small and the parachute will be unstable.
Unless it is an annular chute then does the drag somewhat recover?

Randy

#### amiliv

##### Well-Known Member
Typically a spill hole is 10% the diameter or 1% the surface area. This number is tricky, too big and you loose drag, too small and the parachute will be unstable.
I've seen chutes with spill holes and considerably higher Cd than chutes with no spill hole at all. Examples are Fruity Chutes Iris (claimed Cd of 2.2) and BAMA Aero-spherical (claimed Cd of 2.0). Compared to Cd of 0.7 to 0.8 of average no spill hole hexagon chute. BAMA Tesla has a huge spill hole, and claimed Cd of 0.99. Fruity Chutes Elliptical claims Cd of 1.5 to 1.6. I'm not an expert, far from it. I know little to nothing about chute design. However, surely, there is much more that affects drag than the size of spill hole.

Above examples are from the two chute designers/manufacturers I bought chutes from in the past. I'm sure there are many more other examples.

P.S.
I know you make very fine chutes too, and I've seen them flown at several of local launches. It's just that I'm not as familiar with your chutes as with the other examples I mentioned.

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#### Manoel Victor

##### Active Member
I've seen chutes with spill holes and considerably higher Cd than chutes with no spill hole at all. Examples are Fruity Chutes Iris (claimed Cd of 2.2) and BAMA Aero-spherical (claimed Cd of 2.0). Compared to Cd of 0.7 to 0.8 of average no spill hole hexagon chute. BAMA Tesla has a huge spill hole, and claimed Cd of 0.99. Fruity Chutes Elliptical claims Cd of 1.5 to 1.6. I'm not an expert, far from it. I know little to nothing about chute design. However, surely, there is much more that affects drag than the size of spill hole.

Above examples are from the two chute designers/manufacturers I bought chutes from in the past. I'm sure there are many more other examples.

P.S.
I know you make very fine chutes too, and I've seen them flown at several of local launches. It's just that I'm not as familiar with your chutes as with the other examples I mentioned.
Thanks very much!