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Oct 29, 2012, 11:01 AM
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calculate rounded intake FSA from a square intake

Hi, about to start scratch buliding a jet for a 70mm fan. The jet has two square intakes on either side of the fuesalage and joins to make 1 towards middle of plane. How can I figure out if the 2 square intakes are big enough to get me to 100% fsa of the fan? Each well, i guess its a rectangle not a square intake is 1 inch wide by about 2 inches tall. So overall combined would be 2 x 2 inches.

Edit: Ok I have a 2nd question on fsa. To get 100% of the fsa, you take the inside diameter area and subtract the hub area to get the "open" area squared. So , I did this calculation and came up with about 3044mm squared. So taking the square root of that bring its out to roughly 55mm. That 55mm would be 100% i'm assuming. Now, on a previous build I was reading to do 85% fsa for the thrust tube exit. Now at the time, I didn't know the equations, so I just googled around for the answer and the consensus I got for a 70mm fan at 85% would be 56mm. Now, with the actual math I just did, that 56mm is actually bigger than 100% of my fsa. At 85% based on my numbers it should actually be 46mm. Is this correct, cause I'm super confused now.

Edit 2: Disregard my second question. Was doing the math wrong. You take 85% of the squared open area, then divide by 3.14, and then multiply that by 2.

Edit 3: Ok, trying to figure this out. My 100% FSA is 3044mm squared. So taking my 2x2in combined intake, converted to mm, 50mm x 50mm would be 2500mm squared. So I take it, unless I'm doing something wrong here, my intake is 82% of the FSA of the fan which is in fact too small. Doing some more calculations, if I increase my total intake size to 55mm x 55mm , will give me 3025mm squared, which will be just a tad under the 100%
Last edited by stingeragents; Oct 29, 2012 at 11:49 AM.
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Oct 29, 2012, 01:28 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
You need to read this!
Oct 29, 2012, 03:07 PM
Registered User
Cool thanks. Yea as soon as I posted my frustration I found the answer that I was doing the math wrong. I think my math is correct for figuring out the square area of the intakes to where it is what it needs to be so basically a pointless thread I guess.
Oct 29, 2012, 04:08 PM
Do it Right, the first time!
CoolerByTheLake's Avatar
Perhaps, just a little over thinking? That fan calc in post #9 is a handy tool.

I know you are using a 70mm, but in same post, it was said " (For 90mm fan, motor diameter is usually around 36mm)". A great many are 39mm for the 90.

Good luck on your project. If you want to have some real fun, do a build where you have to do conversions from different scales!!!
Oct 29, 2012, 06:59 PM
Registered User
I have no clue how to scale so thats out. Lol

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