


Joined Aug 2010
69 Posts

Discussion
Easy fsa and thrust tube calculator?
Can someone post on here a method to find the FSA of any given EDF and how to calculate the Thrust Tube diameter? Help please....
search engine here kinda sucks 




FSA is as easy as PI*r^2 of the fan (so, for a 70mm fan, r would be 35) minus PI*r^2 of the motor (70mm fan is usually around 28mm motor, so r would be 14).
The exhaust diameter is then usually around 85% FSA as a rule of thumb for general performance. (so take FSA *0.85, divide by PI, square root it to get r). Simples! 


United States, CA, Corona
Joined Feb 2006
7,469 Posts

C'mon, a dump truck driver doesn't know what Pi is?
I'm an accountant and I just barely remember what Pi is? 3.14 I think. OK, so in what order in the algebraic formula solved? Pretend it's Algebra 101 and list each step to solve the equation, that would be simple. 



Ok.
The FSA is the area of the fan itself, minus the area of the motor (because it blocks the air). The area of the fan is: 3.14*r*r where r is the radius of the fan (so a 70mm fan would be 35, a 90mm fan 45 etc. etc.) The area of the motor is, by the same logic, 3.14*r*r where, r is now the radius of the motor (so, a 70mm fan motor is typically 28mm, so r would be 14) If you then want an 85% FSA thrust tube, the exit diameter is then calculated as follows: You take the FSA that we just calculated and multiply it by 0.85. This then gives you the area of the exit you want. To find the diameter, divide this 85% FSA area by 3.14. Then square root it, which gives the radius. The exit diameter you want, is then simply double the radius. Make sense?? 


Joined Aug 2010
69 Posts

The EDF in question is a 90mm Midi Pro and ARC 3665. Thank you all for the replies!




Here you go, I just knocked up a quick spreadsheet to do it for you. Only type in the green boxes. (For 90mm fan, motor diameter is usually around 36mm).




Does the F in FSA refer to the fan rotor or the complete fan unit?
What happens if I have a rotor with negligible size hub, then a motor mount with a motor tube, and then further back in the air stream sits an outrunner motor. The outrunner bell is wider than the motor tube. Is the FSA then the rotor area minus the motor tube area, or minus the outrunner bell area? 



Awesome post. Thanks for doing that. I just figured mine and will probably make it out of a fountain drink cup.

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