Jul 19, 2011, 09:57 PM Registered User Joined Aug 2010 69 Posts 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
 Jul 19, 2011, 10:04 PM Designer/Scratch Builder St. Louis Intl, Missouri, United States Joined Jan 1997 6,588 Posts What fan do you have?
 Jul 20, 2011, 11:00 AM You win again gravity! UK Joined Jan 2010 1,179 Posts 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!
 Jul 20, 2011, 11:38 AM Man, this stuff is so much fun Anaheim, California Joined Feb 2006 7,138 Posts Leave it to a UK guy to make it simple. How about defining the algebraic formula, that would be simple.
 Jul 20, 2011, 11:40 AM You win again gravity! UK Joined Jan 2010 1,179 Posts It is simple! It just doesn't look it!
 Jul 20, 2011, 11:45 AM Man, this stuff is so much fun Anaheim, California Joined Feb 2006 7,138 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.
 Jul 20, 2011, 12:08 PM You win again gravity! UK Joined Jan 2010 1,179 Posts 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??
 Jul 20, 2011, 12:09 PM Registered User Joined Aug 2010 69 Posts The EDF in question is a 90mm Midi Pro and ARC 36-65. Thank you all for the replies!
Jul 20, 2011, 12:18 PM
You win again gravity!
UK
Joined Jan 2010
1,179 Posts
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).

# Files

 Jul 20, 2011, 12:33 PM Man, this stuff is so much fun Anaheim, California Joined Feb 2006 7,138 Posts Thank You SCMorrisRC, you made my day.
 Jul 21, 2011, 07:24 AM Team30 Micro EDF Stellenbosch, South Africa Joined Apr 2004 2,046 Posts 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?
 Jul 21, 2011, 09:45 AM You win again gravity! UK Joined Jan 2010 1,179 Posts Hmmmmm. A bit of a grey area. I would personally assume the outrunner bell if the duct is still the same diameter as the rotor at that point.
Jul 21, 2011, 10:24 AM
Registered User
United States, TX, San Antonio
Joined May 2009
336 Posts
Quote:
 Originally Posted by SMorrisRC 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).
Awesome post. Thanks for doing that. I just figured mine and will probably make it out of a fountain drink cup.
 Jul 21, 2011, 11:02 AM You win again gravity! UK Joined Jan 2010 1,179 Posts No problem.
 Jul 21, 2011, 08:02 PM Registered User United States, TX, San Antonio Joined May 2009 336 Posts How does the length of the tube come into play ? Does it matter how long if you have the right opening on the end and at the fan ?