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??
