quorneng

jibwalker
It is the area of the EDF minus the area of the hub not the diameters.
In your case the area of the EDF is
Pi x 27 x 27 / 4 = 572 sq mm
The area of your hub is
Pi x 14 x 14 / 4 = 153 sq mm
The FSA is 572 - 153 = 419 sq mm
85% of that is 356 sq mm.
Doing the area calculation backwards from that gives a diameter of 21.3 mm.

TimMartel

Thank you Don.

Tim

birdDog

For those of us, like me, who abhor maths.

http://www.calculator.net/area-calculator.html

EFFlyer

I used SketchAndCalc to calculate the surface area of my EDF along with various other irregular shapes:

http://www.sketchandcalc.com

Watch the video on the homepage first... might save you some time

birdDog

Build a jig for your thrust tube. One plug is sized the diameter of your fan shroud exit and the other plug sized to your preferred exit diameter. Space these on a dowel to the exact length you want (or scribe a line). This is also handy for trimming the duct square.

dahawk

Now that's clever ! Thanks !

Hawk