Originally posted by William Robison
We have a 5" Dynamax fan. The area of the fan circle is 19.63 sq". The hub is 1.5" diameter. area 7.07 sq". Now subtract the hub area from the total fan circle area, you get the "Fan swept area," 12.57 sq". Now working that backwards, we find an open circle 4" in diameter has the same area. So our outlet should be a 4" circle. Pretty close to our 75% at 3.75" diameter, and close enough.
Not to throw a wrench into your calculations but, a hub with diameter of 1.5" has an area of pi(r*2)=1.77 sq" not pi(d*2)=7.07 sq" which makes an outlet diameter of 3.75" only 62% of the Swept Fan Area of the Dynamax fan. I've read that you shouldn't go below 75% SFA for the outlet area but grbaker said he has seen and used outlets smaller than 3.75" with the Dynamax. 100% SFA is plenty for intake area for a model flown off pavement. If you want to fly off grass, more intake area will help.