George:

Let's use some real numbers, maybe it will come clear - not that tou couldn't get it, but that my explanations and pontifications were not as clear as they should have been.

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.

The intake duct needs to have at least the same size as the swept area of the fan, and since pulling the air is not as effiecient as pushing it, combined with air drag in the duct, it works best if the intake area is 1.5 times the swept area. And here we have a pleasant surprise. If you can make the inlet duct a smooth tube the same size as the outside diameter of the fan, you are near as D****t. With the setup we've taken the numbers for, 150% of the swept area is 18.85 sq", and that's a diameter of 4.9". How's that for close to 5"?

But that would look terrible on a Sabrejet, wouldn't it? So make the nose intake 3" diameter, and leave the door off the nose gear, use that as your cheater hole. Total of the two holes should be at least our 18.85 sq".

A good test for duct sizes. Run the engine with fan in the open, check full throttle rpm. Then install the power unit and recheck the rpm. Higher, the intake is restricted. Lower, the exhaust is too small. This is not totally accurate, though, the inlet and exhaust could both be too small, and one would cancel the other.

All this is aimed at having zero restriction on both sides of the fan, and all I have given is empirical, with just a small grounding in real engineering with your calculators and computational machinery. But it works.

Have fun with it, DF is a different world.

Bill.