Rick, I don't know what the OS pipe part # would be, and you know that some of the fan pipes are kinda strange with the outlet coming out the side and stuff.

I had the best luck on the VRDF with a Nelson FAI pipe, and that's what I'll use on the VXDF..I haven't ran it yet..

As for props...well, you are on your own. I ran a 7.4 X 8.25 CF prop on the VRDF, turned about 22K static. Maybe 24K or so in the air. But it would appear my thinking was flawed, guys are getting more speed from larger diameter, less pitch props making me think that these little props don't have enough thrust to fly anything but a real slippery Q40 plane or something along those lines.

And 28K is the upper end of the practical RPM range, so I guess you will want a prop that will turn that unloaded, so now the diameter comes back into play via tip speed, so I have no idea what to use, and am in the same boat as you.