Blower fan axial spacing

[Bill Dufour]

I am trying to figure out the ideal fan axial placement on a dust blower. How far back should the fan wheel be. As close to the inlet side as possible seems logical, but too close may cause a siren effect. Or maybe air flow need a little extra room so long shaving do not jam up. Of course this is after the cyclone so shavings should not be an issue. This is for a standard sheet metal fan wheel with no extra shrouding attached to the wheel.
My google just gets me adds for confined space blowers.
Bill D

[Wes Grass]

Something rattling around in my head says minimum 3x duct diameter. No idea what sort of application that would apply to. Possibly ductwork. Or maybe the distance from the mouth of a carburetor in an airbox?

Trying to visualize this: The center tube is maybe long enough by itself? And the flow rotation reverses from the cyclone to the center tube? So you want the fan running in that direction?

Seems my Oneida has the ports on the same 'side'. Inlet is CCW from the top, outlet CW?

[Bill Dufour]

I think the 3x number is the straight duct into the fan. I looked today and there is about 2" of gap between the front of the fan blades and the housing. Some more efficient blower wheels have a giant washer welded on with a 6" hole in the middle. More efficient but more prone to clogging with stringy stuff.
Like the link which is a small vac fan for high rpm and high pressure. Many vacs stack 2-3 of these in series.
Bill D.