It is amazing to see that when enough engineers, with a few math majors as back up can muddle a subject to a standstill, without producing a few clear usable rules.

 

Things like how big exactly? How long? What radius?

 

You get all of this in your first hour on the flow bench, playing with foam coffee cups and modeling clay.

 

If there plenum was actually drawn accurately then the had a standing restriction at the opening into the box.

The outflow end of a tube will produce a contracta as well as the inlet end.

 

I have determined, (with my home built flow bench) that improvements in radius and smoothness beyond

two diameters of the centerline of the tube is unlikely to produce a measurable improvement in flow.

 

The inlet horn need not be the shape of a trumpet bell to match its performance. You can start with a horn 3 times the diameter of the tube and taper it over 3 diameters and get the same affect flow wise.

 

Where space permits a nicely radiused  horn is best. As in the flat side of a plenum box, for example.

The horn may be oval as opposed to perfectly round and work very well. The rotary, in my opinion has a flow advantage over the piston engines these guys are thinking about. The rotary is more a steady state flow,

rather than a pulsed flow. It is easier to get the rotary to work than it is a piston engine.

 

Build a sample. Hook a shop vac to the engine end. Lower a fat string into the hole. Watch the string.

 

Lynn E. Hanover