Hi guys,

I was reading the other day about a VTOL aircraft that used directional ducts to take off and fly. Apparently, the duct fan was placed many feet away from the tip of the duct to prevent airflow separation.

Can anyone provide some resources as to what airflow separation is in that context, why it is avoided, and how one can minimize the effect?

Thanks in advance! :-)

I think you must be reffering to the need to put the fan some fair distance down the duct away from the mouty. If the front of the opening is too close to the fan blades you get some turbulence from the blades that fouls up the airflow into the mouth of the duct. For the same reason the old Kress ducted fans specified that you needed to add a bell mouth to their fan units if using then outboard from the fuselage like on a Warthog or on airliner like pylons. The fac was just too close to the mouth and the outer housing was a basic tube. Adding a venturi like bell mouth to the forward opening kept the tip turbulence inside the duct and also smoothed the air entering the duct.

That must be it. Thanks a bunch!