Originally Posted by Turbotronic

Jan, the way you described the outlet at rear of fuselage also sounds like it will do a good job of drawing air out of the fuselage volume, good for keeping things cool.
If your issues is on the ground only the Naca ducts will help to let more air into the ducts.
Depending on how well the fuselage is sealed off in flight its difficult to say which way the air is going to go.
The issues with the AW L39 went away when not using the top cover, at least on the ground.
In this case it was compressor stall on spoolup and down.
Andre