being connected at one end to an input conduit and at the opposite end to output conduit so as to allow lubricant oil therein to flow through these tubes airstream 13 flows about and then past those tubes to reach the exit of duct 11 at its downstream end which has an exit cross sectional area that is controlled in magnitude by an exit flap 16 entrance flap 12 u2032 can be selectively rotated forcibly inward or outward from the position thereof shown in fig1 and exit flap 16 can be selectively rotated forcibly inward or outward from the position thereof shown in that figure by angle positioning motor 17 causing a proportional linkage arrangement 17 u2032 to correspondingly move these flaps and the linkage components about pins shown in bold dark circles and about the shaft coupled to that motor alternatively entrance flap 12 u2032 can be so selectively rotated and exit flap 16 can be so selectively rotated by a corresponding one a pair of angle positioning motors 17 u2033 and 17 u2032 u2033 directly coupled thereto by corresponding shafts in turbofan engines that have the airstreams in the fan duct of a sufficiently small mach number flap 12 u2032 may be omitted bifurcation structure 10 is positioned between the bottom of the wall of an engine.