I haven't found an explanation of what this "vent" is in late model Yak-9s. It seems to be moveable from nearly flush to extended as shown. There are two of them just forward of the ventral inlet. Can anyone help?
I see. As memo serves thare were two such outlets with the possibility of opening and closing depending on the temperature. Both were at the fuselage undersides and located symmetrically. The same way of oil cooling was used for the Yak-3.
Just a note .. the main water radiator in the pic is of an early Yak-9 with the oil radiator mounted at the front bottom M-105 ( VK-105 ) engine cowling. Together with the engine M-107A ( VK-107A) introduced for powering of the late Yak-9, I mean the Yak-9U, the main radiator was moved more back and the oil one was relocated almost to the place where the main water cooler had been mounted ealier. In the way the Yakovlev OKB got more streamline front of the fuselage. What is more the main radiator was put deeper into the fuselage in a very similar way the P-51 Mustang had.