[JtD]

The description of the enclosed cowling is that for the V1 prototype model, and as the concept proved unsuccessful, it was dropped for the V2 and later. These aircraft also were powered by a BMW 139, and engine-wise had little in common with serial production models.

The Fw 190 possessed an oil cooler ring, with an adjustable gap to adjust the cooling capacity. The standard size was 10 mm, but it had to be increased to 20 mm for tropical use and F model attack aircraft. This increase cost the plane 15-20 km/h at sea level.

The Fw 190 also had adjustable cooling gills on the side of the aircraft, which allowed the cooling of the engine or more precisely the cylinders. These pretty much worked like any other cooler on any other aircraft, including speed loss as it was opened. One effect I know of was a reduction of 500m in ceiling, therefore about 1 m/s in climb, with open cooling gills.

Lower drag with partially open radiators was a feature of the D-9, where the flush setting was a partially open one. However, even this feature wasn't anything special, other aircraft have this, too. I haven't seen it for the A models.

Il-2 simplifies by treating two cooling mechanisms as one and calling them radiator, and having them both adjustable by the pilot in flight. It also simplifies the drag characteristics of the radiators throughout.

Imho, if the radiator of the Fw 190 gets special attention in terms of cooling and drag relation, all other aircraft need to see the same treatment, as there seems to be nothing especially special about the Fw. A global rework would be nice, but the research alone is a tremendous job.