I spotted something that, to my eye, looks a bit anomalous with the Fiat G.55 Sottoserie 0 / Sottoserie 0 Late.
Specifically, the flight model for these planes seems to have something wrong in them, regarding their turn performance with take-off flaps on.
This graph is produced with 4.101 FM data (available in
http://www.flightsimtesting.com/IL2Compare.aspx), but I also checked with 4.11 FM data on IL-2Compare on my own PC and the results are similar if not the same:
To explain the graph briefly: The two curves for each aircraft are for no flaps, and take-off flaps setting. With the G.55 Serie I Late (Red) you see the high performance turn rate with no flaps, and with take-off flaps engaged, the overall turn time increases while turn rate decreases, but improves it at low speeds compared to turning at slow speed with no flaps. This is to be expected, and seems to be correctly modeled in the flight model.
However, when you look at the Sottoserie 0's corresponding curves in blue, you can notice that take-off flaps actually reduce turn time significantly, at even higher speed than with no flaps, and turn rate shoots sky high, over 30 degrees per second which is apparently more than the designers of this graph expected.
Basically, with the Sottoserie 0, using take-off flaps moves the turn performance curve to the opposite direction than happens with the Serie I (and pretty much all the other aircraft in the sim). Curiously, the same behaviour plagues both the Sottoserie 0, and Sottoserie 0 Late, while Serie I and Serie I Late seem to be normal.
Granted, the take-off flaps will not actually give that much of a performance increase in my opinion, considering that they will jam and slow you down if you use them to turn around in 11 seconds at 320 km/h (plus your plane will disintegrate from the g-forces), but regardless it seems clearly anomalous behaviour that should be sorted out and fixed.
To my amateur eyes it looks almost as if there's a flipped sign somewhere in the FM code for the Sottoserie planes, which makes the performance curves with take-off flaps setting to move in the opposite direction than they should... but I have no idea at all how the FM's work in this game and what actually causes this behaviour.
EDIT: Just to clarify: I know this is not technically a flyable aircraft, but I think the FM still shouldn't be broken... who knows what shenanigans the AI can get up to with this kind of flight model.