Hi,
the 109 flaps could get used without steps by a wheel on the left side of the cockpit, next to the elevator trim wheel, not far away from the throttle.
According to the british 109E test flaps didnt help while turning, though wingis of H.J.Marsaille wrote that he did use them to be able to turn inside of Hurris and P40´s. If this was done only to slow down or to increase the lift is not clear to me.
In general they dont got used in combat, cause main knowledge was "speed is all" and flaps decrease the speed.
If flaps got used in combat, max 15° would be logically, cause only in this area the flaps increase the lift more than the drag. In case of the 109, where the alerons also goes down with the flaps, the flaps goes virtuelly over the whole wing span, so they should have a positive influence, though, i never saw a real turn test regarding this.
Would be intersting to know up to what speed around 0-10° flaps could get used and what impact this have to the flight behaviour.
Greetings,
Knegel