It's a matter of lift and airspeed. The wings make more lift than necessary to keep the plane level. With enough airspeed you can keep going using the remaining aileron to keep that wing down. When you drop below the speed where you can maintain deflection with that aileron (less airflow, less downward force) you can no longer do it.
That's why it's easy enough to fly with part of your wing missing in AH, but not so easy to land. Landing usualy falls well below that controllable state.
P.S. hardly any WW2 planes had hydraulic aileron controls. Some had "boosted" controls, but there was always a physical connection to the ailerons somewhere.
