To add to jacquesm's response, the footage in [2] is definitely from a humid day. That makes condensation happen very readily. If I remember correctly, the tra

To add to jacquesm's response, the footage in [2] is definitely from a humid day. That makes condensation happen very readily. If I remember correctly, the trails from the outboard edges of the flaps are just like wingtip vortices. The airfoil's geometry suddenly changes, acting like a wingtip and creating a vortex.