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@Jan:  You are supposing correctly.  Wing flaps reduce stalling speed and this allows lift-off and climb-out at a lower speed, and hence they reduce the takeoff distance.  During the ground roll and initial takeoff (still in ground-effect) the contribution to drag from the wing flaps is small.  However, wing flaps cause a significant contribution to drag during the climb-out and this explains why there is a limit to the amount of flap used during takeoff, and why pilots retract wing flaps as soon as they achieve the necessary airspeed.  (t) "