Left: the colored contours show B_y at the vertical midplane (y = 0), in an experiment without ambient field. The arrows show the projection of the magnetic field vector onto the midplane. Also shown (with green lines) is the axis of the original flux tube, the internal reconnection site, the central part of the new flux rope and the outskirts of the envelope magnetic field. Right: height-time profiles of the rising rope for experiments with different twist: α = 0.2 (dotted line), α = 0.3 (dashed line), α = 0.4 (solid line), α = 0.5 (dotted-dashed line).