I took this photo in June this year in Zhuhai, a city in the south coast of China. Before we went to the airport, a continuous rainstorm took place there, which rained for more than 10 hours, from evening to noon, and it turned sunny after the rain. A helicopter was taking off, its propellers drew ‘white lines’ in the air. This was due to the different flow direction of the upper and lower airfoil of the propeller, forming wingtip vortices by the movement of the propeller. Because the air pressure inside the wingtip vortex was low, and the air after the rainstorm contained enough water vapor, this water vapor expanded and then cooled, forming water droplets, and ‘white lines’ were formed in the wingtip. Such vortices can destabilize the aircraft and reduce wing lift, and today’s helicopters have winglets on their wingtips to reduce vortices. This helicopter also has wingtip winglets, but because of the particular weather conditions of this day, the air was extremely humid, and the vortex rope had appeared again.
