Vertical wind tunnels aim to re-create the physics of freefall. Air in the tunnel travels at a speed that matches the terminal velocity of a skydiver (about 200 kilometres/120 miles per hour), and produces a column of air up to five metres (16 feet) wide. A skydive typically lasts no longer than seven minutes, so the goal of a wind tunnel is to create a smooth, laminar flow of air which enables skydivers to practise for several hours at a time.
There are two main designs currently in use. Open-circuit wind tunnels use a continuous stream of new air, drawn in at the bottom and expelled at the top. Powerful fans direct the air upwards, producing a jet upon which people can float. These tunnels can be made in portable form and used outdoors too for a more natural skydiving experience.
In contrast, recirculating wind tunnels reuse air in a loop to conserve energy and provide more uniform airflow. Typically, four fans positioned above the tunnel circulate air around a series of tubes in an aerodynamic loop. The air is ‘turned’ through the tubes using vanes, moves up through the flight chamber and is then recirculated to the bottom again. Friction in the mechanism causes the air to become very warm, so often cool fresh air is incorporated in order to make the experience more comfortable for the flyer.