A hardy band of Czech wind tunnel enthusiasts hover and flip in a dazzling display of acrobats inside Prague's Skydive Arena, in this video from the folks at Outrageous Acts of Science. The underlying physics is pretty cool, too.
It's much the same as for regular skydiving. It all comes down to two forces: gravity pulling you down, and friction from air resistance pushing you up. The wind tunnel acrobats are able to hover because they find an equilibrium between those two forces akin to reaching terminal velocity (the point where gravity and air resistance are equal strength). Per the folks at How It Works:
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.
The other important factor is body position, which relates to the cross-section of an object in free fall. Shoot straight down like a bullet, or in a tucked position, and you'll reach a faster terminal velocity than if you fell in a spread-eagled position, because there's less surface area and hence less friction from air resistance.
The Czech acrobats are highly skilled at manipulating those forces by small, seemingly insignificant movements that make a big difference in the wind tunnel environment. Check out the four-person choreographed "dance" in the video below, shot at the same Prague location:
[Via The Kid Should See This]