How heavy things fly

Why do frisbees fly?

A frisbee disc can float through the air for a long time - there are two main reasons for this: firstly, the disc is curved and, secondly, it is set in rotation by throwing it. The curvature creates an upward suction, and the rotation has a stabilizing effect when flying.

The curved plastic discs have been flying around for quite a while. It all started with the cake molds from a bakery in the USA called the “Frisbie Pie Company”, whose flight characteristics were noticeably good. Since the late 1950s, another company has been marketing similarly shaped discs under the name "Frisbee".

Several physical effects enable a Frisbee to float so long and far through the air. First of all, a frisbee is curved. If the air flows over this curvature, there is lift like an airplane wing. The cause is related to the so-called Bernoulli principle of fluid dynamics. The air moves faster above the pane than at the bottom. Because the way at the top is longer due to the curvature. As a result, there is less air pressure above the Frisbee than below it and there is suction upwards [for details see box below the text].

Flight of the frisbee

But this effect is not enough to keep the frisbee in the air for a long time. If you do not give it rotation when throwing it, it immediately begins to flutter and quickly falls to the ground. Typically the disc rotates eight times per second when it leaves the hand. Physicists refer to the inert rotation of a mass as angular momentum. The angular momentum is a conservation quantity and guarantees a stable and calm flight, because it dampens accidental disturbances of the rotational movement. If a tilting force acts on the flying object, it is, so to speak, deflected, which prevents tilting. The same effect also helps that a gyroscope always returns to upright rotation in the event of a malfunction.

This effect can be reinforced by the shape of a frisbee disc: A wide "brim" on the edge not only makes it easier to catch the disc, it also concentrates more weight on the outer edge. This mass distribution also increases the frisbee's angular momentum - and thus leads to more stability. In addition, the brim creates a kind of parachute effect. If the disk flies slowly, then it sinks very slowly to the ground because it offers great air resistance to the fall.

Last but not least, many Frisbee discs have concentric grooves on the top, which have a particularly ingenious purpose: They counteract the so-called stall, which is particularly feared in aircraft. This phenomenon creates large air vortices above the wings, which greatly reduces the speed of the air. As a result, the desired upward pull is reduced. This can even lead to a crash. The grooves on the frisbee disc create small, turbulent air currents on the surface. The air flows much better over these small eddies and the air flow that creates the lift does not tear off as easily.