Q: Why does a Frisbee need to spin in order to fly?
A: When a rock is thrown through the air, one would not consider the resultant motion as 鈥渇lying鈥�. This is because, first, the aerodynamic lift acting on the rock is small compared to its weight, and second, the rock will more than likely tumble as it goes. To fly properly, an object must be able to produce a lift force at least equal to its weight (without causing too much drag) and be able to maintain an orderly attitude as it passes through the air.
The first of these two criteria is quite easy to fulfil. This is essentially what wings are for. The second criterion, which is related to aerodynamic stability and trim, is much harder to fulfil, and usually requires the addition of extra horizontal and vertical wings at the rear of an aircraft, to make things work properly. This is where a Frisbee runs into trouble: it is essentially a single, round wing with no extra surfaces to maintain trim and stability.
Advertisement
If a stationary (nonrotating) Frisbee is tested in a wind tunnel, it is easy to show that it can produce more than enough lift to support its weight, thus fulfilling the first criterion of flying. However, it is also apparent that the Frisbee is very unstable in pitch. That is, if left to its own devices, it would end up tumbling over itself, and hence fails the second criterion of flying. A Frisbee has to spin in order to fly properly because additional pitch stability (strictly speaking, pitch stiffness) is required to overcome the inherent aerodynamic instability of the Frisbee鈥檚 shape 鈥�
A: 鈥� The theory of spinning tops shows that a spinning object is usually unstable unless it is spinning about what is called its major principal axis. If it does so with enough spin then its axis of rotation stays the same, even it there are disturbing forces like air resistance. The faster the spin, the more it can withstand disturbances. This is the principle by which gyroscopes work.
The major principal axis of a Frisbee is the axis through its centre, perpendicular to its face. So if the Frisbee is spun in the normal way with enough force, its axis of spin will remain approximately constant throughout its flight. This also explains why it flies along a plane even when it is thrown at an angle to the horizontal. Its axis, which is at an angle to the horizontal, stays constant. Moreover, Frisbees which have most of their mass at the rim work even better than spinning tops 鈥� they require less spin to achieve the same results.