The Physics of Flight


Forward flight splits rushing air at the disk's leading edge: half goes over the Frisbee; half goes under. Because the edge is tipped up, the disk deflects the lower airstream downward. As the Frisbee pushes down on the air, the air pushes upward on the Frisbee--a force known as aerodynamic lift. The upper airstream is also deflected downward. Like all viscous fluids, flowing air tends to follow curving surfaces--even when those surfaces bend away from the airstream. The inward bend of the upper airstream is accompanied by a substantial drop in air pressure just above the Frisbee, sucking it upward (Bernoulli effect). These two forces taken together tend to LIFT the Frisbee against gravity.

Limits to the airstream's ability to follow a surface explain why a Frisbee flies so poorly upside down. When the upper airstream tries to follow the sharp curve of an inverted Frisbee's hand grip, its inertia breaks it away from the surface. A swirling air pocket forms behind the Frisbee and destroys the suction, raising the air resistance. Once this air resistance has sapped the inverted disk's forward momentum, it drops like a rock. Players can take advantage of this effect in a hard-to-catch throw called the hammer.

Rotation is crucial. Without it, even an upright Frisbee would flutter and tumble like a falling leaf, because the aerodynamic forces aren't perfectly centered. Indeed, the lift is often slightly stronger on the forward half of the Frisbee, and so that half usually rises, causing the Frisbee to flip over. A spinning Frisbee, though, can maintain its orientation for a long time because it has angular momentum, which dramatically changes the way it responds to aerodynamic twists, or torques. The careful design of the Frisbee places its lift almost perfectly at its center. The disk is thicker at its edges, maximizing its angular momentum when it spins. And the tiny ridges on the Frisbee's top surface introduce microscopic turbulence into the layer of air just above the label. Oddly enough, this turbulence helps to keep the upper airstream attached to the Frisbee, thereby allowing it to travel farther.

References

Louis A. Bloomfield Professor of Physics, University of Virginia Author of How Things Work: The Physics of Everyday Life http://www.scientificamerican.com/1999/0499issue/0499working.html