Probing Question: Why do boomerangs return when you throw them?

boomerang

David C. Sommer

Mark Maughmer is determined to make the darned thing fly. Commandeering a hallway of Hammond Building on Penn State's University Park campus, he hurls the plastic boomerang into the air again and again. A passing student cowers. With Maughmer's next attempt, the banana-shaped piece of polystyrene whizzes past my head, spinning as it rotates about its axis. Creating a graceful arc, it swoops back toward its point of origin, hovering in the air for a second before descending into the hands of its thrower as if by magic. "Did you see that?" he asks excitedly.

The first boomerangs were originally little more than hunting sticks thrown by Stone Age humans tens of thousands of years ago, explains Maughmer, a professor of aerospace engineering. Although usually associated with Australian aborigines, boomerangs were used in many different cultures: Ancient specimens have been found in such diverse locations as Eastern Europe, Egypt, and North America. Over time, boomerangs became lighter and were thrown for recreation as well as for hunting. At some unknown point, notes Maughmer, someone discovered that if you threw it just right, a boomerang would come back to you.

Although this circuitous flight path may seem mysterious, he says, it can be easily explained by the laws of aerodynamics.

Notes Maughmer, most boomerangs consist of two blades connected at a central point. Looked at in cross section, the top of each blade is curved and the underside is flat, like the wing of an airplane. This curvature helps the blade produce lift. In addition, the two blades are set at a slight tilt from the horizontal, and this produces more lift by pushing air down as the blades turn, like a propeller. "The boomerang works in a similar way to the seedpods of a maple tree, spinning through the air as they fall," he says.

Unlike a seedpod, however, boomerangs are thrown into the air. This added motion is the key to understanding why a boomerang returns.

"Imagine a boomerang rotating in a wind tunnel, with the force of the wind replacing its movement through the air," suggests Maughmer. As the boomerang rotates, the rearward-facing blade moves in the same direction as the wind, and the forward-facing blade against that direction. The greater wind velocity on the forward-moving blade causes it to produce more lift than the rearward-moving blade. "If you've ever held your hand out of the window of a moving car, you can easily grasp this idea," says Maughmer. When you hold your hand at an angle to the wind, the faster the car is going, the greater the force pushing your hand up will be.

Explains Maughmer, the difference in velocity between the rearward and forward-facing blades creates a difference in the lift on each blade. The effect is that, with each rotation, the boomerang's axis shifts slightly, eventually making its circular path back to the hands of the thrower.

This process is called "gyroscopic precession," says Maughmer. Anyone who has played with a gyroscope as a child has experienced this phenomenon in which a force applied to a rotating object will take effect 90 degrees around the direction of rotation, causing it to tilt its rotational axis while spinning.

As Maughmer's hallway hijinks demonstrate, however, simply hurling a boomerang into the air does not guarantee its return. "You've got to throw it in exactly the right way," he stresses. Not flat, like a Frisbee, but instead at roughly a 15-degree angle. "With the right angle, the right throw, and of course, practice," says Maughmer, "you can get it to come back to you."

Mark D. Maughmer, Ph.D., is a professor of aerospace engineering in the College of Engineering. He can be reached at mdm@psu.edu.

Last Updated October 13, 2008