Why boomerangs return when thrown

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UNIVERSITY PARK, Pa. — The first boomerangs were originally little more than hunting sticks thrown by Stone Age humans tens of thousands of years ago, according to Mark Maughmer, a professor of aerospace engineering at Penn State.

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, noted Maughmer, someone discovered if you threw it just right, a boomerang would come back to you.

Aerodynamics

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

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 fact a boomerang is thrown into the air is the key to understanding why it returns.

“Imagine a boomerang rotating in a wind tunnel, with the force of the wind replacing its movement through the air,” he suggested.

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,” said 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.

The difference in velocity between the rearward- and forward-facing blades creates a difference in the lift on each blade, he explained.

Gyroscopic precession

The effect is, 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.

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.

Simply hurling a boomerang into the air does not guarantee its return. You have to throw it exactly the right way, he stressed.

Not flat, like a Frisbee, but instead at roughly a 15-degree angle. With the right angle, the right throw and practice, you can get it to come back to you.

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