In an acrobatic performance, the performer throws a crisscross wooden flake like the impeller of a fanner to the audience. Surprisingly, the impeller does not fall among the audience but returns to the performer after drawing a big circle. What's the matter?

This strange wooden flake is called whirling dart. It is said to be a hunting tool used by Australian aborigines. The whirling dart is made of hard wood and shapes like a reaping hook. After being thrown, the dart flies along an arc route so that it can hit horses behind trees or small animals behind huge stones and hillocks. If it misses the target, the tart can return itself. Therefore, the whirling dart is also called Flying Picker or Australian Stick.

The whirling dart can return itself because it is designed according to the principle of atmospheric mechanics. Most whirling darts are crisscross wooden impellers while some are three-leaf, six-leaf and lambdoidal shaped. Their laminas, just like wings of planes, are streamlined. Under the laminas there are a platform. When the laminas whirl, the speed of air is different above and below the platform and combination force is thus produced. 

After being thrown out perpendicularly, the dart begins rotating with the performer's wrist as the axis. The track of the whirling dart is horizontal at the first beginning and gradually becomes inclined upwards. Due to different speed of the dart's rotation and flight forward, the airflow speed on the two impellers becomes different. As a result, the lifting force becomes unbalanced, which makes the whirling dart move on a round track like a peg-top and then return to the performer's hand.