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The aerodynamic properties of the boomerang have been grasped for some time now. Similar qualities were put to practical use in aviation technology for much of the twentieth century. Yet curiosity, even bewilderment about the boomerang's performance persists. It is as though it was a secret quality, concealed and mystified, eluding our comprehension. So, let's see how a typical returning boomerang actually flies.
What happens to a boomerang when thrown by a right-handed person? Typically the boomerang is thrown almost vertically with its arms pointing out forwards and the convex surface (the ‘top’) facing the thrower's body. At the moment of release, the boomerang flies in a near-vertical position, flying so fast that we need to think of it in slow-motion to work out what is really going on.
Flight consists of two vital components. One is forward motion, causing the boomerang to move, at first away from the thrower. The second is rotation, with the upper arm moving forwards and the lower arm backwards.
The upper arm of the boomerang moves faster because its speed consists of forward motion plus the speed of rotation. The lower arm moves more slowly, for it moves with the speed of forward motion less the speed of rotation. Once this is understood, the boomerang is no longer mysterious.
The speed difference means that the upper arm is in a stronger air current than the lower arm. The difference in pressure exerts a more powerful pulling and pushing force on the upper arm. This force becomes strongest when the upper arm reaches its most forward, almost horizontal position. It pulls the boomerang left, on its circular flight path.
Just a slight upwards tilt of the convex surface when throwing, is enough to make the boomerang ‘lie down’, that is, to move into a horizontal position. Once this happens, the strong pulling and pushing forces change from dragging the boomerang left and begin lifting it upwards. It is the speed of rotation that lifts the boomerang high in the air. Boomerangs usually reach their highest position at up to 15 metres, after which they slow down. Although the rotation may not be enough to lift the boomerang any further upwards, it is often enough to keep it in the air.
The boomerang ‘lying’ in flight shows a similar speed difference. The arm that rotates in the direction of flight moves much faster than the other arm. Imagine you were walking faster with your right leg than your left. The unavoidable result would be walking in a circle. So the boomerang is continually turning left, with its faster rotating arm staying on the outer side of its circular flight path.
To summarise: the boomerang is hurled into the air almost vertically. It flies roughly parallel to the ground as the flight path curves. Soon it lies down before spinning upwards, all the time still turning left. From its highest rise the boomerang descends in a glide as it gradually loses forward speed and rotation.
Observing returning boomerangs in flight is to enjoy a fascinating phenomenon of physics. They often seem to take a long time to complete a full circle although, in fact, a typical flight lasts only about eight seconds.