What Makes a Boomerang Come Back?
It is the built-in skew or twist in a boomerang combined with its spinning motion that makes it return to the thrower. At first people believed that air, pressing on the lower flat surface and passing over the upper rounded face, was responsible for the return flight. But T. L. Mitchell, a Scottish explorer of Australia, gave the true explanation early in the 19th Century. The curved throwing stick is used chiefly by the aborigines of Australia for hunting and warfare. (They also use a non-returning kind of boomerang.)
The boomerang is held at one end, above and behind the thrower’s shoulder, with the concave edge to the front and swung forward rapidly with the flat side underneath. Just before it is released, it is given extra power with a strong wrist movement. If thrown downward or parallel to the ground it sweeps upward to a height of 50 feet or more. When thrown so that one end strikes the ground, it ricochets into the air at terrific speed, spinning endwise. It completes a circle 50 yards or more wide and then several smaller ones, up to five, before it drops to the ground near the thrower.
A returning boomerang is a rotating wing. Though it is not a requirement that the boomerang be in its traditional shape, it is usually flat. A falling boomerang starts spinning, and most then fall in a spiral. When the boomerang is thrown with high spin, a boomerang flies in a curve rather than a straight line. When thrown correctly, a boomerang returns to its starting point.
Returning boomerangs consist of two or more arms, or wings, connected at an angle. Each wing is shaped as an airfoil. As the wing rotates and the boomerang moves through the air, this creates airflow over the wings and this creates lift on both “wings”. However, during one-half of each blade’s rotation, it sees a higher airspeed, because the rotation tip-speed and the forward speed add, and when it is in the other half of the rotation, the tip speed subtracts from the forward speed. Thus if thrown nearly upright each blade generates more lift at the top than the bottom.
While it might be expected that this would cause the boomerang to tilt around the axis of travel, because the boomerang has significant angular momentum, gyroscopic effect causes the plane of rotation to tilt about an axis that is 90 degrees to the direction of flight, and this is what curves the flight in such a way that it will tend to return.
Thus gyroscopic precession is what makes the boomerang return to the thrower when thrown correctly. This is also what makes the boomerang fly straight up into the air when thrown incorrectly. With the exception of long-distance boomerangs, they should not be thrown sidearm or like a Frisbee, but rather thrown with the long axis of the wings rotating in an almost-vertical plane.
Fast Catch boomerangs usually have three or more symmetrical wings (seen from above), whereas a Long Distance boomerang is most often shaped similar to a question mark. Maximum Time Aloft boomerangs mostly have one wing considerably longer than the other. This feature, along with carefully executed bends and twists in the wings help to set up an ‘auto-rotation’ effect to maximize the boomerang’s hover-time in descending from the highest point in its flight.
Some boomerangs have turbulators—bumps or pits on the top surface that act to increase the lift as boundary layer transition activators (to keep attached turbulent flow instead of laminar separation). Historical evidence also points to the use of non-returning boomerangs by the Native Americans of California and Arizona, and inhabitants of southern India for killing birds and rabbits. Indeed, some boomerangs were not thrown at all, but were used in hand to hand combat by Indigenous Australians. Ancient Egyptian examples, however, have been recovered, and experiments have shown that they functioned as returning boomerangs.
Boomerangs can be variously used as hunting weapons, percussive musical instruments, battle clubs, fire-starters, decoys for hunting waterfowl, and as recreational play toys. The smallest boomerang may be less than 10 centimetres (4 in) from tip to tip, and the largest over 180 cm (5.9 ft) in length. Tribal boomerangs may be inscribed and/or painted with designs meaningful to their makers. Most boomerangs seen today are of the tourist or competition sort, and are almost invariably of the returning type.
Modern boomerangs used for sport are often made from thin aircraft plywood, plastics such as ABS, polypropylene, phenolic paper, or even high-tech materials such as carbon fibre-reinforced plastics. Boomerangs come in many shapes and sizes depending on their geographic or tribal origins and intended function. Many people think of a boomerang as the Australian type, although today there are many types of more easily usable boomerangs, such as the cross-stick, the pinwheel, the tumblestick, the boomabird and many other less common types.