|The Boomerang||page 1|
Most of us are familiar with the boomerang, the wonderful stick that returns when you throw it. However, most of us don't know the history of the boomerang or the complex physics and aerodynamics involved. To understand why a boomerang returns, we must look at the shape of the boomerang, the aerodynamics and physics. Finally, the correct throwing technique will be discussed.
The boomerang is often times thought of as a weapon. However, the boomerang has always been primarily a recreational toy. The real weapon used by the Aborigines was the killer-stick. The killer-stick shares many properties with the boomerang except one. The killer-stick does not return!
The killer-stick was simply a stick honed to have a cross-section similar to a modern day airfoil. This stick actually flew through the air at high speeds. It was given a rotation at launch for stability much like the discus and frisbee of today. The killer-stick could be thrown very far and with great accuracy.
The boomerang, on the other hand, is smaller and lighter than a killer-stick. There is also a more pronounced elbow between the "wings." The boomerang, as previously mentioned, was not used to kill game, but was used to hunt birds.
When a flock of birds was spotted, an Aboriginal hunter imitated the call of a hawk. The hunter would then throw the boomerang above the birds. The birds would then swoop down to elude the hawk and fly directly into the waiting nets of the hunter.
The boomerang was most likely derived from the killer-stick. Imagine yourself as an Aboriginal hunter. You've studied birds and decided that your killer-stick would fly faster with a sharper angle between the two "wings." With your newly fashioned stick, you spot your prey and launch your weapon. Then a breeze catches your killer-stick and carries it into the air. Then your weapon begins to turn around and fly back towards you! This certainly isn't a decipherable property in a weapon, but it does make a wonderful toy.
As can be seen in the figure, the boomerang consists of a leading wing and a trailing wing connected at the elbow. Each wing has the typical cross section of a airfoil. Therefore, each wing has a leading and trailing edge arranged so as the leading edge strikes the air first as the boomerang rotates. Because of this configuration, there are right-handed and left-handed boomerangs. The figure above is a right-handed boomerang. A left-handed boomerang is simply a mirror image of the right-handed boomerang. The typical angle between the wings is 105 degrees to 110 degrees.
When a boomerang is tossed in the correct manner, the wings rotate through the air and react to the aerodynamic and gyroscopic forces. These forces cause the boomerang to circle around and lay down as it returns, until it descends in a horizontal hover. During the flight of the boomerang, the following principles come into play: Bernoulli's relation, gyroscopic stability, gyroscopic precession, and Newton's laws of motion. We shall examine how these forces cause a boomerang to return to the thrower.
As the boomerang flies through the air, each wing produces lift. Once again, Bernoulli's principle is used to explain how the lift is formed. The air moves faster over the upper surface than the air moving over the lower surface. This means that a pressure differential exists between the lower and upper surface which translates into lift.
A boomerang is thrown with a spin in a similar manner as the discus and frisbee. This spin has two effects on the boomerang as it travels through the air. The first being a stabilizing force known as gyroscopic stability. This phenomenon has been previously discussed in the discus and frisbee sections. The second effect of the spin results in the curved flight of the boomerang.
The turning force imposed on the boomerang comes from the unequal air speed of the spinning wings. If we start with a stationary, spinning boomerang, both wings would produce the same amount of lift. Now give that same spinning boomerang a forward velocity and the speed of the air traveling over the wings differs. Thus, the forward moving wing experiences more lift than the retreating wing. The net result is a force which turns the boomerang. Due to a phenomenon known as gyroscopic precession, this force is felt 90 degrees from where it was applied.
Gyroscopic precession is the principle governing the "no hands" bicycle turn. When riding a bike, the spinning motion of the wheels gives the bike stability at speed. To execute a "no hands" bicycle turn, one simply leans to the side of the direction that they wish to turn. The wheels have a delayed reaction to the force of the lean. This way, the wheels feel the force a quarter turn from where the force was applied. So instead of falling over, the bicycle turns in the desired direction.
Unlike the "no hands" turn, the boomerang experiences a continuous turn as the force is applied for the duration of the flight. The boomerang is thrown with a slight tilt from vertical. This causes the boomerang to also lay down as it turns. Thus the boomerang returns to the thrower in a horizontal hover.
The duration of flight is determined by the force with which it was thrown as well as the spin applied at launch. As with anything flying through the air, a boomerang is subject to drag and its own weight. The drag slows the boomerang down, thereby limiting the flight time. However, given enough spin and initial velocity, the boomerang might circle above the throwers head a few times before landing.
Now that we have a good understanding of how a boomerang works, we should also know how to properly throw a boomerang for many happy returns. A boomerang is launched almost vertically (see figure). The angle depends on the speed of the wind. If a boomerang were to be launched horizontally, it would begin to climb until the wings stalled. At this point, the boomerang would simply fall to the ground.
The boomerang is also thrown at an angle to the wind. The thrower starts by facing the wind and turns about 50 degrees to their right or left, depending on whether the thrower is right or left-handed. With the proper angle to the wind, the boomerang will return to you as planned.
The boomerang is such a simple device and yet it relies on complex aerodynamics and physics. Thanks to our understanding of the boomerang, more shapes have been explored. The angle between the wings may be altered to change the characteristics. For example, a sharper angle would decrease the tip speed, thus making the boomerang easier to catch. A modern boomerang might have several wings joined at a common juncture. Alternatively, a boomerang might be fashioned to represent an object like a bird or straight-edged razor. There are even boomerangs shaped in the form of the letters of the alphabet. All of these boomerangs use the same principles discussed above to return to the thrower at the end of its flight.
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