Boomerang

Origins

The boomerang would be derived from a old Arme of jet (of modern name “killing stick” ), which is presented in the form of a stick to launching highly sophisticated, whose form Aérodynamique makes it possible to increase the range (approximately 150 meters). Initially cut in wood or bone, a killing stick of war or hunting is conceived to fly in straight line while rotating. The energy accumulated in rotation stabilizes the trajectory of the object. It does not return towards its launcher, even if it misses its goal.

The name, used originally by the Indigenous of an area of the News-Wales of the South, extended to the weapons of war or from hunting analogues in other areas of Australia (where the boomerang is known since millenia) or besides. The boomerang is not specific to Australia: one found very similar objects in Egypt in the tomb of the Pharaon All-Anhk-Amon and other old men of 23 000 years at the time of archaeological excavations in Poland.

An alternative of this projectile has the property to return towards its launcher if it is launched correctly; this quality was to be for the indigenous S Australian the occasion of exercises of play and address. For the same reason, one is interested today in this type of flying object, which explains why the term “boomerang” generally gets along for a flying object returning towards its starting point.

Characteristics

A “traditional” boomerang of sport has two blades, connected by a more or less round elbow, opened of a variable angle (between 60 and 120 degrees). The blade chord is of approximately 50 mm, the thickness of the profile approximately 5 with 8 mm (either 10 with 16 % relative thickness). The under-surface of the profile (the lower part for a wing) is generally flat. The scale is about 40 with 50 cm, the weight between 80 and 140 grams, on average 120 grams. Industrial production out of lamellate wood or plastic.

To launch the boomerang

Precautions

To pay attention to the spectators who must remotely remain sufficient; a boomerang flies quickly, it can surprise and wound people not knowing the trajectory in curve of a boomerang,
to make sure of a large sufficient free space not to lose your boomerang in trees,
Estimer the force of the wind: not the throw if the wind is too strong.

Technique

To locate the direction of the wind and to place themselves vis-a-vis this one. The throw is carried out under an angle of 45° compared to the wind.
To hold the boomerang so that, arm with horizontal in front of oneself, it is perpendicular on the ground.
To bring back the arm behind the head, then to launch the boomerang while seeking so that it has a trajectory parallel on the ground, while printing a rotation movement to him on itself.
To analyze the trajectory of return:
1. the boomerang returns directly in the hands: cheer! .
2. if the boomerang falls in front of the launcher , to start again it in a direction which forms an angle more closed with the direction of the wind .
3. if the boomerang falls behind the launcher , to start again it in a direction which forms a more open angle with the direction of the wind .

The return of the boomerang

A boomerang has a curved trajectory, it is built and launched so that it returns towards its starting point.

Bearing pressure of blades

The blades of the boomerang produce aerodynamic loads:

the Bearing pressure, perpendicular to the plan of the blade;
the Trailed , in the plan of the blade and backwards, slows down the machine and slows down its rotation (but does not intervene in the return).

These aerodynamic loads depend speed compared to the air (translation) and on the number of revolutions.

With a launching in a strictly vertical plane, the bearing pressure results in a strictly side force, in a direction depend on the profile of the wing. Typically, a boomerang for droitier launched in a vertical plan produces a bearing pressure towards the left. One will see now how this force will curve the trajectory towards the left.

Advancing and moving back blade

As the boomerang moves in rotation, speed in the air is more important for the higher blade (advancing) that for the lower blade (moving back). The higher blade develops a greater aerodynamic bearing pressure than the lower blade.

This phenomenon is known, in particular because of the problem of stability of the Hélicoptère S which it and which posed should have been solved: because of the difference in bearing pressure, the plan of rotation of blades is inclined on the side. If the Calage (the incidence) of the blades of a helicopter in translation were fixed, it would rock on the side.

The same thing occurs for the boomerang: the plan of rotation is inclined around the axis parallel at the speed of the machine; if the plan of rotation is vertical, it tends to lie down, and the bearing pressure, initially directed towards the left, rocks downwards.

Gyroscopic effect

But (and fortunately) the things do not occur like that because of the gyroscopic precession : thanks to the effects of inertia which had with rotation, the swing on the side is accompanied by a movement known as of " Precession " , shifted of 90 degrees in the direction of rotation, resulting in making turn towards the left the plan of rotation. What reorientates the force of bearing pressure towards the interior of the turn. It is thanks to this effect that the boomerang follows a trajectory curves and returns towards the launcher.

Trajectory of the boomerang

In short, for a boomerang of vertically launched droitier:

initial rotation is on a horizontal axis, blade advancing in top, folding blade in bottom,
the bearing pressure is a force directed towards the " gauche" (compared to the initial trajectory),
the bearing pressure being asymmetrical (stronger in top than in bottom), it produces a rotation of the axis of rotation of the boomerang: the top is inclined towards the left, bottom is inclined towards the line (to anticipate this effect, launching should not be strictly vertical)
this slope causes a precession: before inclines itself towards the left, the back inclines itself towards the line
what moves the axis of the bearing pressure: she is not exerted any more towards the " gauche" , but towards " the intérieur" trajectory, which makes him play the part of a force of recall and brings back the boomerang towards its launcher.

The trajectory is not circular, because the force of recall is not fixed but depends on many parameters.

The adjustment of the return

The parameters which influence the phenomenon of return are:

physical characteristics of the boomerang:

the weight (like any projectile),
moment of inertia of the boomerang (the stronger it is, the more the precession and thus the feed-back effect is strong),
profile of the blades (which conditions the bearing pressure),

parameters of launching:

the initial speed, which also influences the bearing pressure (more speed = more bearing pressure)
the plan of launching (the more vertical it is, the more the curve of the trajectory occurs early)
the number of revolutions (the stronger it is, the larger the difference in bearing pressure is, and the more the return is marked).

Of course, it is also necessary to take account of the wind.

Sports activity

The sporting practice of the boomerang is the subject of competitions a little everywhere in the world, comprising various types of tests. According to those, the classification is established according to the imposed correction or speed, its duration, its precision, diffusion lenght. There exists a world cup since 1988 to both in a different country and an international federation manages the payment of the tests as well as the homologation of the records.

Manufacture

The manufacture of a boomerang can be done starting from a plan. An owner of cutting, a dense plywood board (marine quality, gasoline mahogany tree or moabi, or standard birch aviation), tools with wood and a few work hours are enough to obtain a model for beginner.

The manufacture of a plywood boomerang passes by the following stages: cut out, formatted, sandpapering, completion. Tools necessary: for cutting a saw hand saw or rather a jigsaw; for working a rasp or an electric file (or disqueuse, but that requires a good technique and makes fly much of wood dust); for the sandpapering of the hold and sandpaper to be sandpapered, for the completion several layers of varnished or the gloss paint (with a preliminary underlayer).

The boomerang in the literature

The boomerang is evoked rather seldom in the literature. It appears there sometimes as a weapon almost supernatural, able to strike and to return nevertheless, which is physically impossible in the event of shock against a body of mass comparable or higher than that of the boomerang:

In this extract of a novel of Jules Verne appeared in 1868, the Children of the captain Grant , it is reported an episode of a hunting for the boomerang being held in Australia; the weapon returns to its sender after having struck a dozen Cacatoès…
In Tintin in America (Hergé, 1932) a gangster neutralizes an accomplice, in order to prevent it from speaking with Tintin, by the jet of a boomerang which returns to him then in the hands.

Anecdotes

a known image circulates, showing an apple posed on the head of the launcher (Barnaby Ruhe, the USA) crossed into two by the return of its boomerang. The apple was precut, but that does not reduce of anything the merit the launcher which makes a success of a rather spectacular turn there. However, it is not useless to recall that a badly caught up with boomerang can wound the hands of the launcher: to avoid hurting itself, better is worth to catch up with a boomerang with the hands with horizontal, if need be with Gant S.
in the film Mad max 2 (1981), one of the characters uses a boomerang at the sharp edges. At the time of an attack on the fort defended by max one of the henchmen of the band of Humungus has the fingers sliced while trying to catch the boomerang in full vol.

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