What is the principle of making boomerang?

Take a closer look at the following article and introduce the background knowledge of boomerang and LZ questions very clearly ~ ~ ~

I hope I can help you ~ ~ * *

Principle and realization of boomerang

When it comes to boomerang, an animation will pop up in most people's minds: the primitive aborigines (or the legendary Mount Tai) look at the prey in the distance, raise the boomerang and throw it to the right (if he is right-handed), and the boomerang flies out in a beautiful arc. After hitting the prey, the boomerang continues to turn around in this beautiful curve and fly to its beloved owner. At this time, the master can catch darts with one hand. Such an amazing tool, it

History:

It can be traced back to the Stone Age 15000 years ago. Perhaps at that time, people often threw their prey with stone sticks or bones, and accidentally found that some special-shaped stone sticks or bones had the ability to swing. ..............

Use boomerang:

Australian aborigines, ancient Egyptians, South American aborigines …

-But the oldest unearthed boomerang: it was discovered in Poland, about 20,000 years ago!

Classification:

The English name of Boomerang is "boomerang", which is divided into two types:

(1) rotary joint (so-called boomerang)

Rotating darts are usually heavier and longer to increase the ability to kill prey.

(2) Do not turn (called "spin dart" in ancient times)

Why can boomerang fly back?

-A mixture of propeller and gyroscope

Careful observation of the thickness of the two wings shows that the two wings are not mirror symmetrical with the center line. Generally speaking, the thicker end is the leading edge and the thinner end is the tail end. According to the cross section, the thickness configuration of the (right-handed) boomerang is: the outer edge of the right wing and the inner side of the left wing are the leading edges, and the inner edge of the right wing and the outer edge of the left wing are the tail ends. Therefore, after the right hand throws the dart, the boomerang will rotate counterclockwise, as shown in the figure. At this time, the thick end will always be in front, while the thin end will

If you straighten the curved wings of the boomerang, drill a small hole in the middle and insert a bamboo stick and dragonfly.

Helicopter propeller darts are imagined as the wings of bamboo dragonflies. The section of the propeller or wing of the aircraft is the same as that in Figure 3. When the airfoil passes through the air, the upper airflow is pushed out and deviates far away, but at the same time it is combined with the lower airflow at the tail end, as shown in Figure 4. For an observer who is stationary relative to the wing surface, this means that the upper air flow is faster and the lower air flow is slower. Generally, the pressure of air with faster flow rate is lower than that of the flow surface. This is the so-called Bernoulli principle [3], so the airfoil feels an upward net lift.

However, we have only seen the fallen bamboo dragonfly, but we have never seen the bamboo dragonfly coming back in circles. It can be seen that this analogy can only explain the floating ability of boomerang, but can't fully explain its rotating force. Similar to the boomerang, everyone is familiar with the example of spinning by itself and rotating around a central axis at the same time. Gyroscope is such a model: when the central axis of the gyroscope is not vertical but inclined, don't we often see the whole gyroscope rotating around the lead passing through the fulcrum? This is the so-called precession [3], as shown in Figure 5. If the gyro itself does not rotate, when the gyro tilts, the gravity is no longer parallel to the central axis, and the gyro will tend to tip over. another

Figure 5. Precession of gyro.

On the other hand, the tilting motion caused by gravity can also be regarded as the rotation of the center of mass of the gyro around the fulcrum, so we can say that there is a rotation trend at this time, and physics quantifies this trend as the so-called "moment". This rotation direction is circled with four fingers of the right hand, and the direction of thumb is the direction of torque. Generally speaking, the moment is parallel to the rotation axis of the object, and the rotation it drives will be the same or opposite to the original rotation direction of the object. Therefore, the rotation is accelerated or decelerated. However, as shown in fig. 5, if the torque is perpendicular to the rotation axis of the object, the rotation speed will not change, on the contrary, the direction of the rotation axis will change. Because no matter how much the direction of torque changes, it will always be perpendicular to the rotating shaft, and the rotating shaft will change direction continuously, and finally it will come back once, resulting in the so-called precession [3].

After understanding the lift generated by helicopter propeller and the precession caused by the moment perpendicular to the gyro axis by Bernoulli principle, the flight principle of boomerang is to combine them into one. number

Figure 6. The influence of Bernoulli principle on darts.

At this time, the propeller is not placed horizontally, but vertically, and its corresponding gyro rotation axis makes an angle of 90 degrees with the vertical line. As shown in the left side view of Figure 6, because the boomerang rotates during flight, the speed of the upper half is faster than that of the lower half. According to Bernoulli's principle, the upper part will be larger than the lower part.

The rear view on the right side of Figure 6 shows half of the lateral force. If the boomerang does not rotate, this force will generate a rotating force through a moment perpendicular to the rotation axis in fig. 7.

Cause the dart to roll over, that is, there is a moment acting on the boomerang. But the boomerang is rotating, and the rotating shaft is perpendicular to this moment, as shown in Figure 7, so the rotating shaft will keep rotating and the boomerang will rotate around the circle relatively. Because the thrower is in the rotating circle, the boomerang will fly back to the thrower.

Rotating darts DIY

From the discussion in the last section, it can be seen that the key point of boomerang rotation lies in whether its wing surface is like a wing, which can make the airflow speed on both sides unbalanced, and then produce the torque of lateral rotation. Today, the design of boomerang is very diverse, and Figure 8 shows some boomerang different from traditional boomerang.

The method to be introduced here is a boomerang making method developed by Professor Zhou Jianhe's laboratory of National Kaohsiung Normal University, which is suitable for indoor flight [5]:

As shown in fig. 9, the chip board weighing more than 500 kilograms was cut into three pieces, with a length of 13.7 cm and a width of 2.7 cm.

At one end of each rectangular cardboard, cut a cut about 1 cm long and 0. 1 cm wide from the center.

The included angle of each wing should be 120o as far as possible, so as to combine staples as shown in Figure 9.

Figure 9. Simple three-wing boomerang.

The wing surface is slightly concave to increase the turning power.

Please follow the following principles when testing:

When firing boomerangs, make sure that no one is on the flight path.

Hold the dart body so that its front is vertical. For right-handed people, the convex surface is to the left; Left-handed, convex to the right.

Only the wrist level.

If there is wind on the site, please face the direction in which the wind blows. If you are right-handed, shoot at a right angle of 45o. Left-handers shoot 45 degrees to the left.

Try to increase the weight evenly on the outer edge of the wing (for example, tape it), and take another shot to compare with the previous flight trajectory.

Spinning darts golf tournament

It is said that aborigines in Australia once threw the boomerang nearly 100 m away, with a height of nearly 50 m, and circled back and forth for up to 5 times! [6] Although the boomerang demonstrated here is not so magical, its short-range characteristics are suitable for flying indoors and in small venues. Here is an interesting boomerang playing method: put some marks at several appropriate distances, let the contestants throw darts and swing above the pole, and each pole counts as a hole. This kind of play is quite similar to golf, so it is called boomerang golf. Another way of boomerang clearance is to shoot boomerang at the tree and calculate the score of entering the hole by selecting dense shrubs as the benchmark. Because boomerang has the ability to fly in curves, if some obstacles are properly selected between benchmarks, it will increase the difficulty of customs clearance and increase the interest.

Boomerang

As the name implies, it will fly back after flying out. Its shapes are trefoil, cross, multi-leaf and so on. As early as the Neolithic Age, cross hunting gear made of hardwood chips appeared in China. Hunters of primitive tribes often use this weapon to attack birds and animals. They found different crosses in constant throwing, which can come and go under the influence of the wind. Acrobatics scholars believe that the earliest acrobatic program in China is boomerang. I'm sure many of you have seen acrobats fly boomerangs over the audience's heads on TV, and then fly back to the performers with a scream, which won a full house applause. (Actually, you can do it with more practice. After the hunter sends a boomerang to his prey, if it doesn't hit the target, the boomerang will magically return to the sender. Nowadays, boomerang has become the darling of Australians today. People use it as a toy and sports equipment for throwing competitions. This sport has been popular in Europe and America, and the World Rebound Championship is held regularly in the northern German town of Kiel. Not only that, it even penetrated into the mainstream culture of Australia, so a special custom appeared. When the guest left, the host bought a boomerang as a parting gift. The gift contains the meaning of blessing, that is, I hope he will fly away and then fly back.

The boomerang is very suitable for modern people as an outdoor leisure and fitness exercise, and can fully experience the unique flying pleasure during the flight. When the flying level improves day by day, it will generate stronger interest and make people tired of playing for a long time.

Indigenous Australians

About40,000 years ago, indigenous people settled in Australia.

Before the arrival of European immigrants, indigenous tribes lived in most parts of mainland Australia. They are good at adapting to the environment, creative and mastering simple and effective technologies. Although their material culture is relatively simple, they have created, preserved and disseminated a lot of detailed knowledge and skills, and "boomerang" is one of them.

The patent of "boomerang" belongs to Australian aborigines and is one of the greatest inventions of aborigines in their long hunting practice. In the early days, the aborigines only used sticks to hunt or catch birds. There are many kinds of sticks, thick, short and blunt, but they can be roughly divided into two types: one is for hitting and the other is for throwing. In the process of using these sticks, indigenous people gradually invented and created to meet the needs of survival and make the quality of life of indigenous people reach a new level. Therefore, "boomerang" has become a symbol of aborigines, which shines brightly in the history of aborigines.

1770, Captain james cook of HMS Endeavour landed on an island near York Island, Australia, and raised the British flag. 1788, Captain arthur phillip led the first fleet of 1500 people to land in Sydney Bay, and began the history of the New British colony, which was desperately resisted by the local aborigines. Backward indigenous people can't compete with guns and gunpowder of civilized people. However, a mysterious and deadly weapon in the hands of indigenous people-"boomerang", has made Europeans pay a heavy price. After studying this mysterious weapon carefully, they found that this mysterious thing actually used very advanced aerodynamic principles. Europeans who love sports began to carefully transform this mysterious thing, so it became popular today.