The weather is too hot and you want to "fly up to the sky" to teach the sun a lesson? Here are three ways to do it

"I want to fly up to the sky and stand shoulder to shoulder with the sun..."

Flying has always been a dream hidden deep in the hearts of human beings. Because of this dream, we have kites, hydrogen balloons, and... modern aircraft.

If you look through books and documents related to aerospace, you will find that: currently, there are only three ways for humans to fly into space!

No.1

Becoming lighter than air

Under standard conditions, the average relative molecular mass of air is 29. As long as you can find something with a smaller molecular weight than air, it can float in the air. If it is "light" enough, it can even make people "fly", such as hydrogen balloons, helium balloons, etc.

Lighter-than-air aerostat, Image source: Chinese Academy of Sciences

China's first hydrogen balloon was made by Hua Hengfang, a military expert in the late Qing Dynasty. His method of making hydrogen balloons seems very simple today. He used strong acid and metal to react to produce hydrogen, and then filled the balloon with hydrogen.

"Balloon Release at the Military Academy" - Hua Hengfang's balloon launch

The hot air balloons we are familiar with use hot air to replace hydrogen or helium, which are lighter than air. Since the density of the gas is inversely proportional to the absolute temperature, the density and mass of the gas can be reduced by increasing the temperature, thus achieving the purpose of being lighter than air. The ancient Kongming lantern is the earliest hot air balloon.

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Be it hydrogen balloons or hot air balloons, although both are good, it is difficult to control their flying direction. In order to better control the flying posture and direction, people designed and manufactured airships based on the idea of ​​"becoming lighter".

The airship is filled with a gas lighter than air (usually helium, which is safer) to provide lift, and then engines are installed on the airship to provide propulsion power for the airship's horizontal movement and thereby control its flight attitude.

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No.2

Create air pressure difference to get airborne

When an object is flying in the air, making the air flow rate on the upper surface greater than that on the lower surface, forming an air pressure difference, is also a way to achieve flight.

When there is an air pressure difference between the upper and lower surfaces, the object can obtain a lift. If this lift is large enough to overcome the earth's gravity, the object can leave the ground!

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Birds use the difference in air pressure to fly. Birds have natural physical conditions for flying: wings that cleverly use aerodynamics, a skeleton structure composed of hard and light hollow bones, well-developed chest muscles, a unique respiratory system, and light and smooth feathers.

The main reason why birds can fly is because they have mastered the flying knowledge of "creating air pressure difference". When a bird glides in the air, the air forms a pressure difference between the upper and lower parts of its wings, generating an upward lift, which can hold the bird in the air and prevent it from falling.[6]

In addition, birds use active flapping flight to generate a certain amount of aerodynamic lift. When the wings of birds flap, they are equivalent to the propellers or jet propulsion devices of an airplane, allowing birds to maintain a continuous flight state.

In fact, in the early days, the wooden bird made by Lu Ban and the flapping-wing aircraft made by Leonardo da Vinci both attempted to imitate the flying state of birds to achieve the purpose of taking off, but their flight plans all ended in failure. The reason was that they lacked understanding of the principles of aerodynamics and blindly imitated the geometric shape and flapping form of bird wings.

Leonardo da Vinci's ornithopter drawing. Image source: Britannica

However, when people began to understand aerodynamics and developed fixed-wing aircraft like kites, the modern means of transportation that successfully flew into the sky, the airplane, was born. The flying principle of airplanes and kites is the same, both of which create a pressure difference through the different airflow speeds on the upper and lower surfaces of the wing, thereby obtaining lift.

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In short, whether it is birds, kites, propellers, jets or helicopters, they all fly by "creating pressure difference", but the ways they obtain the conditions for ascent are different.

No.3

Spraying matter downwards to gain momentum

If our goal is to fly out of the earth, then the first two methods may not be applicable, because space is a weightless environment, there is basically no air and no pressure difference. Therefore, we need to use other forces to achieve flight.

The rocket takes off. Image source: China's manned spaceflight official website

For rockets, since the rocket carries not only fuel but also oxidizer, the engine does not need air to work.

In addition, the fuel and oxidizer burn in the engine combustion chamber to produce a large amount of high-pressure gas, which is ejected from the engine nozzle at high speed. The rocket then obtains a reaction force opposite to the gas jet, allowing the rocket to take off.

This method of ejecting matter downward to achieve flight was actually clearly described by Newton as early as the 17th century: "If a certain mass is thrown backward at a certain speed, it will be pushed by a reaction force and accelerate forward."

In modern aerospace, in order to achieve higher flight goals, designers do not simply use one of these three "flying" methods to build aircraft, but use them in combination. For example, in order to achieve faster flight speeds, supersonic fighter aircraft use ramjet engines that eject material backwards.

Although the flying methods that humans have mastered so far are nothing more than "becoming lighter than air", "creating air pressure differences", and "jetting matter downward to gain power for flying", in the near future, more people will surely discover more practical ways to fly into the sky!

The sky is like a dome, covering the vast expanse of land. A large part of our activity map should be the sky. There are too many unknown things in the sky. When we work hard to eliminate these unknowns, human civilization will be greatly developed!

This article is produced by Science Popularization China, produced by Xiao Junbai (Institute of Optoelectronics Technology, Chinese Academy of Sciences), and supervised by China Science Popularization Expo

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