After packing up the paragliders floating in the sea, the group hurriedly rowed ashore.

As soon as I got ashore, I saw hundreds of people standing on the beach in an orderly manner.

"What's wrong?"

Liu Jinghong jumped off the boat and asked, saw Li Zhen and Qi stunned, and quickly turned around: "Teacher Lin, are you okay?" Alas, I told you not to be impulsive, and not to take risks rashly after listening to Master Li. You just don't listen. ”

Lin Xiufeng said angrily: "What do you mean by me taking risks? What kind of bullshit did you design? ”

"Teacher Lin, that's not right. You asked me to design it. ”

"When will I ......" Lin Xiufeng said, seeing Li Zhen, he reacted in his heart that Liu Jinghong was throwing the pot. Crashed on the spot: "Get out!" ”

A group of people stood there in disdain, looking at Li Zhen whose face was sinking like water.

Li Zhen glanced at Lin Xiufeng, who was constantly shivering, and sighed: "Old Lin, what are you doing with them." ”

Lin Xiufeng's old face turned red, and he didn't say anything for a long time.

Li Zhen sighed helplessly, looked at the wreckage of the paraglider, and waved his hand: "Gather in the classroom." ”

“……”

Everyone watched Li Zhen leave first, looking at each other, and they were all a little uncertain.

Liu Jinghong sighed: "It's a problem." ”

“……”

Along the way, Li Zhen kept thinking, it seemed that it was really better to be blocked.

I couldn't stop this group of people at all, just like Renaissance University couldn't stop itself in the first place.

After thinking about it for a while, Li Zhen went back to have breakfast and came to the Kyushu Institute of Physics. In the physics institute, everyone had already arrived, and Lin Xiufeng was trembling and holding a bottle of hot soup and drinking it. It's not lightly frozen.

"Do you know what's wrong?"

Li Zhen's first sentence made everyone stupid.

Liu Jinghong shouted: "You shouldn't ignore Master Li's advice......"

"Don't talk nonsense."

Liu Jinghong lowered his head: "I don't know." ”

"Bring me the paragliding drawings."

Someone sent the drawings over, Li Zhen looked at them twice, and threw them away: "Who drew it?" ”

"I ......," Liu Jinghong said with some shame.

Li Zhen looked at him, looked at him for a while, and said to everyone's surprise: "The idea is right." The direction is wrong, there is a problem with the design. There are too many loopholes in it, and you didn't come here for paragliding. ”

The crowd rioted.

This breath ...... There is a problem.

They're all excited, are you going to explain the real thing?

"How did you get that?"

Someone asked.

Li Zhen looked back and wrote four words on the blackboard: gravity and lift.

"I've been thinking about the phenomenon of parachutes these days and have come to a conclusion. It's gravity and lift. You have to get this into account first. ”

"First of all, the parachute has an aerodynamic problem in it, and the pressure per unit area is proportional to the speed of the air flow to the third power. Then open a parachute, and his parachute area is set. The falling process can be understood as the air blowing upwards at the speed at which you land. In other words, you can think of the wind resistance you experience during your descent as if there is wind blowing upwards below. This creates lift.

The lifting force is related to the 3rd power of the landing speed. When the parachute is first opened, the lower the speed is at first, and the greater the lift force will be received by the speed. When the lifting force is greater than the weight of the body + the weight of the parachute, the speed will slow down until the two sides reach equilibrium, so this is why when you jump, there will be a sudden pause. ”

“……”

Qin Enze slapped the table suddenly, but forgot that he still had a plaster cast on his hand, gritted his teeth and stood up in pain, and said with some excitement:

"I see. Is it because when I jumped down, the umbrella was not fully opened at that time. And then after it was completely opened, there was lift immediately, so there was such a stutter? ”

"Yes, so to speak."

Li Zhen said again: "The idea of paragliding is no problem. Aerodynamically, this wing with forward momentum is established. ”

Liu Jinghong stood up excitedly: "So, I'm right." But what's wrong? ”

"The design is going in the wrong direction. You're just thinking about how to move it forward, but you're ignoring that there's a big problem here. It's lift. ”

"It's lift. I've designed an umbrella that's that big. ”

"It doesn't have much to do with size. The shape is wrong. Your theoretical knowledge has not arrived at home. ”

Li Zhen said seriously:

"The paraglider itself can be unpowered, but it can fly, which is theoretical. The reason why it fails is because your paraglider is not full of air at all. The inside has to be filled with air for it to be established. It's like a parachute, and when you jump down, you can imagine which one will be true, a flat parachute, or a parachute that is filled with air inside. ”

The crowd pondered for a while, but still couldn't get an answer.

Li Zhen also carefully recalled the conclusions he had drawn from his research in the past two days, and said:

"My thinking is this: since the paraglider generates speed and lift as it flies through the air, it has to be so that its speed and lift are much greater than its resistance. Therefore, in terms of structure, there should be an airbag in the inner layer of the paraglider jacket. Before it is filled with air, the paraglider has no substantial edges and corners, and once the inner airbag is filled with air, the leading edge of the paraglider will be angular. In this way, when the paraglider flies in the air, the opposing air flow is diverted from the upper and lower parts of the wing surface, the drag is parallel to the wind force of the opponent, and the weight is combined with the air above the wing, so that the paraglider can generate speed to move forward.

Fluid mechanics says this, combined with aerodynamics, you should know this. ”

Liu Jinghong slapped his thigh: "Oops, I know." I don't have an airbag installed. I set it up with a double layer, but I don't have an airbag inside. It's leaking. ”

Li Zhen gave him a blank look and continued:

"I think the upper and lower levels of the paraglider are different in length, and when there is forward speed, the air flows through the upper and lower surfaces of the paraglider, and there are different pressure differences on the different lengths of the planes, and the side with the more pressure pushes against the side with less pressure. When the air flows through the convex surface of the upper layer, the pressure becomes smaller due to the long distance, the flow velocity is faster, and the air flowing through the concave surface of the lower layer becomes larger due to the short distance and the slower flow velocity, so the lift force of the air below pushes the wing face upward, and the pressure difference between the upper and lower layers is the total lift, which is the most basic flight principle. ”

'Rustle'

Everyone below began to take notes.

Li Zhen continued: "The phenomenon that Mr. Lin appeared today should be called a stall. ”

"Stall?"

"Mr. Lin jumped off the cliff with a paraglider on his back today, and the process of running and landing was actually a relative movement with a certain angle of attack and the air, generating aerodynamic force to make him glide in the air for a short time. But he had to manipulate the corner of the umbrella to change direction. Because the paraglider designed by Liu Jinghong is unstable and has big problems in itself. Therefore, when Mr. Lin flies at a certain angle of attack, the airflow through the wing surface is smoothly "attached" to the upper surface.

When he further increases the angle of attack to a certain position, the air flow close to the upper surface begins to separate from a certain position, thus creating an unstable vortex behind it, which will cause the drag to increase rapidly and the lift force to disappear, this phenomenon is called stalling. After the stall, the maneuvering deteriorates, and as the operation gets worse and worse, and Liu's paraglider does not have a device to limit the angle of attack at all, so it will fall faster and faster, becoming a vertical landing.

In the conditions of the sea breeze, the angle of attack has almost become a folded shape. So he started spinning like crazy. Eventually fell into the sea. ”

“……”

Everyone listened to this explanation and was speechless for a while.

There are so many reasons for this?

Lin Xiufeng looked at Liu Jinghong with a resentful expression.

Liu Jinghong looked at Li Zhen speechlessly: "Then Master Li, since you know, why didn't you say it earlier." ”

Li Zhen gave him a blank look and didn't pay attention to him at all.

There's no way to answer. Because of these problems, it didn't take long for Li Zhen himself to figure it out......