Back in 1978, I was admitted to Northwestern Polytechnical University.

Chapter 4 Senior brothers, you don't understand? Let me explain.
The surname Sha is very rare. At Northwestern Polytechnical University, there is probably only one professor with the surname Sha: Sha Bonan!
Mr. Sha was born in a time of war. After joining the army, he served in the naval aviation force. In the 1950s, as the chief maintenance officer of the aircraft squadron, he participated in the war against North Korea. He was commended three times and also received two Korean military medals.

This experience gave Mr. Sha a special affection for combat aircraft, and he always paid attention to what kind of aircraft the troops needed.

After the war, Mr. Sha went to Harbin Engineering University for further studies, where he studied diligently and worked hard.

Now, Professor Sha is a professor at Northwestern Polytechnical University, but he still pays close attention to front-line fighter jets. When he took his students to Factory 132 for an internship and discovered the defects of the J-7 aircraft, he began to ponder: how exactly can the performance of the aircraft be improved?

After a long period of thought, a gradual approach took shape in his mind: based on the original wing, keep the inner section unchanged, reduce the sweep angle of the outer section, change the delta wing to a double delta wing, increase the wingspan, and improve performance!

With minor modifications and significant results, the project first received support from the school leaders after being reported to the university. Then, the project was reported to Chengdu, where it received support from Factory 132. After several years of strong support from the Air Force, the aircraft, codenamed J-7E, was finally approved by the General Staff. Moreover, it was the first time in the country that an aircraft factory and a university had collaborated on the research and development!
After the J-7E was built, test flights revealed that its close-range air combat capability reached 80% of that of the F-16!

The Air Force was very satisfied and equipped the aircraft in large quantities. The famous August 1st Aerobatic Team used this aircraft before switching to the J-10, which is enough to show how excellent its maneuverability is!

Now, it should be the time when Mr. Sha is just starting to think about how to improve the J-7!

Since I've encountered this situation myself... of course I have to help!

Watching Niu Guishan walk towards the cafeteria window to get his food, Qin Liang plopped down opposite Professor Sha.

"Teacher Sha, is this model a MiG-21? The spine isn't bulging, and the air intake cone isn't protruding either. It should be an early model of the MiG-21..."

This sentence surprised Professor Sha. This young man knows quite a lot.
"The MiG-21 was an aircraft developed by the Russians to reach Mach 2. The entire aircraft design was almost entirely centered around this goal, which resulted in many flaws. Its small fuselage and limited internal fuel, coupled with the nose air intake further restricting fuel capacity, led to its short range. However, these were not the biggest problems. The key issue was the use of a large-sweep delta wing in order to reach Mach 2, which resulted in very poor low-speed performance and maneuverability that was even inferior to the MiG-19..."

If you prioritize high altitude and high speed, you can't prioritize maneuverability. It's like trying to have your cake and eat it too!

Just then, two students walked over, each carrying an aluminum lunchbox containing a vegetarian dish with two steamed buns on top. Upon hearing Qin Liang's words, they immediately stopped and looked at him.

"Young man, you're showing off your skills in front of Guan Yu! Talking about such superficial things in front of our Teacher Sha, have you been flipping through newspapers in our library?"

"That's right, what you're saying are things that everyone already knows. If you're really knowledgeable, then tell us something we don't know... For example, why are the wings of the MiG-21 different from those of the MiG-17 and MiG-19?"

Professor Sha frowned. These two senior students were his students; they had recently returned from Factory 132. They had learned a lot while at the factory, and now…

He instinctively wanted to speak up and stop him, but... he really wanted to see if the young man in front of him was truly capable or just parroting others, so he remained silent.

Which department are you from?

"Aircraft-related!"

"Oh, so you studied aerodynamics, right?"

"Of course, we learned it in our first-year major courses!" "Which textbook did you use? Or rather, which professor wrote it?"

Why are you asking this? Both senior students were completely baffled.

"Liu Zhizhong, published in 1966," Professor Sha began.

"Oh, in that case, Chapters 8 and 9 of this book introduce the aerodynamic characteristics of subsonic and supersonic airfoils, including airfoils. If you had listened carefully in school, you would know why the Russians chose delta wings back then!"

The two senior students opened their eyes wide.

"On page 232 of the textbook, the advantages and disadvantages of various airfoils are introduced. On page 235, the first-order approximation theory of the supersonic aerodynamic characteristics of thin airfoils is introduced. In this theory, we approximate the shock wave as a Mach wave, and the expansion wave is also approximated as a Mach wave. We also assume that all Mach waves are parallel to each other. Then, from formula 7-44 in Chapter 7, we can obtain the pressure coefficient at any point on the wing, and then..."

Professor Sha nodded. This young man is absolutely right!

"Okay, that's the theoretical formula. Now, let's put it into practice. After World War II, supersonic aircraft generally adopted swept wings. The higher the Mach number, the greater the sweep angle of the wing. However, there is a limit. If it is exceeded, it is easy to cause wingtip stall and wing deformation, resulting in a crash. In order to overcome the problems caused by swept wings, designers began to look for various methods."

The two senior students no longer showed their disdain and listened attentively.

"One option is the F104, with a low aspect ratio and high wing loading. This results in a loss of maneuverability, turning the aircraft into a manned rocket. Another option is the delta wing. Do you remember the formulas I just mentioned?"

The two nodded.

"The delta wing has a large sweep angle, which, according to the formula we just derived, reduces drag during supersonic flight. It also has a long mean chord and a light wing loading, thus ensuring high-speed maneuverability. However, because of its low aspect ratio, it has a low lift-to-drag ratio at subsonic speeds and poor low-speed performance… If you still don't understand, go back and review aerodynamics… You're all very smart, but you just can't apply theory to practice…"

The two senior students blushed, their hands trembling slightly as they held their lunchboxes: "Teacher, we need to go to the library..."

As they talked, the two of them ran away as fast as they could.

Professor Sha's gaze was approving: "Young man, your basic knowledge is very solid, and you can also connect theory with practice. Not bad. You're also in the aircraft department, right? What major? It shouldn't be aerodynamics, right? I didn't see you when I was lecturing..."

"Old Sha, he's... a freshman who just got into Northwestern Polytechnical University..."

Niu Guishan was stunned!

He went to get food, intending to get whatever Qin Liang wanted to eat, but when he turned around, Qin Liang had actually gone to chat with Professor Sha. Well, let them chat, might as well get a couple more dishes so Professor Sha can eat with them.

When he returned after getting his food, he heard Qin Liang explaining to two senior students how aerodynamics theory connects with practice...

This Qin Liang is absolutely a prodigy!
(End of this chapter)