Chapter 29 The Regret of Domestic Airliners

You know, the J-7 improved design completed by Chang Haonan only provided 20 points of engineering experience. Although this fan used some aero-engine design ideas, it cannot be regarded as a big project after all, even if it is added with The experiment only took two days.

Then the only possible explanation is obvious - a research that has been experimented and finally implemented will gain more experience than a research that only performed computational simulations.

That's quite reasonable.

Therefore, if you want to accumulate experience quickly, you must get in touch with actual projects as soon as possible.

Gotta join the group as soon as possible!

Thinking of this, Chang Haonan turned back to the public phone booth and dialed Du Yishan's office.

……

Early the next morning, in the Aircraft Department Building, in the student office of Du Yishan's research group.

"Junior brother, this will be your place from now on."

Fang Zhen brought Chang Haonan to a desk by the window and patted him on the shoulder.

It can be seen that this table has been specially taken care of. Although mottled marks can still be seen on the table legs and drawers, it can be seen that the green velvet cloth covering the tabletop and the transparent glass pressed on the top are all new. Up.

"Okay, thank you, senior brother."

Chang Haonan said as he put some of his personal items such as pen holders, cups, draft papers, etc. on the table.

"Junior brother, you can read the books or the materials on the bookshelf over there first. Just be careful not to get them dirty or broken. I'll go and get started first."

After arranging Chang Haonan's position, Fang Zhen returned to his position and started working hard.

The former came to the bookshelf, ready to look for materials related to the wing flutter analysis.

With good luck, he quickly found two file boxes labeled "Yun-7-200A Passenger Aircraft Wings" on the second floor of the bookshelf.

Although Yao Mengna did not clearly state the object of their research that day, it seems that the only one that can be matched at this point in time is the passenger aircraft project later known as the MA-60.

The MA-60 is the first domestic passenger aircraft designed to fully adopt standards that are in line with the world's advanced levels. It can basically be regarded as the starting point of China's modern passenger aircraft industry.

But on the other hand, the first time means no experience, so this model actually left a lot of regrets in the original timeline.

Overly conservative wing design is one of them.

But this time, he was ready to make up for this regret personally.

After spending half a minute confirming that there were no words such as "confidential" on the two file boxes, Chang Haonan returned to his desk holding the two boxes of information.

"Senior Brother Fang, on the subject of flutter analysis of large-aspect-ratio wings, what step have you taken now?"

Fang Zhen raised his head and immediately saw the file box in Chang Haonan's hand.

"According to Yao Bo... oh no, according to your previous ideas, we completed the finite element model of the wing on MSC.NASTRAN software, and then calculated the first five natural modes and natural frequencies of the wing. Now Yao Bo and I are trying to use the pk method and the vg method for flutter analysis respectively."

It must be said that as students of Du Yishan, Fang Zhen and Yao Mengna do have something. It has only been less than a month since they changed their new ideas, and they have to learn and do at the same time. Their progress is already quite good.

Chang Haonan was secretly glad that he had asked this question in advance, otherwise he would have spent a lot of time doing repetitive work.

"What, junior brother is going to help us finish the rest of the work?" Fang Zhen asked half-jokingly.

"Well... you should be almost done with the linear flutter analysis. I'm going to try to see if I can get rid of the nonlinear part."

Chang Haonan's answer made Fang Zhen, who had just lowered his head, instantly look at him again.

The difference between linear analysis and nonlinear analysis is one word, but the applicable theoretical basis is completely different.

In some special cases, the elastic deformation amplitude of the aircraft wing will be very large. At this time, the relationship between strain and displacement is no longer linear. The linear theory based on the small deformation assumption that Chang Haonan and Yao Mengna said before is no longer applicable. , new theoretical tools are needed for guidance.

There is no particularly reliable method in engineering.

You must know that once flutter is induced, the best result is permanent damage to the aircraft structure. If you are not careful, the aircraft will be destroyed and people will be killed. It is almost impossible to rely on pilot skills to save it like a stall or spin.

Therefore, a correction is generally added directly to the linear analysis results and a sufficient safety margin is left in the results.

Of course, if you have unlimited money and time, you can build every type of wing and pile up test flight data to create miracles.

But apparently even Americans can’t bear this consumption...

This is one of the reasons why wing designs are generally conservative.

Chang Haonan opened his mouth to do nonlinear flutter analysis.

Although the previous linear analysis ideas were provided by him, using the dipole grid method is, after all, a summary of the existing theory.

Nonlinear analysis relies heavily on the engineering experience of the analyst

In short, Fang Zhen doesn't think Chang Haonan has really made any breakthrough in this area, but he will not dampen the opponent's enthusiasm.

"That's okay. You can try different correction methods. When the linear analysis results come out, we can compare them with the experimental results and make the correction factors more accurate."

This time, Chang Haonan did not answer, but lowered his head and silently picked up the pen.

As a reborn person, he knows very well that the current wild approach that relies on correction factors is impossible to be accurate.

Moreover, all flutter analyzes in this era are based on the overall calculation of the wing as a fixed structure.

But in fact, there are aerodynamic control surfaces such as flaps and ailerons on the wings of the aircraft.

They also affect the aeroelasticity of the wing.

More importantly, later aircraft designers have proven that the distribution of aerodynamic loads in the aircraft flow field can be changed through the control torque generated by the deflection of the control surface, thus destroying the original self-oscillation coupling mechanism and making the entire closed-loop system regain its normal state. to stability.

The principle is actually very simple:

The motion signal of the aircraft is input to the control system through the sensor, and the control signal generated by the control system is input to the actuator to cause the deflection of the control surface. The control force generated by the deflection of the control surface is finally fed back to the aircraft, thereby suppressing the unstable vibration of the system. .

Therefore, it is also called active vibration suppression.

The biggest advantage of this idea is that there is no need to modify the wing design at all, just upgrade the flight control and sensors.

According to the experience of later generations, it is not a problem to increase the critical speed of flutter by about 10%.

And what he has to do now is to reproduce everything in his memory!

……

Time passed by, and after the regular breakfast time, a few more people walked into the office.

Fang Zhen gave a brief introduction, but at this time Chang Haonan's thoughts were already immersed in the materials spread out on the table. He simply said hello and continued to write and calculate on the draft paper.

Because he discovered that the wing design provided in the information had far greater potential than he had previously imagined!

"What a great idea..."

Compared with the wing actually installed on the MA-60, the wing in front of you is lighter, has a larger aspect ratio, and also has a winglet.

This means lower flight resistance, faster flight speed and lower fuel consumption.

For a passenger aircraft ready for commercial operations, this is the core competitiveness.

Of course, as a price, the aerodynamic characteristics of such a wing are more complex, the design is more difficult and risky, and it is more prone to flutter.

Perhaps it is for this reason that in the previous life, MA60 finally chose the original conservative plan.

But now, such regret will never happen again!