You know, Chang Haonan completed the J-7 improved design, but only provided 20 points of engineering experience, although this fan uses some aero engine design ideas, but after all, it can't be regarded as a big project, even if you add the time to do experiments, it took two days of effort.

Then the only possible explanation is obvious - a study that has been experimented and finally landed has more experience than a study that has only done computational simulations.

It makes perfect sense.

Therefore, if you want to accumulate experience points quickly, you need to get in touch with the actual project as soon as possible.

Gotta join the group as soon as possible!

Thinking of this, Chang Haonan turned around and returned to the public phone booth just now and dialed the phone number of Du Yishan's office.

……

Early the next morning, in the aircraft department building, in the student office of Du Yishan's research group.

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

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

You can see that the table has been specially cared for, although the legs and drawers can still be mottled, but the green flannel cloth on the top and the transparent glass pressed on the top can be seen to be fresh.

"Okay, thank you, brother."

Chang Haonan said and put some of his personal belongings, such as pen holders, water cups, scratch paper, etc., on the table.

"Then junior brother, you can take a look at the books or the materials on the bookshelf over there, just be careful not to get dirty and broken, I'll get busy first."

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

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

As luck would have it, he soon found two archival boxes on the second level of the bookshelf labeled "Y-7-200A Airliner Wings".

Although Yao Mengna did not clearly state the object of their research that day, the only one who could match the current point in time was this passenger aircraft project that was later called the MA60.

MA60 is the first time in China to fully adopt the world's advanced standards for passenger aircraft design, which can basically be regarded as the starting point of China's modern passenger aircraft industry.

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

An overly conservative wing design is one of them.

And this time, he is ready to make up for this regret himself.

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

"Senior Brother Fang, what step have you achieved now on the topic of wing flutter analysis of the large aspect ratio?"

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 idea, we built the finite element model of the wing on the MSC.NASTRAN software, and then calculated the first five natural mode shapes and natural frequencies of the wing, and now Yao Bo and I are trying to perform flutter analysis using the p-k method and the v-g method, respectively. ”

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

Chang Haonan was secretly glad that he had asked such a sentence in advance, otherwise he would have to spend a lot of effort doing repetitive work.

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

"Hmm...... You should be almost done with the linear flutter analysis, and I'm going 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 very different.

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

And there is no particularly safe way to do it.

You know, once flutter is induced, the best result is also permanent damage to the aircraft structure, the slightest carelessness will destroy the aircraft and kill people, and there is almost no way to rely on pilot skills to save it like stalling or tailspin.

Therefore, it is common to add a correction directly to the results of the linear analysis and leave enough safety margin in the results.

Of course, if there is an infinite amount of money and time, it is possible to build every type of wing and pile up test flight data to produce miracles.

But apparently this consumption is not enough even for Americans......

This is one of the reasons why wing design tends to be conservative in general.

Chang Haonan opened his mouth to do nonlinear flutter analysis.

Although he provided the previous idea of linear analysis, the use of the dipole grid method is after all a summary of the existing theory.

Nonlinear analysis, on the other hand, relies heavily on the analyst's engineering experience

In short, Fang Zhen didn't think Chang Haonan had really made any breakthroughs in this regard, but he wouldn't discourage the other party's enthusiasm.

"That's fine, you can try different correction methods, wait for the linear analysis results to come out, let's compare with the experimental results, and then make the correction factor more accurate."

This time, Chang Haonan didn't answer, just lowered his head and silently picked up the pen.

As a reborn, he knew very well that the current wild path of relying on correction factors could not be accurate.

Also, all flutter analyses in this era were 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 have an impact on the aeroelasticity of the wing.

More importantly, later aircraft designers have proved that the distribution of aerodynamic loads in the flow field of the aircraft can be changed by the control torque generated by the deflection of the control surface, and then the original self-excited oscillation coupling mechanism can be destroyed, so that the entire closed-loop system can be re-stabilized.

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, and the control force generated by the deflection of the control surface is finally fed back to the aircraft, so as to suppress the unstable vibration of the system.

Therefore, it is also called active flutter suppression.

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

Based on 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 all this in his memory!

……

As the minutes ticked by, after the regular breakfast time had passed, a few more people walked into the office.

Fang Zhen made a brief introduction, but at this time, Chang Haonan's thoughts were already immersed in the information spread out on the table, and he simply said hello and continued to bow his head and write on the scratch paper.

Because he found that the wing design provided in the information has far more potential than he imagined!

"Very thoughtful......"

The wing in front of me is lighter than the one actually mounted on the MA60, has a larger aspect ratio, and has a winglet.

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

For an airliner ready for commercial operation, this is the core competitiveness.

Of course, at the trade, the aerodynamic characteristics of such a wing are more complex, the design is more difficult and risky, and flutter is more prone to occur.

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

But now, such regrets will not happen again!