Chapter 29 The Regret of Domestic Passenger Aircraft
You must know that 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 , but after all, it cannot be regarded as a big project. Even if the time for experiments is included, it 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 conducted computational simulations.
That's quite reasonable.
So if you want to accumulate experience points quickly, you have to 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, the green velvet cloth covering the tabletop and the transparent glass pressed on the top can still be seen. It can be seen that they are all newly replaced.
"Okay, thank you, senior brother."
Chang Haonan said and 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 busy 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.
His luck was good, and 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.
And 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, what step have you taken on the subject of flutter analysis of large aspect ratio wings?"
Fang Zhen raised his head and immediately saw the file box in Chang Haonan's hand.
“Following Yao Bo...oh no, following your previous ideas, we completed the finite element model of the wing on the MSC.NASTRAN software, and then calculated the first five natural vibration modes of the wing and Natural frequency, now Yao Bo and I are trying to use the p-k method and the v-g method to conduct flutter analysis respectively. "
It must be said that as students of Du Yishan, Fang Zhen and Yao Mengna do have something to say, starting from the replacement of new ideas. It has been less than a month so far, 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 wasted a lot of time on 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 again. Got him.
Linear analysis and nonlinear analysis are one word different, but the applicable theoretical foundations are vastly 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 relationship based on the small deformation assumption that Chang Haonan and Yao Mengna said before The theory is no longer applicable and 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 obviously even the Americans cannot bear this consumption...
This is also one of the reasons why wing design is 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 analysts
In short, Fang Zhen does not feel that Chang Haonan has really made any breakthrough in this area, but he will not dampen the other party'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.
Furthermore, 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 have an impact on 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, causing the control surface to deflect, and the control surface deflects The control force generated is finally fed back to the aircraft, thereby suppressing the unstable vibration of the system.
So it is also called chatter active 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 just said hello briefly and continued to write on the draft paper with his head down. Do the math.
Because he discovered that the wing design provided in the information had far greater potential than he had previously imagined!
“What an idea...”
Compared with the wing actually installed on the MA-60, the wing in front of me is lighter, has a larger aspect ratio, and has A winglet.
This means lower flight resistance, faster flight speed and lower fuel consumption.
For a passenger aircraft that is 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 for this reason, in the previous life, MA60 finally chose the original conservative plan.
But now, such a regret will never happen again!
(End of this chapter)