Chapter 983 Mach number 0.985
Although the most critical turbulence model in the whole process was not developed by Liu Hongbo himself.
But this did not affect his feeling of pleasure at this time.
Refreshing.
Anyone dissatisfied?
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For a moment, a room of French people looked at each other in silence, while Liu Hongbo was still under the effect of the "Mr. Chang Experience Card".
The entire computer room became inexplicably quiet.
Only the humming sound from the air conditioning and fans running.
Of course, an experience card is an experience card after all.
In fact, what makes Liu Hongbo feel proud is different from what shocks the French.
For Europeans, naming a phenomenon or equation after a person's surname is a common thing.
What's more, this is not a formula or theorem that is universally applicable. Even if it is very effective, it can only make some reputation in the aviation industry... and at most the chemical industry.
It’s really not worth the fuss.
What really surprised them was the picture itself.
To be more precise, that "dimple" part.
“This flutter boundary curve...is actually discontinuous?”
Ed Steiner put his eyes closer, as if he wanted to get into the screen:
< br>"And the shape of the depression is also a bit... strange..."
I didn't feel it at first glance, but after zooming in and taking a closer look, you can find that the entire curve is actually divided into different parts. Three connected sections.
Moreover, the degree of "denting" was much higher than they had estimated beforehand.
Almost 20% of the equivalent airspeed.
If the situation is as shown in the image, then the predetermined goal, which is to achieve the maximum usable speed of the wing to Mach 0.95, seems to be theoretically impossible to achieve.
This is not good news.
Compared with those large passenger aircraft that can easily weigh several hundred tons, business jets have greater restrictions in terms of structure and volume, and are often forced to adopt more radical airfoil designs in pursuit of range. Therefore, in terms of speed, On the contrary, it is more likely to be restricted.
For example, the current signature Falcon 900 series of Dassault Aviation’s civil division has a cruising speed of only about Mach 0.8.
Not even as good as the Boeing 747.
And Ed Steiner hopes to solve this pain point on Falcon Z.
In fact, the time of rich people may not be very valuable.
But they are often more willing to spend money to save time.
So it's a pretty good gimmick.
Even the early publicity that has been released so far has focused on high speed.
Although no specific numerical value was mentioned.
But if you operate like a tiger at once and only come up with a product with a cruising speed similar to that of an ordinary passenger plane, it is obviously impossible to meet the needs of potential customers.
However, this cannot be blamed on Ed Steiner for not knowing.
The study of unsteady aerodynamic forces is itself a very complex multidisciplinary problem.
Normally, not to mention calculations, even ground simulations cannot fully restore the actual situation.
But if there is a problem like supersonic pitting that needs to be tested, it is impossible to really let the pilot reach the flutter boundary. It will be settled once the situation is about the same.
Therefore, the results of all current empirical models are conservative.
“That’s right.”
Liu Hongbo also returned to work at this moment and immediately replied:
"Although supercritical airfoils can achieve a higher lift-to-drag ratio, the flow conditions in the speed range near the speed of sound are more complicated than conventional airfoils. Much more, and the final manifestation in the flutter characteristics is a more serious flutter speed sag."
"Doesn't that mean..."
Ed Steiner wanted to say something. What, but Liu Hongbo stretched out his hand to stop him, indicating that he still had something to say:
"In addition, flutter speed sag is not the only abnormal problem that occurs in supercritical airfoils in the transonic range. .”
"We conducted wind tunnel tests on several wings with typical characteristics and found that the flutter phenomenon that occurs in the transonic region, especially near the bottom of the dimple, is more complex, such as at certain Mach numbers and specific angles of attack. , a series of phenomena such as beat response, limit cycle oscillation, stall flutter, single degree of freedom flutter, burst response, etc. appeared successively or simultaneously..."
This sentence made some people feel a little embarrassed. ——
The Chinese are obviously not only good at numerical calculations, but they are not far behind in terms of wind tunnel testing...it can even be said to be ahead of others.
Fortunately, Liu Hongbo had already forgotten the little incident and quickly continued:
“Our model can relatively accurately restore the test results of limit cycle oscillation. It is the discontinuous section of the curve you just mentioned, but it is also powerless against other phenomena with worse regularity.”
"And Doctor, you should also know that even wind tunnel testing can only simulate the real response of this speed range qualitatively, not quantitatively..." Listening to Liu Hongbo's explanation, Ed Steiner felt a little numb. .
He even began to feel a little lucky that he had made the decision and got rid of the whole task of wing design.
In fact, France was the first country in the world to use a two-degree-of-freedom rudder model for ground flutter simulation.
Or strictly speaking, Dassault.
But because of this, it was easier for Ed Steiner to imagine what kind of hell drawing the other party was describing.
Considering that today’s time is precious, he decided to temporarily abandon the middle part and go straight to the topic:
“So...since the design requirement of Mach number 0.95 cannot be achieved, what are the specific results of your design? How much is it? "
Of course Ed Steiner has never read the Analects of Confucius, but he also knows that it depends on it and the truth behind it.
Therefore, although the target proposed on the surface is 0.95, there is actually a slightly lower psychological expectation.
It's just that I've never mentioned it to anyone else before.
If the Chinese people can achieve this minimum threshold, then in order not to delay the project schedule, he is willing to accept this design plan.
However, Liu Hongbo's reaction was completely beyond Ed Steiner's expectation:
"Wait..."
The former frowned:
< br>
"I just introduced some problems encountered during the design process...but I didn't say that the design requirements were not met?"
"Ah?"
Ed Steiner's side Even the psychological preparation was done, but I heard this answer.
The great sorrow and great joy in just half an afternoon made the old man's heart feel a little overwhelmed.
He subconsciously knocked on his somewhat stuffy chest and pointed at the screen in front of him:
"But this picture..."
"This picture is the result of our simulation using the original airfoil provided by you. It is used to verify the accuracy of the calculation method, not the design result..."
Liu Hongbo raised his forehead and shook his head helplessly, suddenly feeling a sense of powerlessness as when he returned to the classroom to teach students:
"If you continue to scroll down, you can see that after optimization in different directions There are several design plans..."
Until this moment, everyone remembered that this seemed to be just the first picture they saw.
But because the amount of information in it was too large, it took a long time...
As the document scrolled down, design plans of different shapes and corresponding calculation results were presented to everyone one after another. .
“At the beginning, our idea was to try to use limit cycle oscillation, that is, to control the oscillation within the linear range, so that it can be offset by the active suppression system. In theory, the chatter can be reduced The critical speed is infinitely close to Mach 1, which is about... 0.998."
A group of people sat in front of the screen, and Liu Hongbo stood behind them, introducing the design process slowly:
"However, later we realized that the above requirements can only be met when the curve of the limit cycle oscillation amplitude changing with the airflow speed is perpendicular to the velocity axis. And on this basis, as long as there is a slight deviation, it will enter Subcritical bifurcation, thereby inducing explosive limit cycle oscillations, which is too risky for aircraft..."
"So, we finally decided not to solve the problem of transonic dents for the time being, but to squeeze the location of the dents backward so that the aircraft can be relatively safely on the supercritical bifurcation curve, so that harmonic balance can be used The flow control equation can be solved by the method, and then combined with the structural motion equation, the nonlinear flutter can be solved."
"Considering that the flutter velocity sag is caused by the influence of air compressibility, only the airfoil needs to be optimized. The compression effect on the forward flow..."
"..."
After excluding some core secrets that cannot be disclosed, Liu Hongbo spent about 40 minutes giving a general introduction to the entire optimization process. Again.
At this time, the design file was also flipped to the bottom position.
"Although we have made great efforts, the results still cannot reach the data of linear limit cycle oscillation. After all, this is determined by theoretical limitations and cannot be broken through with simple engineering means..."
Liu Hongbo used Said in a very regretful tone.
Everyone else was listening in rapt attention, and their hearts naturally sank following his tone.
However, before sinking to the bottom, Ed Steiner turned to the final design result.
At the same time, Liu Hongbo also changed the topic:
"Unfortunately, the ultimate speed of our final optimized design only reached Mach 0.985..."
A dozen pairs of eyes that were originally staring at the computer screen turned to look at Liu Hongbo almost simultaneously.
“???”
Do you think this is human language?
Only 0.985?
Although due to power limitations, economy and safety considerations, the aircraft cannot really fly at the limit speed that the wings can withstand.
But a supercritical wing with such a high ultimate speed basically means that it can achieve a maximum cruise speed of Mach 0.9-0.95...
Compared to the previous generation of business jets , which is already an improvement of almost 15%.
Ed Steiner took a deep breath and decided not to care about the Chinese man's seemingly weird tone:
"That's enough...0.985...that's enough..."< br>
(End of this chapter)