Chapter 983 Mach Number 0985
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 being happy at the moment.
Refreshed.
Anyone dissatisfied?
Then you can also try to find a great junior fellow student!
For a moment, the French people in the room looked at each other in silence, while Liu Hongbo was still enjoying the effect of "Mr. Chang Experience Card".
The entire computer room suddenly became inexplicably quiet.
The only sound left was the hum of the air conditioner and fan.
Of course, the experience card is just an experience card after all.
In fact, what made Liu Hongbo feel proud was not the same as what shocked the French.
For Europeans, naming a phenomenon or an equation after a person's surname is a very common thing.
What's more, this is not a universal formula or theorem. Even if it is very useful, it can only make a name for itself in the aviation industry... and at most in the chemical industry.
It's really not worth making a fuss about.
What really surprised them was the picture itself.
To be more precise, it's the "pit" part.
"This flutter boundary curve... is actually discontinuous?"
Ed Steiner put his eyes close to the screen, as if he wanted to get into it:
"And the shape of the depression is a bit... strange..."
You didn't notice it at first glance, but after zooming in and taking a closer look, you can see that the entire curve is actually divided into three unconnected segments.
Moreover, the extent of the "sag" was much higher than they had previously estimated.
Almost 20% of the equivalent airspeed was reached.
If the situation is really as shown in the image, then the goal determined in advance, that is, to make the wing's maximum usable speed reach Mach 0.95, seems theoretically impossible to achieve.
This is not good news.
Compared with large passenger aircraft that weigh hundreds of tons, business jets have greater limitations in structure and size, and are often forced to adopt more radical wing designs in pursuit of range, so they are more easily restricted in speed.
For example, the current signature Falcon 900 series of Dassault Aviation's civil division has a cruising speed of only around Mach 0.8.
Not even as good as a Boeing 747.
Ed Steiner hopes to solve this pain point on the Falcon Z.
In fact, the time of rich people is not necessarily valuable.
But they are often more willing to spend money to save time.
So it was a pretty good gimmick.
Even in the early publicity that has been released, high speed has been emphasized.
Although no specific values were mentioned.
But if you work hard and finally 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 being clueless.
The study of unsteady aerodynamics is itself a very complex multidisciplinary problem.
Normally, not to mention calculations, even ground simulations cannot fully restore the actual situation.
However, if we really want to test a problem like a supersonic pit, it is impossible to let the pilot reach the flutter limit. We can only stop when the situation is almost the same.
Therefore, the results of all current empirical models are conservative.
"Yes."
Liu Hongbo also returned to work mode 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 much more complicated than those of conventional airfoils, which ultimately manifests itself in the flutter characteristics as a more severe flutter speed sag phenomenon."
"Doesn't that mean..."
Edsteiner wanted to say something, but Liu Hongbo stopped him with his hand, indicating that he had something else to say:
“In addition, flutter speed sag is not the only abnormal problem that occurs with supercritical airfoils in the transonic range.”
"We conducted wind tunnel tests on several wings with typical characteristics and found that the flutter phenomenon in the transonic region, especially near the bottom of the pit, is more complicated. For example, 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, and burst response appear successively or simultaneously..."
This sentence made some people feel awkward.
It is clear that the Chinese are not only good at numerical calculations, they are also not behind in the level of wind tunnel testing...it can even be said that they are ahead of others.
Fortunately, Liu Hongbo had already forgotten about the little thing just now and quickly continued:
"Our model can relatively accurately reproduce the test results of the limit cycle oscillation, which is the discontinuous part of the curve you just mentioned, but it is also powerless for other phenomena with less regularity."
"And you should also know, Doctor, that even a wind tunnel test can only qualitatively, but not quantitatively, simulate the actual response of this speed range..."
Listening to Liu Hongbo's explanation, Edsteiner felt a little scalp tingling.
He even began to feel fortunate that he had made the decision to offload the wing design work.
In fact, France was the first country in the world to use a two-degree-of-freedom control surface model for ground flutter simulation.
Or strictly speaking, it was Dassault.
But because of this, it was easier for Edsteiner to imagine what kind of picture of hell the other party was describing.
Considering that today's time is precious, he decided to temporarily abandon the middle part and get straight to the point:
"So... since we can't meet the design requirement of Mach number 0.95, what is your design result?"
Of course, Edsteiner has never read the Analects, but he also knows the principle of "aiming high and achieving high".
Therefore, although the apparent target is 0.95, there is actually a slightly lower psychological expectation.
But I've never mentioned it to anyone else before.
If the Chinese 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 Edelsteiner's expectations:
"and many more……"
The former frowned:
"I just introduced some problems encountered during the design process...but I didn't say that the design requirements could not be met?"
"Ah?"
Edsteiner had even prepared himself mentally, but in the end he heard this answer.
The great sorrow and joy that had occurred in just half an afternoon made the old man's heart a little overwhelmed.
He subconsciously tapped his chest, which was a little stuffy, and pointed at the screen in front of him:
“But this picture…”
"This picture is the result of our simulation using the original airfoil you provided. It is used to verify the accuracy of the calculation method. It is not the design result..."
Liu Hongbo held his head and shook his head helplessly. He suddenly felt as powerless as if he was back in the classroom teaching his students:
"If you continue to scroll down, you will see several design solutions that have been optimized in different directions..."
It was not until this time that everyone remembered that this seemed to be just the first picture they saw.
It's just that because the amount of information in it is too large, it took a long time...
As the document scrolls down, various design plans and corresponding calculation results are presented to everyone.
"At first, our idea was to try to use the limit cycle oscillation, that is, to control the oscillation response within the linear range, so that it can be offset by the active suppression system. In theory, the critical flutter speed can be infinitely close to Mach 1, which is about... 0.998."
A group of people sat in front of the screen, while Liu Hongbo stood behind them, slowly introducing the design process:
"However, we later realized that the above requirements can only be met when the curve of the change of the limit cycle oscillation amplitude with the airflow velocity is perpendicular to the velocity axis. If there is any deviation from this basis, it will enter the subcritical bifurcation, thereby inducing explosive limit cycle oscillations, which is too risky for aircraft..."
"So, we finally decided not to solve the transonic pit problem for the time being, but to squeeze the location where the pit appears backward so that the aircraft can be relatively safely on the supercritical bifurcation curve. In this way, the harmonic balance method can be used to solve the flow control equations, and then combined with the structural motion equations to solve the nonlinear flutter."
“Considering that the flutter speed sag is caused by the compressibility of the air, we only need to optimize the compression effect of the wing surface on the forward flow…”
"..."
After excluding some core secrets that could not be disclosed, Liu Hongbo spent about 40 minutes to give a brief introduction to the entire optimization process.
At this time, the design file was also flipped to the bottom.
"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 by simple engineering means..."
Liu Hongbo said this in a tone of great regret.
The others were already listening intently, and their hearts naturally sank as they followed his tone.
However, before it sank to the bottom, Edsteiner turned to the part about the final design results.
At the same time, Liu Hongbo also changed the subject:
"Unfortunately, the design we finally optimized only reached a maximum speed of Mach 0.985..."
More than a dozen pairs of eyes that were originally staring at the computer screen turned to look at Liu Hongbo almost at the same time.
"???"
Listen to what you are saying, is this human language?
Only reached 0.985?
Although due to power limitations, economy and safety considerations, it is impossible for an aircraft to actually fly at the maximum speed that the wings can withstand.
But a supercritical wing with such a high ultimate speed basically means that it is completely possible to achieve a maximum cruising speed of 0.9-0.95 Mach...
Compared with the previous generation of business jets, this is an improvement of almost 15%.
Edsteiner took a deep breath and decided not to bother with the Chinese man's strange tone:
"That's enough... 0.985... That's enough..."
