Chapter 983 Mach number 0.985
Although the most critical turbulence model in the entire process was not developed by Liu Hongbo himself.
But this did not affect his feeling of pleasure at this time.
Feels refreshed.
Is anyone dissatisfied?
?Then you can also find a way to find an awesome junior brother!
For a time, 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.
?The only sound left is the hum of the air conditioner and fans running.
?Of course, an experience card is an experience card after all.
Actually, 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 useful, it can only make some reputation in the aviation industry... and at most the chemical industry.
It’s really not worth making any fuss about.
What really surprised them was the picture itself.
More precisely, it’s the “dimple” part.
“This flutter boundary curve...is actually discontinuous?”
?Ed Steiner moved his eyes closer, as if he wanted to get into the screen:
“And the shape of the depression is also a little... strange...”
?At first glance, I didn’t feel it, but after zooming in and taking a closer look, I can see that the entire curve is actually divided into three non-connected segments.
Furthermore, the degree of “denting” was much higher than they had estimated beforehand.
Nearly 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 unachievable.
This is not good news.
Compared to those large passenger aircraft that weigh several hundred tons, business jets have greater restrictions in terms of structure and volume. They are often forced to adopt more radical airfoil designs to pursue range, so they are more likely to be restricted in terms of speed.
?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.
?Ed Steiner hopes to solve this pain point on Falcon Z.
In fact, rich people’s time may not be very valuable.
But they are often more willing to spend money to save time.
So this is a pretty good gimmick.
Even the early publicity that has been launched has focused on high speed.
Although no specific numerical value is 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.
??However, if a problem like supersonic pitting is really required 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.
Hence, the results of all current empirical models are conservative.
“That’s right.”
Liu Hongbo also entered working mode again 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 conventional airfoils, and the final manifestation in the flutter characteristics is a more severe flutter velocity sag. "
“Doesn’t that mean…”
?? Ed Steiner wanted to say something, but Liu Hongbo reached out to stop him, indicating that he still had something to say:
“In addition, flutter velocity sag is not the only abnormal problem that occurs with supercritical airfoils in the transonic speed 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, and burst response 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 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, which is the discontinuous section of the curve you just mentioned, but it is equally powerless for other phenomena with less regularity.”
“And Doctor, you should also know that even wind tunnel testing can only simulate the real response in 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 himself 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 precisely because of this, it was easier for Ed Steiner to imagine what kind of **** 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?”
Of course Ed Steiner has never read The Analects of Confucius, but he also knows 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 he is willing to accept this design plan in order not to delay the project schedule.
However, Liu Hongbo’s reaction was completely beyond Ed Steiner’s expectations:
"etc…"
The former frowned:
“I just introduced some problems encountered during the design process...but I didn’t say that I couldn’t meet the design requirements?”
“Ah?”
Ed Steiner had already done some psychological preparation, but he heard this answer.
The great sorrow and 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. It is not a design result..."
Liu Hongbo raised his forehead and shook his head helplessly. Suddenly, he felt as helpless as when he returned to the classroom to teach students:
“If you continue to scroll down, you will see several design solutions optimized in different directions...”
It was only then that everyone remembered that this was just the first picture they saw.
It just took a long time because the amount of information in it was too large...
?As the document scrolls down, design plans of different shapes and corresponding calculation results are 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 a linear range, so that it can be offset by the active suppression system. In theory, the critical speed of flutter can be infinitely close to Mach 1 , about...0.998."
A group of people sat in front of the screen, while Liu Hongbo stood behind them and introduced the design process in a leisurely manner:
"However, we later realized that the above requirements can only be met when the curve of the limit cycle oscillation amplitude changing with the airflow velocity is perpendicular to the velocity axis. On this basis, as long as there is a slight deviation, it will enter the subcritical fraction. 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 this 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 impact of air compressibility, it is only necessary to optimize the compression effect of the airfoil 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.
At this time, the design file has also been flipped to the bottom position.
“Despite our 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 simple engineering means..."
Liu Hongbo said in a very regretful tone.
??As for everyone else who was listening, 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 subject:
“Unfortunately, the ultimate speed of our final optimized design only reached Mach number 0.985…”
??A dozen pairs of eyes that were originally staring at the computer screen turned to look at Liu Hongbo almost at the same time.
???”
Do you hear what you are saying is human?
Only reached 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 a maximum cruising speed of Mach 0.9-0.95 can be achieved...
Compared with the previous generation of business jets, this 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…”
(End of this chapter)