Chapter 381 Designing a DSI air intake duct to practice your skills
Young people who have just joined the workforce are often very passionate, not to mention after being given direction by Chang Haonan himself.
The nose wing is a relatively independent aerodynamic component. Due to its relatively small size, it will be in a contracted state during level flight or low angle of attack flight, and will not affect the normal aerodynamics of the aircraft. The shape will only start and take effect when the angle of attack exceeds a certain threshold.
And this process is obviously impossible to do manually by the pilot.
So where to set this threshold and what the length and width of the nose side strips are to obtain the best side strip vortex is the "homework" left to Lin Tongxin.
As for Sun Huizhong himself, he is naturally responsible for a more important task -
Under the guidance of Chang Haonan, complete the design and optimization of the DSI air inlet of the advanced trainer aircraft.
As a completely non-adjustable type of inlet, the biggest problem with the DSI inlet is that it only has one configuration. Therefore, the optimization interval is constant and must have a large optimization interval. width.
It doesn’t matter if the optimal optimization range of the rectangular baffle air inlet is only Mach 0.01. Anyway, as long as the flight control is strong enough, you can ensure that the air intake efficiency is always in the optimal state by adjusting the position of the baffle. Down.
But if the fixed air inlet is made like this, then slightly changing the flight speed will cause the air intake efficiency to drop sharply, and even the boundary layer cannot be blown away, resulting in strong air intake distortion, which will affect the air intake efficiency. Fly safely.
In short, it requires a high level of design and experience to achieve a satisfactory optimization state for aircraft using DSI inlets.
Besides, Project No. 10 is a key project named at the highest level, and every move it makes will attract the attention of countless people.
After the first flight of the No. 01 prototype using a three-way adjustable air inlet, it will need to undergo major changes, so it is necessary to produce sufficiently convincing data.
As it happens, the common speed range of advanced trainer aircraft is much smaller than that of fighter jets, and the optimization difficulty of the relatively traditional aerodynamic layout is also lower than that of the J-10 using a canard + delta wing configuration.
< br>A perfect opportunity to practice your hands.
"Gong Sun, you must make sure you fully understand what I am going to say next. If there is anything you don't understand, ask me immediately."
Chang Haonan turned on his computer and called up the After preparing several design documents, he motioned to Sun Huizhong to move two chairs from the side.
For him nowadays, many times he no longer has to do everything by himself, especially when there is a helper around him.
“Understood.”
Sun Huizhong quickly sat next to him.
"As for the basic principle of DSI inlet, I have already talked about it when I was in Chengdu. Simply put, it uses the characteristics of the cone shock wave itself to form a strong pressure gradient on the convex hull surface at the lip of the inlet. , blow the fuselage boundary layer at this location directly out of the air intake opening to reduce the negative impact of the boundary layer on the air intake efficiency."
Speaking of this, Chang Haonan called up a picture of his. The pressure distribution map I asked Yao Mengna to help draw some time ago was tested using a DSI inlet model under ideal conditions.
It can be clearly seen from the above that the convex hull structure uses the pressure difference to directly "push" the shock wave out.
After confirming that both Sun Huizhong and Lin Tongxin had no doubts, he officially entered the key design part today:
“It can be seen from this principle that the core of the DSI air intake design is The shape of this convex hull structure, and through theoretical and numerical analysis of the waverider, I summarized two methods of designing the convex hull.”
"I remember you said before that you should select a number of tracking points from the leading edge of the conical surface, and start tracking from these tracking points to the downstream of the flow field..."
Not allowed Needless to say, Sun Huizhong definitely used his heart when he was in Rongcheng.
Even such details can be remembered.
“Yes, this is the generating body method, also called the cone guide method. It is also the basic principle of the most traditional method for designing DSI inlets. It is relatively intuitive and easy to understand, and the design process requires less calculations. , so at the beginning of this year, it was the first method that I took into consideration."
Chang Haonan nodded, but then changed his tone and said:
"But recently I just happened to design a new numerical calculation tool whose calculation efficiency is much higher than that of traditional software, so the limit on the amount of calculation becomes much smaller, so I developed a new numerical calculation tool based on the generative volume method. The second method is the close cone method.”
“The second method?”
Sun Huizhong felt that his brain was a little weak at this time.
Designing a DSI air intake is inherently a difficult task. Even the United States has only installed a relatively simple early version on an improved F16 technology verification aircraft.
The JSF Joint Strike Fighter, the first mass-produced model planned to use DSI inlets, has just begun to be selected.
In short, it is still in the exploratory stage of design.
As a result, you have already summarized a complete set of methodology, and it also includes two solutions?
It just makes no sense...
"Yes, the second method is more accurate than the generating body method, because this method does not need to presuppose a geometry, but directly gives Draw a curve describing the shape of the shock wave at the exit of the waverider, and divide this curve into countless small arcs. Each arc can be considered a part of the shock curve, and the cone that generates the shock wave in this section is the close cone, and the overclose cone The plane that is orthogonal to the axis and is orthogonal to the arc is called an oblique surface..."
"..."
"The corresponding close cone is obtained on each arc of the shock wave curve, and finally a series of streamlines are obtained by defining the leading edge curve tracing. When combined, they form the lower surface of the DSI inlet convex hull."
“This design method is more flexible and can meet the requirements of various flow sizes and various air inlet geometries. That is, it can adapt to any engine by adjusting different shock wave curves without having to let the engine adapt to the intake. Tao, the performance of the final product will naturally be better."
At this point, Chang Haonan turned around and saw two pairs of simple eyes.
Obviously, the jump in thinking just now seems a bit too big.
It is really difficult for engineers with average mathematical theory knowledge to fully accept the principles in a short period of time.
"Let's calculate it using two methods."
He was going to demonstrate it in the simplest way:
"Let's first assume that the shape of the inlet shock wave is a quadratic curve z =-ax^2+b(a>0).”
Chang Haonan wrote a quadratic function directly on the paper.
"Uh... Director Chang, the shape of the shock wave is so simple..."
While Chang Haonan was lowering his head to write, Lin Tongxin originally wanted to ask why the shock wave curve was in such a simple form.
But the words stopped abruptly in the middle of the sentence. Because Chang Haonan wrote the second function on the paper.
“And such a combination of parametric equations and curves.”
Chang Haonan stopped writing and looked at Lin Tongxin standing next to him:
"What did you want to ask just now?"
"No, it's okay."
The latter quickly shook his head:
"I was just thinking, Director Chang, how can you It’s really amazing to come up with such a complicated shock wave curve.”
It hurts.
It hurts.
My face hurts...
Sun Huizhong next to him had an expression of "I don't really want to know this person."
Actually, Chang Haonan didn’t hear clearly what the other party was saying just now, so he just continued to introduce:
“The design of the convex hull surface is originally the core step of the DSI air inlet. I had previously thought about expressing it simply by conic equations, but it was still too difficult to derive the form of the shock curve from a mechanism based on the current computing power, so I had to add some parameterized parts. ”
< br>
"I understand."
Lin Tongxin now just hopes that this topic will be over soon.
It was so embarrassing, he just wanted to escape from this world.
Fortunately, Chang Haonan did not continue to pester:
“Then all we need to do next is...”
He spent more than ten minutes on the computer and actually demonstrated the steps he just said.
Of course, for convenience, all parameters are temporarily set to 1, so the designed convex hull shape is a bit strange.
But judging from the reactions of the two people next to him, at least the purpose of the explanation was achieved.
So Chang Haonan picked up his pen again and circled a, b, l, β and other symbols:
“Our next work, in summary, is based on the characteristics of the aircraft. Characteristics, choose appropriate parameters for this shock curve. ”
Then he looked at Lin Tongxin next to him again, but this time he said in a more relaxed tone:
“This is another reason why I don’t want to extend the length of the side strips to the bottom of the canopy. The inflow compression effect of the strip wing will destroy the relatively clean shock wave shape of the air inlet, which makes it difficult for me to conduct characteristic analysis on the convex hull shape corresponding to different shock wave curves during the design process, and the air inlet is designed directly on There won’t be this problem right in front of the edge.”
“Understand now?”
Lin Tongxin nodded quickly.
He has learned not to express opinions without finalizing his ideas.
"That's good. If there are no other questions, we are ready to start work. But even if there is anything you don't understand later, you can always come and ask me."
Chang Haonan said, lifting the lid of the laptop Buttoned it up, stood up and prepared to take the two people to the computer room:
"Although I initially proposed that this model is indeed for the purpose of technical verification for the No. 10 Project, if nothing happens, you, Guihang, will It will be the first aircraft manufacturer to use DSI inlets on mass-produced aircraft.”
He knows that China in this era has a deep obsession with striving for “firsts” in all aspects. .
By the 2020s, after the aviation industry had developed, it no longer deliberately promoted and guided in this direction.
It can only be said that the more something is lacking, the more something is desired.
Sure enough, when Sun Huizhong and Lin Tongxin heard his words, their spirits were lifted, and the slight fatigue they had just shown disappeared instantly.
“Can it be faster than the United States?”
The latter immediately asked excitedly.
“Of course, the JSF project has not yet been revealed. There is a high probability that your model will make its first flight before the end of this year.”
Chang Haonan packed up his things and motioned for the other two to follow him.
"This..."
Lin Tongxin was stunned by this time point.
He remembered that his teacher said in school that the design cycle of an aircraft often takes several years or even more than ten years.
“Master, is our Guizhou Airlines so efficient?”
Because Chang Haonan had already walked to the front, he had to whisper to Sun Huizhong next to him.
The latter was almost amused by him:
“There are indeed people who are very efficient, but they are not our Guihang.”
After speaking, he looked at Chang Haonan’s back. , followed quickly.
"Hiss..."
Lin Tongxin took a breath of air:
"Director Chang is a bit scary..."
(End of this chapter)