Chapter Eighty-Seven: Braving Wind and Rain

  "As the projects we are studying have already exceeded our current understanding of atmospheric physical properties, when flying speeds exceed a certain extent, we believe that everything changes and we must take these changes seriously, otherwise supersonic flight cannot be achieved. Without such flight capabilities, what is the point of studying jet aircraft? Therefore, from the beginning of our research, we have placed great emphasis on studying flight environments and conditions."

  The state parameters of the atmosphere refer to its pressure, temperature and density. These parameters change with the flight altitude and have an impact on the aerodynamic forces acting on the aircraft as well as a significant influence on the thrust produced by the jet engine.

  Each atmospheric molecule has its own position, speed and energy. The connection between molecules is very weak and there is no fixed external shape. However, when a flying object moves in this medium, the size of its outer dimensions far exceeds the free path of the molecules, so when studying the relative motion between the flying object and the atmosphere, the distance between the molecules can be considered as zero, that is to say, the gas is treated as a continuous medium.

  The traction force generated when adjacent atmospheric layers move relative to each other is called the internal frictional force of the atmosphere, also known as the viscous force of the atmosphere. This viscosity of the atmosphere is a common physical property exhibited by air in motion, and the frictional resistance generated when airflow passes over an object is related to the viscosity of the atmosphere. For objects moving rapidly through the air, such as aircraft, the frictional resistance caused by air viscosity on the surface of the aircraft can no longer be ignored and must be taken seriously into consideration.

  The compressibility of a gas is the property that its density and volume change when its pressure changes. It is generally considered to be incompressible, but gases react greatly to such changes, so it is believed that all gases are compressible. When airflow passes over the surface of an aircraft, due to the compression effect of the aircraft on the air, the air pressure will produce changes, and the density will also fluctuate accordingly. When the airflow speed is relatively low, the change in pressure is not large, and the change in density is also small, so when studying problems related to low-speed airflow, the compressibility of the air can be ignored. However, when the airflow speed is high, it should be taken into consideration.

  The speed of sound refers to the speed at which a sound wave propagates through a medium, producing a series of compressions and rarefactions. Humans can hear various sounds transmitted through the air because the air is compressed and expanded. When an airplane flies through the air, it pushes forward the small masses of air in its path, compressing them, and as the plane continues to move forward, the pushed and compressed masses expand back to their original position. Therefore, when an airplane flies, the surrounding air will constantly produce vibrating compressions and rarefactions.

  The speed of sound is an important factor in considering the degree of air compression, and the greater the speed of sound, the more the air will be compressed. Another factor is the speed of the aircraft, the faster the motion, the greater the pressure applied to the air, and the more severe the air will be compressed. Combining these factors together results in a very special mathematical equation.

  The Mach number (Ma) is equal to the speed of the aircraft at a certain altitude divided by the local speed of sound. The higher the Mach number, the more severe the compression of air. When the Mach number is less than or equal to 0.4, the compressibility of air has little effect, and it can be considered that the air is incompressible. However, when the Mach number exceeds 0.4, the study of aircraft propulsion must take into account the effects of air compressibility, especially after entering transonic flight, because compressibility produces a unique flow phenomenon called shock waves, which will have a significant impact on the aerodynamics and shape design of the aircraft.

  "At low speeds, the interaction between the aircraft and the air is a gradual process, with the air considered incompressible. At high speeds, however, the aircraft suddenly arrives at the front, and the air cannot get out of the way quickly enough, resulting in a sudden and intense compression, with pressure, density, and temperature all increasing sharply, while the flow velocity relative to the aircraft suddenly decreases. This boundary between no change and change is called a shock wave."

  "Shock waves are further divided into normal shock waves, oblique shock waves and conical shock waves. However, during supersonic flight, the airflow produces shock waves due to obstruction and expansion waves due to expansion. Shock waves can be said to be a phenomenon that is usually produced when supersonic airflow slows down, while expansion waves are a necessary phenomenon when it accelerates. The shock wave causes the parameters before and after the wave to change suddenly, and the airflow passing through the shock wave undergoes sudden compression, with pressure and density temperature increasing, and speed and Mach number decreasing. However, the parameters before and after the expansion wave change continuously."

  "There is another point, although the thickness of the shock wave is very small, when the airflow passes through the shock wave, the internal friction caused by the viscosity of the gas inside the shock wave is very strong. Part of the mechanical energy of the airflow will be converted into heat energy due to the consumption of friction, causing its temperature to rise sharply. This phenomenon is called aerodynamic heating. However, expansion waves do not have this loss, which is similar to the boundary layer, where the viscosity of the gas converts kinetic energy into heat energy, resulting in a loss of kinetic energy. The resistance caused by this loss can be referred to as shock wave resistance, abbreviated as wave resistance."

  Zhang Yu rarely inquired about the progress of Wang Zhu's research, knowing that Wang Zhu was more interested in jet aircraft and must have understood a lot about the Kondor project team's research progress. However, with the rare opportunity to stay at China Aviation, his strong curiosity still prompted him to ask about the research progress. Before that, Wang Zhu had carefully explained some theoretical knowledge to him, probably because he was worried that Zhang Yu would not understand later, so he lectured on high-speed flight-related knowledge theory.

  "I think that if nothing unexpected happens, the sound barrier will certainly become a huge and insurmountable obstacle in the development of aircraft."

  "So we gave it a special name, called sonic barrier." Wang Zhu looked at Zhang Yu, and in the large conference room, there were only two people left, and anything could be said. "Actually, the sonic barrier problem was not discovered when researching this project, but when we studied the piston-type aircraft, we found that when the speed reached over 700 during level flight, when the plane entered a dive, it would inevitably approach the speed of sound, at which point the plane would produce intense vibrations, and flight would become extremely unstable, almost causing the pilot to lose control of the plane. If the plane was not designed well, there would be a risk of structural damage, even disintegration in mid-air."

  "Didn't you say that sonic booms are caused by the shock waves and wave resistance generated when an airplane is flying? If we study and solve these two influencing factors, won't we break through this difficult problem of sound barrier?" Zhang Yu was indeed pretending to be confused at this time. For these kinds of questions that were almost like popular science knowledge in later generations, even now they are considered high-tech confidential matters. Of course, Zhang Yu could ask about these questions, but outsiders couldn't.

  "We have just mentioned the production of shock waves and wave resistance. In fact, objects with different shapes produce different shock waves under supersonic conditions, resulting in varying degrees of wave resistance. The stronger the shape's obstruction to airflow, the stronger the produced shock wave, and naturally, the greater the wave resistance."

  "Objects similar to rectangles often produce detached shock waves, which are strong positive shock waves that occur at a certain distance from the front end. Detached shock waves have a strong blocking effect on airflow, resulting in large wave drag. On the other hand, objects with sharp heads often produce attached oblique shock waves at the front end of the sharp head, and these shock waves have a relatively weak blocking effect on airflow. Therefore, the sharper the object is, the smaller the blockage to airflow will be, the more inclined the shock wave will be, and the smaller the wave drag will be. Therefore, when designing the forebody, wing, etc. of supersonic aircraft, we should make them sharp, which can reduce the intensity of the shock wave and thus reduce the wave drag."

  "Can you talk about your research results on supersonic aerodynamic shapes?"

  "We have already started researching the aerodynamic layout of aircraft, and we have also considered certain aspects of the aircraft's geometric shape and parameters," Wang said, pulling out a few papers from his folder and handing them to Zhang Yu. "We all know that different aircraft and speeds have different aerodynamic layouts. According to the connection position between the wing and fuselage, they can be divided into upper single-wing, middle single-wing, and lower single-wing. If classified according to whether there is an upward angle on the wing chord plane, they can be divided into upper inverted wing, non-inverted wing, and lower inverted wing. Of course, if classified by the number of vertical tails, they can be divided into single tail, double tail, and no vertical tail, with the latter having a V-shaped tail instead."

  "Of course, if we refer to the general aerodynamic layout, which is usually the arrangement of the horizontal tail relative to the wing in the longitudinal position, that is, the longitudinal startup layout form of the aircraft, we have normal type, duck type and no horizontal tail type. Different layout types have a significant impact on the flight performance and stability, controllability of the aircraft."

  "These are still somewhat understood, the geometric shape of an airplane is composed of the shapes of main components such as fuselage, wing and tail, among which the wing is the main component that produces lift and drag. The planar shape of the wing includes span, chord, sweep angle and taper ratio. And the main parameters affecting the aerodynamic characteristics of an airplane are sweep angle, aspect ratio, taper ratio and relative thickness of airfoil..."

  "That's right!" Wang Zhuo timely gave a pat on the back, but he didn't have the habit of continuing to flatter, pointing to the drawing in Zhang Yu's hand and saying: "In low-speed flight, the lift coefficient of a flat wing with a large aspect ratio is larger, and the induced drag is smaller. In subsonic flight, the sweptback wing can delay the generation of shock waves and reduce their intensity, thereby reducing wave resistance. When entering the supersonic flight stage, shock waves are already inevitable, and designs such as small aspect ratio, triangular wings, and edge strips that are beneficial to reducing deficiencies can be adopted."

  "To reduce the wave drag generated during supersonic flight, we can adopt wing planforms such as sweptback wings, triangular wings, small aspect ratio wings, variable sweepback wings, and edge-aligned wings. Often, a ducktail or no horizontal tail configuration is used instead of the conventional layout."

  "Let's hear it!" Zhang Yu put down the blueprint, poured a cup of cool tea for Wang Zhuo to soothe his throat, and then sat quietly like a little student, waiting for Wang Zhuo's explanation.

  "I told you earlier that the swept wing can increase the critical Mach number, and even after exceeding it, it can further reduce wave drag. However, it is not perfect, when a flow of a certain speed blows over the swept wing, there will be a part of the flow that flows along the direction of the wing, causing the boundary layer to thicken gradually from the root to the tip of the wing, and causing flow separation at the wingtip."

  "When the angle of attack increases to a certain extent, tip stall occurs, which then spreads to the mid-span and root sections of the wing, resulting in large-scale stall. This process is estimated to be extremely rapid, also causing the aircraft to pitch up violently, making flight extremely unstable, such that the pilot may experience these terrifying phenomena without warning. Of course, what can be done is to install wing fences on the surface of the wing and make serrations or notches at the leading edge of the wing, so that the airflow forms vortices or dynamic wing fences, thereby preventing the flow from moving along the direction of the wing."

  "The increase in the angle of attack of the wing's leading edge will drastically worsen the load-bearing structure of the swept-back wing root, and also increase its structural weight. Moreover, the aerodynamic characteristics at low speeds will deteriorate, causing the aircraft's lift to decrease and drag to increase. Therefore, when the flight Mach number reaches 2, the effective speed of the wing is less than 1, and only a triangular wing can be adopted."

  "The triangular wing and large sweepback angle are basically similar, with the characteristics of a larger leading edge sweepback angle, smaller aspect ratio and smaller relative thickness. The root part is relatively long, which helps to increase the relative thickness at the root under the condition of unchanged relative thickness, thereby improving the load-bearing situation at the root and reducing structural weight. The wing structure height remains unchanged, reducing the wing's relative thickness to reduce wave drag."

  "The triangular wing has good aerodynamic performance, the focal point of the wing changes from transonic to supersonic flight, with a smaller change than other flat wings, which helps to ensure the lateral stability of the aircraft. However, during subsonic flight, the lift-to-drag ratio is relatively low and cruise characteristics are poor. Sufficient lift coefficient can only be obtained at high angles of attack, and during landing, in order not to obstruct the pilot's downward view, the nose cannot be raised too high, so the angle of attack cannot be too large, therefore the triangular wing aircraft must have very poor landing performance."

  "So, supersonic aircraft adopt small aspect ratio and large sweptback wing, the larger the sweep angle, the lower the wave drag, which is beneficial to supersonic flight. However, due to the small aspect ratio and wingspan, the low-speed flight performance is poor, and the takeoff and landing distances are very long. Therefore, as we move forward, the aircraft's performance requirements between high speed and low speed are contradictory. The variable swept wing can solve this problem well. During takeoff and landing, a smaller sweep angle is adopted, and the maximum aspect ratio is beneficial to low-speed cruise economy and larger takeoff and landing lift. During supersonic flight, a larger sweep angle is adopted, and the wing's aspect ratio decreases accordingly, which helps reduce flight resistance. But it's clear that such an aircraft is destined to have a complex structure, and after the aerodynamic center changes greatly, just balancing the problem is enough to be troublesome."

  "To resolve the contradiction between supersonic high-speed flight and low-speed flight, the best approach is to adopt a wing with a leading edge. It should consist of two parts: the leading edge and the trailing edge. Due to the large sweepback angle of the leading edge, the effective sweepback angle of the entire wing increases, reducing wave drag. With the presence of the basic wing, the effective aspect ratio of the entire wing increases, reducing induced drag during low-subsonic flight and transonic flight. It can be said that it has relatively perfectly solved the difficulty of high-low speed compatibility, of course, how to determine a good leading edge wing design will be an extremely difficult engineering task."

  "The canard aircraft is actually the horizontal tail moved to the front of the wing. Since the horizontal tail is located in front of the center of gravity, during normal angle-of-attack flight, the canard produces positive lift to maintain the balance of the aircraft, so it has a positive effect on the overall lift. And when flying at high angles of attack, the vortex generated by the leading edge of the canard flows along the upper surface of the wing and produces a beneficial interference similar to that of a wingtip device, which increases the lift of the wing and is very beneficial for improving the takeoff and landing performance of the aircraft. To achieve the design requirements of short-range takeoff and landing at supersonic speeds, adopting a canard layout is necessary."

  "The fuselage and wings of a tailless layout aircraft are both longer and thinner, with a larger wing area. The center of gravity is located at the rear, and adopting a tailless layout can reduce the drag generated by the horizontal stabilizer. Pitch control is achieved through the elevons on the trailing edge of the wing. When the elevons on both wings move up or down simultaneously, they produce a pitching moment, acting as an elevator. When the elevons on both wings deflect in opposite directions, they produce a rolling moment, acting as an aileron. This type of aircraft has a simple external structure but has control difficulties..."

  Wang Zhu obviously didn't know that the last layout he mentioned, the tailless aircraft, was actually the aerodynamic layout of stealth aircraft in later generations. The duck layout and edge wing layout were also classic aerodynamic layouts for fighter jets in later generations. In another time and space, the aviation powerhouse America had its F-14 fighter jet as a variable swept wing aircraft, and its F-16 aircraft as an edge wing aircraft. Of course, the Republic's J-10 and Sweden's JA-37 aircraft were also duck layout designs. These advanced design concepts and some of the drawings had already been stolen by Zhang Yu from Wang Zhu's database. The drawings that Wang Zhu showed Zhang Yu at this time were the results of their detailed research and adaptation to their own situation.

  "The design on this drawing is pretty good! It adopts a relatively small thickness symmetrical wing type, with the maximum thickness position near the middle of the wing chord. The curvature radius of the leading edge is smaller, and the change in the outline of the wing section profile is relatively gentle, which directly helps to increase the Mach critical number and delay the generation of shock waves. Even after exceeding the critical value, the wing section forms an oblique shock wave at high supersonic speeds, which helps to reduce wave drag."

  "Yes, this is a genius design. Airfoils with smaller wave resistance have double-arc, diamond, wedge and double-diamond shapes. Research has shown that the relative thickness of the airfoil is indeed closely related to wave resistance, and wave resistance is roughly proportional to the square of the relative thickness. If the thickness is doubled, the wave resistance will increase four times. Adopting a relatively small relative thickness is an inevitable trend in wing development, but we don't have the technical means to determine how large the relative thickness should be. Just like those wing styles I mentioned earlier, it's easy to talk about, but if you really want to turn theory into practical drawings, just calculating various data will make us laborious..."

  "This problem I can't solve, the aviation industry is originally a huge systematic project, I know that just to determine whether a design is reasonable, the amount of calculation required is equivalent to the annual workload of the statistics bureau. Have you ever thought... " Zhang Yu said to this point, gesturing for Wang Zhuo to lean in closer. "I'm saying have you ever thought of inventing a calculating machine to assist you in solving these massive calculation problems, or even helping you design airplanes and use data simulation to replace tedious experimental verification..."

  "You're talking about mechanical computers? This thing can help with some pure calculation problems, but human-assisted design, I think it's not something we can achieve right now!" Wang Zhuo said, shaking his head, apparently he had been troubled by those messy data for a long time. If there was really a way to solve these problems, they wouldn't have taken three or four months to come up with some rough drafts.

  "So you're saying that these supersonic aircraft on the blueprints won't be able to take to the skies for over a decade?"

  "Maybe it won't take that long, it should be possible within five years. Of course, the premise is that our China Aviation Power Company makes progress smoothly, and if we can have a continuous supply of high-quality talents joining the research team, I believe the time will be shorter and the effect will be better."

  What Wang Zhuo said is the truth, the current aircraft and those in the future are very different, there are no complicated electronic devices that need to be researched and matched, so far wireless radio communication systems are the most advanced electronic equipment, the aircraft of the air combat era are definitely not comparable to those that can fight beyond visual range, however, as a person one should have pursuits, when doing things one should have goals, since China Aviation has made up its mind to achieve something in the field of jet aircraft, then there is no reason to retreat.

  Since you have chosen your direction, go all out and make progress without any hindrance!

  PS: Today is Thursday again, and the updated chapter has arrived. Thank you for your support, especially to Eternal Destruction, Yi Fan Liu Ying and other friends who have rewarded me, thank you.