Have you heard a loud sound like an explosion when a jet plane flies overhead? This sound occurs when an airplane breaks the sound barrier. What is the sound barrier and why does a plane make such a sound?
As you know, sound travels at a certain speed. Speed depends on altitude. At sea level, the speed of sound is approximately 1220 kilometers per hour, and at an altitude of 11,000 meters - 1060 kilometers per hour. When an airplane flies at speeds close to the speed of sound, it is subjected to certain stresses. When it flies at normal (subsonic) speeds, the front of the plane pushes a pressure wave in front of it. This wave travels at the speed of sound.
The pressure wave is caused by the accumulation of air particles as the aircraft moves forward. The wave moves faster than the plane when the plane flies at subsonic speeds. And as a result, it turns out that air passes unhindered over the surfaces of the aircraft’s wings.
Now let's look at an airplane that flies at the speed of sound. There is no pressure wave in front of the plane. What happens instead is that a pressure wave forms in front of the wing (since the aircraft and the pressure wave are moving at the same speed).
Now a shock wave is formed, which causes large loads in the aircraft wing. The expression “sound barrier” dates back to before airplanes could fly at the speed of sound—and was thought to describe the stresses an airplane would experience at those speeds. This was considered a "barrier".
But the speed of sound is not a barrier at all! Engineers and aircraft designers overcame the problem of new loads. And all we have left from the old views is that the impact is caused by a shock wave when the plane flies at supersonic speeds.
The term "sound barrier" misleadingly describes the conditions that occur when an aircraft is traveling at a certain speed. One might think that when the plane reaches the speed of sound, something like a “barrier” appears - but nothing like that happens!
To understand all this, consider an airplane flying at a low, normal speed. As the aircraft moves forward, a compression wave is formed in front of the aircraft. It is formed by an aircraft moving forward, which compresses air particles.
This wave moves ahead of the aircraft at the speed of sound. And its speed is higher than the speed of an airplane, which, as we have already said, flies at low speed. Moving ahead of the plane, this wave forces air currents to flow around the plane of the plane.
Now imagine that the plane is flying at the speed of sound. No compression waves are formed ahead of the plane, since both the plane and the waves have the same speed. Therefore, the wave forms in front of the wings.
As a result, a shock wave appears, which creates large loads on the aircraft's wings. Before the planes reached sound barrier and exceeded it, they believed that such shock waves and overloads would create something like a barrier for the aircraft - a “sound barrier.” However, there was no sound barrier, as aeronautical engineers developed a special aircraft design for this.
By the way, the strong “blow” that we hear when an airplane passes the “sound barrier” is the shock wave that we have already talked about - when the speed of the airplane and the compression wave are equal.
Sometimes when a jet plane flies through the sky, you can hear a loud bang that sounds like an explosion. This "burst" is the result of the aircraft breaking the sound barrier.
What is the sound barrier and why do we hear an explosion? AND who was the first to break the sound barrier ? We will consider these questions below.
What is the sound barrier and how is it formed?
The aerodynamic sound barrier is a series of phenomena that accompany the movement of any aircraft (airplane, rocket, etc.) whose speed is equal to or exceeds the speed of sound. In other words, the aerodynamic “sound barrier” is a sharp jump in air resistance that occurs when an aircraft reaches the speed of sound.
Sound waves travel through space at a certain speed, which varies depending on height, temperature and pressure. For example, at sea level the speed of sound is approximately 1220 km/h, at an altitude of 15 thousand m – up to 1000 km/h, etc. When the speed of an aircraft approaches the speed of sound, certain loads are applied to it. At normal speeds (subsonic), the nose of the aircraft “drives” a wave of compressed air in front of it, the speed of which corresponds to the speed of sound. The speed of the wave is greater than the normal speed of the aircraft. As a result, air flows freely around the entire surface of the aircraft.
But, if the speed of the aircraft corresponds to the speed of sound, the compression wave is formed not at the nose, but in front of the wing. As a result, a shock wave is formed, increasing the load on the wings.
In order for an aircraft to overcome the sound barrier, in addition to a certain speed, it must have a special design. That is why aircraft designers developed and used a special aerodynamic wing profile and other tricks in aircraft construction. At the moment of breaking the sound barrier, the pilot of a modern supersonic aircraft feels vibrations, “jumps” and “aerodynamic shock”, which on the ground we perceive as a pop or explosion.
Who was the first to break the sound barrier?
The question of the “pioneers” of the sound barrier is the same as the question of the first space explorers. To the question “ Who was the first to break the supersonic barrier? ? You can give different answers. This is the first person to break the sound barrier, and the first woman, and, oddly enough, the first device...
The first person to break the sound barrier was test pilot Charles Edward Yeager (Chuck Yeager). On October 14, 1947, his experimental Bell X-1 aircraft, equipped with a rocket engine, went into a shallow dive from an altitude of 21,379 m above Victorville (California, USA), and reached the speed of sound. The speed of the plane at that moment was 1207 km/h.
Throughout his career, the military pilot made a major contribution to the development of not only American military aviation, but also astronautics. Charles Elwood Yeager ended his career as a general in the US Air Force, having visited many parts of the world. The experience of a military pilot came in handy even in Hollywood when staging spectacular aerial stunts in the feature film “The Pilot.”
Chuck Yeager's story of breaking the sound barrier is told in the film "The Right Guys," which won four Oscars in 1984.
Other "conquerors" of the sound barrier
In addition to Charles Yeager, who was the first to break the sound barrier, there were other record holders.
- The first Soviet test pilot - Sokolovsky (December 26, 1948).
- The first woman is American Jacqueline Cochran (May 18, 1953). Flying over Edwards Air Force Base (California, USA), her F-86 aircraft broke the sound barrier at a speed of 1223 km/h.
- The first civilian aircraft was the American passenger airliner Douglas DC-8 (August 21, 1961). Its flight, which took place at an altitude of about 12.5 thousand m, was experimental and was organized with the aim of collecting data necessary for the future design of the leading edges of the wings.
- First car to break the sound barrier - Thrust SSC (October 15, 1997).
- The first person to break the sound barrier in free fall was American Joe Kittinger (1960), who parachuted from a height of 31.5 km. However, after that, flying over the American city of Roswell (New Mexico, USA) on October 14, 2012, Austrian Felix Baumgartner set a world record by leaving balloon with a parachute at an altitude of 39 km. Its speed was about 1342.8 km/h, and its descent to the ground, most of which was in free fall, took only 10 minutes.
- The world record for breaking the sound barrier by an aircraft belongs to the X-15 air-to-ground hypersonic aeroballistic missile (1967), currently in service Russian army. The rocket's speed at an altitude of 31.2 km was 6389 km/h. I would like to note that the maximum possible speed of human movement in the history of manned aircraft is 39,897 km/h, which was reached in 1969 by the American spaceship"Apollo 10".
The first invention to break the sound barrier
Oddly enough, the first invention that broke the sound barrier was... a simple whip, invented by the ancient Chinese 7 thousand years ago.
Before the invention of instant photography in 1927, no one would have thought that the crack of a whip was not just a strap hitting the handle, but a miniature supersonic click. During a sharp swing, a loop is formed, the speed of which increases several tens of times and is accompanied by a click. The loop breaks the sound barrier at a speed of about 1200 km/h.
What do we imagine when we hear the expression “sound barrier”? A certain limit can seriously affect hearing and well-being. Usually the sound barrier is correlated with the conquest of airspace and
Overcoming this obstacle can provoke the development of old diseases, pain syndromes and allergic reactions. Are these ideas correct or do they represent established stereotypes? Do they have a factual basis? What is the sound barrier? How and why does it arise? All this and some additional nuances, as well as historical facts We will try to find out what is associated with this concept in this article.
This mysterious science is aerodynamics
In the science of aerodynamics, designed to explain the phenomena accompanying movement
aircraft, there is the concept of “sound barrier”. This is a series of phenomena that occur during the movement of supersonic aircraft or rockets that move at speeds close to the speed of sound or greater.
What is a shock wave?
As a supersonic flow flows around a vehicle, a shock wave appears in a wind tunnel. Its traces can be visible even to the naked eye. On the ground they are expressed by a yellow line. Outside the shock wave cone, in front of the yellow line, you can’t even hear the plane on the ground. At speeds exceeding sound, bodies are subjected to a flow of sound flow, which entails a shock wave. There may be more than one, depending on the shape of the body.
Shock wave transformation
The shock wave front, which is sometimes called a shock wave, has a fairly small thickness, which nevertheless makes it possible to track abrupt changes in the properties of the flow, a decrease in its speed relative to the body and a corresponding increase in the pressure and temperature of the gas in the flow. In this case, the kinetic energy is partially converted into internal energy of the gas. The number of these changes directly depends on the speed of the supersonic flow. As the shock wave moves away from the apparatus, the pressure drops decrease and the shock wave is converted into a sound wave. It can reach an outside observer, who will hear a characteristic sound resembling an explosion. There is an opinion that this indicates that the device has reached the speed of sound, when the plane leaves the sound barrier behind.
What's really going on?
The so-called moment of breaking the sound barrier in practice represents the passage of a shock wave with the increasing roar of the aircraft engines. Now the device is ahead of the accompanying sound, so the hum of the engine will be heard after it. Approaching the speed of sound became possible during the Second World War, but at the same time pilots noted alarming signals in the operation of aircraft.
After the end of the war, many aircraft designers and pilots sought to reach the speed of sound and break the sound barrier, but many of these attempts ended tragically. Pessimistic scientists argued that this limit could not be exceeded. By no means experimental, but scientific, it was possible to explain the nature of the concept of “sound barrier” and find ways to overcome it.
Safe flights at transonic and supersonic speeds are possible by avoiding a wave crisis, the occurrence of which depends on the aerodynamic parameters of the aircraft and the altitude of the flight. Transitions from one speed level to another should be carried out as quickly as possible using afterburner, which will help to avoid a long flight in the wave crisis zone. The wave crisis as a concept came from water transport. It arose when ships moved at a speed close to the speed of waves on the surface of the water. Getting into a wave crisis entails difficulty in increasing speed, and if you overcome the wave crisis as simply as possible, then you can enter the mode of planing or sliding along the water surface.
History in aircraft control
The first person to reach supersonic flight speed in an experimental aircraft was the American pilot Chuck Yeager. His achievement was noted in history on October 14, 1947. On the territory of the USSR, the sound barrier was broken on December 26, 1948 by Sokolovsky and Fedorov, who were flying an experienced fighter.
Among civilians, the passenger airliner Douglas DC-8 broke the sound barrier, which on August 21, 1961 reached a speed of 1.012 Mach, or 1262 km/h. The purpose of the flight was to collect data for wing design. Among aircraft, the world record was set by a hypersonic air-to-ground aeroballistic missile, which is in service with the Russian army. At an altitude of 31.2 kilometers, the rocket reached a speed of 6389 km/h.
50 years after breaking the sound barrier in the air, Englishman Andy Green achieved a similar achievement in a car. American Joe Kittinger tried to break the record in free fall, reaching a height of 31.5 kilometers. Today, on October 14, 2012, Felix Baumgartner set a world record, without the help of transport, in a free fall from a height of 39 kilometers, breaking the sound barrier. Its speed reached 1342.8 kilometers per hour.
The most unusual breaking of the sound barrier
It’s strange to think, but the first invention in the world to overcome this limit was the ordinary whip, which was invented by the ancient Chinese almost 7 thousand years ago. Almost until the invention of instant photography in 1927, no one suspected that the crack of a whip was a miniature sonic boom. A sharp swing forms a loop, and the speed increases sharply, which is confirmed by the click. The sound barrier is broken at a speed of about 1200 km/h.
The mystery of the noisiest city
It’s no wonder that residents of small towns are shocked when they see the capital for the first time. Plenty of transport, hundreds of restaurants and entertainment centers confuse and unsettle you from your usual rut. The beginning of spring in the capital is usually dated to April, rather than the rebellious, blizzardy March. In April there are clear skies, streams are flowing and buds are blooming. People, tired from the long winter, open their windows wide towards the sun, and street noise bursts into their houses. Birds chirp deafeningly on the street, artists sing, cheerful students recite poetry, not to mention the noise in traffic jams and the subway. Hygiene department employees note that staying in a noisy city for a long time is harmful to health. The sound background of the capital consists of transport,
aviation, industrial and household noise. The most harmful is car noise, since planes fly quite high, and the noise from enterprises dissolves in their buildings. The constant hum of cars on particularly busy highways exceeds all acceptable standards twice. How does the capital overcome the sound barrier? Moscow is dangerous with an abundance of sounds, so residents of the capital install double-glazed windows to muffle the noise.
How is the sound barrier stormed?
Until 1947, there was no actual data on the well-being of a person in the cockpit of an airplane that flies faster than sound. As it turns out, breaking the sound barrier requires certain strength and courage. During the flight, it becomes clear that there is no guarantee of survival. Even a professional pilot cannot say for sure whether the aircraft’s design will withstand an attack from the elements. In a matter of minutes, the plane can simply fall apart. What explains this? It should be noted that movement at subsonic speed creates acoustic waves that spread out like circles from fallen stone. Supersonic speed excites shock waves, and a person standing on the ground hears a sound similar to an explosion. Without powerful computers, it was difficult to solve complex problems and one had to rely on blowing models in wind tunnels. Sometimes, when the plane's acceleration is insufficient, the shock wave reaches such a force that windows fly out of the houses over which the plane flies. Not everyone will be able to overcome the sound barrier, because at this moment the entire structure shakes, and the mountings of the device can receive significant damage. That's why it's so important for pilots good health and emotional stability. If the flight is smooth and the sound barrier is overcome as quickly as possible, then neither the pilot nor any possible passengers will feel any particularly unpleasant sensations. A research aircraft was built specifically to break the sound barrier in January 1946. The creation of the machine was initiated by an order from the Ministry of Defense, but instead of weapons it was stuffed with scientific equipment that monitored the operating mode of mechanisms and instruments. This plane was like a modern cruise missile with a built-in rocket engine. The plane broke the sound barrier at a maximum speed of 2736 km/h.
Verbal and material monuments to conquering the speed of sound
Achievements in breaking the sound barrier are still highly valued today. So, the plane in which Chuck Yeager first overcame it is now on display at the National Air and Space Museum, which is located in Washington. But technical specifications this human invention would be worth little without the merits of the pilot himself. Chuck Yeager went through flight school and fought in Europe, after which he returned to England. The unfair exclusion from flying did not break Yeager’s spirit, and he achieved a reception with the commander-in-chief of the European troops. In the years remaining until the end of the war, Yeager took part in 64 combat missions, during which he shot down 13 aircraft. Chuck Yeager returned to his homeland with the rank of captain. His characteristics indicate phenomenal intuition, incredible composure and endurance in critical situations. More than once Yeager set records on his plane. His further career was in the Air Force units, where he trained pilots. The last time Chuck Yeager broke the sound barrier was 74 years old, which was on the fiftieth anniversary of his flight history and in 1997.
Complex tasks of aircraft creators
The world-famous MiG-15 aircraft began to be created at the moment when the developers realized that it was impossible to rely only on breaking the sound barrier, but that complex technical problems had to be solved. As a result, a machine was created so successful that its modifications entered service with different countries. Several different design bureaus entered into a kind of competitive struggle, the prize in which was a patent for the most successful and functional aircraft. Aircraft with swept wings were developed, which was a revolution in their design. The ideal device had to be powerful, fast and incredibly resistant to any external damage. The swept wings of airplanes became an element that helped them triple the speed of sound. Then it continued to increase, which was explained by an increase in engine power, the use innovative materials and optimization of aerodynamic parameters. Overcoming the sound barrier has become possible and real even for a non-professional, but this does not make it any less dangerous, so any extreme sports enthusiast should sensibly assess their strengths before deciding to undertake such an experiment.
Passed the sound barrier :-)...
Before we start talking about the topic, let's bring some clarity to the question of the accuracy of concepts (what I like :-)). Nowadays two terms are in fairly wide use: sound barrier And supersonic barrier. They sound similar, but still not the same. However, there is no point in being particularly strict: in essence, they are one and the same thing. The definition of sound barrier is most often used by people who are more knowledgeable and closer to aviation. And the second definition is usually everyone else.
I think that from the point of view of physics (and the Russian language :-)) it is more correct to say the sound barrier. There is simple logic here. After all, there is a concept of the speed of sound, but, strictly speaking, there is no fixed concept of supersonic speed. Looking ahead a little, I will say that when an aircraft flies at supersonic speed, it has already passed this barrier, and when it passes (overcomes) it, it then passes a certain threshold speed value equal to the speed of sound (and not supersonic).
Something like that:-). Moreover, the first concept is used much less frequently than the second. This is apparently because the word supersonic sounds more exotic and attractive. And in supersonic flight, the exotic is certainly present and, naturally, attracts many. However, not all people who savor the words “ supersonic barrier“They actually understand what it is. I have already been convinced of this more than once, looking at forums, reading articles, even watching TV.
This question is actually quite complex from a physics point of view. But, of course, we won’t bother with complexity. We’ll just try, as usual, to clarify the situation using the principle of “explaining aerodynamics on your fingers” :-).
So, to the barrier (sound :-))!... An airplane in flight, acting on such an elastic medium as air, becomes a powerful source of sound waves. I think everyone knows what sound waves in air are :-).
Sound waves (tuning fork).
This is an alternation of areas of compression and rarefaction, spreading in different directions from the sound source. Something like circles on water, which are also waves (just not sound ones :-)). It is these areas, acting on the eardrum of the ear, that allow us to hear all the sounds of this world, from human whispers to the roar of jet engines.
An example of sound waves.
The points of propagation of sound waves can be various components of the aircraft. For example, an engine (its sound is known to anyone :-)), or parts of the body (for example, the bow), which, compacting the air in front of them when moving, create certain type pressure (compression) waves traveling forward.
All these sound waves propagate in the air at the speed of sound already known to us. That is, if the plane is subsonic, and even flies at low speed, then they seem to run away from it. As a result, when such an aircraft approaches, we first hear its sound, and then it itself flies by.
I will make a reservation, however, that this is true if the plane is not flying very high. After all, the speed of sound is not the speed of light :-). Its magnitude is not so great and sound waves need time to reach the listener. Therefore, the order of sound appearance for the listener and the plane, if it flies high altitude can change.
And since the sound is not so fast, then with an increase in its own speed the plane begins to catch up with the waves it emits. That is, if he were motionless, then the waves would diverge from him in the form concentric circles like ripples on the water caused by a thrown stone. And since the plane is moving, in the sector of these circles corresponding to the direction of flight, the boundaries of the waves (their fronts) begin to approach each other.
Subsonic body movement.
Accordingly, the gap between the aircraft (its nose) and the front of the very first (head) wave (that is, this is the area where gradual, to a certain extent, braking occurs free stream when meeting with the nose of the aircraft (wing, tail) and, as a consequence, increase in pressure and temperature) begins to contract and the faster the higher the flight speed.
There comes a moment when this gap practically disappears (or becomes minimal), turning into a special kind of area called shock wave. This happens when the flight speed reaches the speed of sound, that is, the plane moves at the same speed as the waves it emits. The Mach number is equal to unity (M=1).
Sound movement of the body (M=1).
Shock shock, is a very narrow region of the medium (about 10 -4 mm), when passing through which there is no longer a gradual, but a sharp (jump-like) change in the parameters of this medium - speed, pressure, temperature, density. In our case, the speed decreases, pressure, temperature and density increase. Hence the name - shock wave.
In a somewhat simplified way, I would say this about all this. It is impossible to abruptly slow down a supersonic flow, but it has to do this, because there is no longer the possibility of gradual braking to the speed of the flow in front of the very nose of the aircraft, as at moderate subsonic speeds. It seems to come across a subsonic section in front of the nose of the aircraft (or the tip of the wing) and collapses into a narrow jump, transferring to it the great energy of movement that it possesses.
By the way, we can say the other way around: the plane transfers part of its energy to the formation of shock waves in order to slow down the supersonic flow.
Supersonic body movement.
There is another name for the shock wave. Moving with the aircraft in space, it essentially represents the front of a sharp change in the above-mentioned environmental parameters (that is, air flow). And this is the essence of a shock wave.
Shock shock and shock wave, in general, are equivalent definitions, but in aerodynamics the first one is more used.
The shock wave (or shock wave) can be practically perpendicular to the direction of flight, in which case they take approximately the shape of a circle in space and are called straight lines. This usually happens in modes close to M=1.
Body movement modes. ! - subsonic, 2 - M=1, supersonic, 4 - shock wave (shock wave).
At numbers M > 1, they are already located at an angle to the direction of flight. That is, the plane is already surpassing its own sound. In this case, they are called oblique and in space they take the shape of a cone, which, by the way, is called the Mach cone, named after a scientist who studied supersonic flows (mentioned him in one of them).
Mach cone.
The shape of this cone (its “slimness,” so to speak) depends precisely on the number M and is related to it by the relation: M = 1/sin α, where α is the angle between the axis of the cone and its generatrix. And the conical surface touches the fronts of all sound waves, the source of which was the plane, and which it “overtook”, going to the top sound speed.
Besides shock waves may also be annexed, when they are adjacent to the surface of a body moving at supersonic speed, or moving away, if they are not in contact with the body.
Types of shock waves during supersonic flow around bodies of various shapes.
Usually shocks become attached if the supersonic flow flows around any pointed surfaces. For an airplane, for example, this could be a pointed nose, a high-pressure air intake, or a sharp edge of the air intake. At the same time they say “the jump sits”, for example, on the nose.
And a detached shock can occur when flowing around rounded surfaces, for example, the leading rounded edge of a thick airfoil of a wing.
Various components of the aircraft body create quite complex system shock waves. However, the most intense of them are two. One is the head one on the bow and the second is the tail one on the tail elements. At some distance from the aircraft, the intermediate shocks either catch up with the head one and merge with it, or the tail one catches up with them.
Shock shocks on a model aircraft during purging in a wind tunnel (M=2).
As a result, two jumps remain, which, in general, are perceived by an earthly observer as one due to the small size of the aircraft compared to the flight altitude and, accordingly, the short period of time between them.
The intensity (in other words, energy) of a shock wave (shock wave) depends on various parameters (the speed of the aircraft, its design features, environmental conditions, etc.) and is determined by the pressure drop at its front.
As it moves away from the top of the Mach cone, that is, from the aircraft, as a source of disturbance, the shock wave weakens, gradually turns into an ordinary sound wave and ultimately disappears completely.
And on what degree of intensity it will have shock wave(or shock wave) reaching the ground depends on the effect it can produce there. It’s no secret that the well-known Concorde flew supersonic only over the Atlantic, and military supersonic aircraft reach supersonic speed at high altitudes or in areas where there are no settlements(at least it seems like they should do it :-)).
These restrictions are very justified. For me, for example, the very definition of a shock wave is associated with an explosion. And the things that a sufficiently intense shock wave can do may well correspond to it. At least the glass from the windows can easily fly out. There is enough evidence of this (especially in the history of Soviet aviation, when it was quite numerous and flights were intense). But you can do worse things. You just have to fly lower :-)…
However, for the most part, what remains of shock waves when they reach the ground is no longer dangerous. Just an outside observer on the ground can hear a sound similar to a roar or explosion. It is with this fact that one common and rather persistent misconception is associated.
People who are not too experienced in aviation science, hearing such a sound, say that the plane overcame sound barrier (supersonic barrier). Actually this is not true. This statement has nothing to do with reality for at least two reasons.
Shock wave (shock wave).
Firstly, if a person on the ground hears a loud roar high in the sky, then this only means (I repeat :-)) that his ears have reached shock wave front(or shock wave) from an airplane flying somewhere. This plane is already flying at supersonic speed, and has not just switched to it.
And if this same person could suddenly find himself several kilometers ahead of the plane, then he would again hear the same sound from the same plane, because he would be exposed to the same shock wave moving with the plane.
It moves at supersonic speed, and therefore approaches silently. And after it has had its not always pleasant effect on the eardrums (it’s good, when only on them :-)) and has safely passed on, the roar of running engines becomes audible.
Approximate flight pattern of an aircraft at different meanings M numbers using the example of the Saab 35 "Draken" fighter. The language, unfortunately, is German, but the scheme is generally clear.
Moreover, the transition to supersonic sound itself is not accompanied by any one-time “booms”, pops, explosions, etc. On a modern supersonic aircraft, the pilot most often learns about such a transition only from instrument readings. In this case, however, a certain process occurs, but it is subject to certain rules piloting is practically invisible to him.
But that's not all :-). I'll say more. in the form of some tangible, heavy, difficult-to-cross obstacle that the plane rests on and which needs to be “pierced” (I have heard such judgments :-)) does not exist.
Strictly speaking, there is no barrier at all. Once upon a time, at the dawn of the development of high speeds in aviation, this concept was formed rather as psychological belief about the difficulty of transitioning to supersonic speed and flying at it. There were even statements that this was generally impossible, especially since the prerequisites for such beliefs and statements were quite specific.
However, first things first...
In aerodynamics, there is another term that quite accurately describes the process of interaction with the air flow of a body moving in this flow and tending to go supersonic. This wave crisis. It is he who does some bad things that are traditionally associated with the concept sound barrier.
So something about the crisis :-). Any aircraft consists of parts, the air flow around which during flight may not be the same. Let's take, for example, a wing, or rather an ordinary classic subsonic profile.
From the basic knowledge of how lift is generated, we know well that the flow speed in the adjacent layer of the upper curved surface of the profile is different. Where the profile is more convex, it is greater than the overall flow velocity, then, when the profile is flattened, it decreases.
When the wing moves in the flow at speeds close to the speed of sound, a moment may come when in such a convex area, for example, the speed of the air layer, which is already greater than the total speed of the flow, becomes sonic and even supersonic.
Local shock wave that occurs at transonics during a wave crisis.
Further along the profile, this speed decreases and at some point again becomes subsonic. But, as we said above, a supersonic flow cannot quickly slow down, so the emergence of shock wave.
Such shocks appear in different areas of the streamlined surfaces, and initially they are quite weak, but their number can be large, and with an increase in the overall flow speed, the supersonic zones increase, the shocks “get stronger” and shift to the trailing edge of the profile. Later, the same shock waves appear on the lower surface of the profile.
Full supersonic flow around the wing profile.
What does all this mean? Here's what. First– this is significant increase in aerodynamic drag in the transonic speed range (about M=1, more or less). This resistance grows due to a sharp increase in one of its components - wave resistance. The same thing that we previously did not take into account when considering flights at subsonic speeds.
For the formation of numerous shock waves (or shock waves) during the deceleration of a supersonic flow, as I said above, energy is wasted, and it is taken from kinetic energy aircraft movements. That is, the plane simply slows down (and very noticeably!). That's what it is wave resistance.
Moreover, shock waves, due to the sharp deceleration of the flow in them, contribute to the separation of the boundary layer behind itself and its transformation from laminar to turbulent. This further increases aerodynamic drag.
Swelling of the profile when different numbers M. Shocks, local supersonic zones, turbulent zones.
Second. Due to the appearance of local supersonic zones on the wing profile and their further shift to the tail part of the profile with increasing flow speed and, thereby, changing the pressure distribution pattern on the profile, the point of application of aerodynamic forces (the center of pressure) also shifts to the trailing edge. As a result, it appears diving moment relative to the aircraft's center of mass, causing it to lower its nose.
What does all this result in... Due to a rather sharp increase in aerodynamic drag, the aircraft requires a noticeable engine power reserve to overcome the transonic zone and reach, so to speak, real supersonic sound.
A sharp increase in aerodynamic drag at transonics (wave crisis) due to an increase in wave drag. Сd - resistance coefficient.
Further. Due to the occurrence of a diving moment, difficulties arise in pitch control. In addition, due to the disorder and unevenness of the processes associated with the emergence of local supersonic zones with shock waves, control becomes difficult. For example, in roll, due to different processes on the left and right planes.
Moreover, there is the occurrence of vibrations, often quite strong due to local turbulence.
In general, a complete set of pleasures, which is called wave crisis. But, the truth is, they all take place (had, concrete :-)) when using typical subsonic aircraft (with a thick straight wing profile) in order to achieve supersonic speeds.
Initially, when there was not yet enough knowledge, and the processes of reaching supersonic were not comprehensively studied, this very set was considered almost fatally insurmountable and was called sound barrier(or supersonic barrier, if you want to:-)).
There have been many tragic incidents when trying to overcome the speed of sound on conventional piston aircraft. Strong vibration sometimes led to structural damage. The planes did not have enough power for the required acceleration. In horizontal flight it was impossible due to the effect, which has the same nature as wave crisis.
Therefore, a dive was used to accelerate. But it could well have been fatal. The diving moment that appeared during a wave crisis made the dive protracted, and sometimes there was no way out of it. After all, in order to restore control and eliminate the wave crisis, it was necessary to reduce the speed. But doing this in a dive is extremely difficult (if not impossible).
The pulling into a dive from horizontal flight is considered one of the main reasons for the disaster in the USSR on May 27, 1943 of the famous experimental fighter BI-1 with a liquid rocket engine. Tests were carried out for maximum flight speed, and according to the designers' estimates, the speed achieved was more than 800 km/h. After which there was a delay in the dive, from which the plane did not recover.
Experimental fighter BI-1.
In our time wave crisis is already quite well studied and overcoming sound barrier(if required :-)) is not difficult. On airplanes that are designed to fly at fairly high speeds, certain design solutions and restrictions are applied to facilitate their flight operation.
As is known, the wave crisis begins at M numbers close to one. Therefore, almost all subsonic jet airliners (passenger ones, in particular) have a flight limit on the number of M. Usually it is in the region of 0.8-0.9M. The pilot is instructed to monitor this. In addition, on many aircraft, when the limit level is reached, after which the flight speed must be reduced.
Almost all aircraft flying at speeds of at least 800 km/h and above have swept wing(at least along the leading edge :-)). It allows you to delay the start of the offensive wave crisis up to speeds corresponding to M=0.85-0.95.
Swept wing. Basic action.
The reason for this effect can be explained quite simply. On a straight wing, the air flow with a speed V approaches almost at a right angle, and on a swept wing (sweep angle χ) at a certain gliding angle β. Velocity V can be vectorially decomposed into two flows: Vτ and Vn.
The flow Vτ does not affect the pressure distribution on the wing, but the flow Vn does, which precisely determines the load-bearing properties of the wing. And it is obviously smaller in magnitude of the total flow V. Therefore, on a swept wing, the onset of a wave crisis and an increase wave resistance occurs significantly later than on a straight wing at the same free-stream speed.
Experimental fighter E-2A (predecessor of the MIG-21). Typical swept wing.
One of the modifications of the swept wing was the wing with supercritical profile(mentioned him). It also makes it possible to shift the onset of the wave crisis to higher speeds, and in addition, it makes it possible to increase efficiency, which is important for passenger airliners.
SuperJet 100. Swept wing with supercritical profile.
If the plane is intended for passage sound barrier(passing and wave crisis too :-)) and supersonic flight, it usually always differs in certain design features. In particular, it usually has thin wing profile and empennage with sharp edges(including diamond-shaped or triangular) and a certain form wing plan (for example, triangular or trapezoidal with overflow, etc.).
Supersonic MIG-21. Follower E-2A. A typical delta wing.
MIG-25. An example of a typical aircraft designed for supersonic flight. Thin wing and tail profiles, sharp edges. Trapezoidal wing. profile
Passing the proverbial sound barrier, that is, such aircraft make the transition to supersonic speed at afterburner operation of the engine due to the increase in aerodynamic resistance, and, of course, in order to quickly pass through the zone wave crisis. And the very moment of this transition is most often not felt in any way (I repeat :-)) either by the pilot (he may only experience a decrease in the sound pressure level in the cockpit), or by an outside observer, if, of course, he could observe it :-).
However, here it is worth mentioning one more misconception associated with outside observers. Surely many have seen photographs of this kind, the captions under which say that this is the moment the plane overcomes sound barrier, so to speak, visually.
Prandtl-Gloert effect. Does not involve breaking the sound barrier.
Firstly, we already know that there is no sound barrier as such, and the transition to supersonic itself is not accompanied by anything extraordinary (including a bang or an explosion).
Secondly. What we saw in the photo is the so-called Prandtl-Gloert effect. I have already written about him. It is in no way directly related to the transition to supersonic. It’s just that at high speeds (subsonic, by the way :-)), the plane, moving a certain mass of air in front of itself, creates a certain amount of air behind rarefaction region. Immediately after the flight, this area begins to fill with air from the nearby natural space. an increase in volume and a sharp drop in temperature.
If air humidity sufficient and the temperature drops below the dew point of the surrounding air, then moisture condensation from water vapor in the form of fog, which we see. As soon as conditions are restored to original levels, this fog immediately disappears. This whole process is quite short-lived.
This process at high transonic speeds can be facilitated by local shock waves I, sometimes helping to form something like a gentle cone around the plane.
High speeds favor this phenomenon, however, if the air humidity is sufficient, it can (and does) occur at fairly low speeds. For example, above the surface of reservoirs. Most, by the way, beautiful photos of this nature were made on board an aircraft carrier, that is, in fairly humid air.
This is how it works. The footage, of course, is cool, the spectacle is spectacular :-), but this is not at all what it is most often called. nothing to do with it at all (and supersonic barrier Same:-)). And this is good, I think, otherwise the observers who take this kind of photo and video might not be happy. Shock wave, do you know:-)…
In conclusion, there is one video (I have already used it before), the authors of which show the effect of a shock wave from an aircraft flying at low altitude at supersonic speed. There is, of course, a certain exaggeration there :-), but general principle understandable. And again impressive :-)…
That's all for today. Thank you for reading the article to the end :-). Until next time...
Photos are clickable.
At present, the problem of "breaking the sound barrier" appears to be essentially a problem for high-power propulsion engines. If there is sufficient thrust to overcome the increase in drag encountered up to and immediately at the sound barrier, so that the aircraft can pass quickly through the critical speed range, then no particular difficulty should be expected. It might be easier for an aircraft to fly in the supersonic speed range than in the transition range between subsonic and supersonic speeds.
The situation is thus somewhat similar to that which prevailed at the beginning of this century, when the Wright brothers were able to prove the possibility of powered flight because they had a light engine with sufficient thrust. If we had the proper engines, supersonic flight would become quite common. Until recently, breaking the sound barrier in horizontal flight was only possible using rather uneconomical propulsion systems, such as rocket and ramjet engines with very high fuel consumption. Experimental aircraft such as the X-1 and Sky-rocket are equipped with rocket engines that are reliable only for a few minutes of flight, or turbojet engines with afterburners, but at the time of writing there are few aircraft that can fly with supersonic speed for half an hour. If you read in a newspaper that a plane "passed the sound barrier," that often means it did so by diving. In this case, gravity supplemented the insufficient traction force.
Exists strange phenomenon related to these aerobatic maneuvers that I would like to point out. Let's assume that the plane
approaches the observer at subsonic speed, dives, reaching supersonic speed, then exits the dive and again continues to fly at subsonic speed. In this case, an observer on the ground often hears two loud booming sounds, fairly quickly following each other: “Boom, boom!” Some scientists have proposed explanations for the origin of the double hum. Ackeret in Zurich and Maurice Roy in Paris both proposed that the hum was due to the accumulation of sound pulses, such as engine noise, emitted while the aircraft was passing through sound speed. If an airplane is moving towards an observer, then the noise produced by the airplane will reach the observer in a shorter period of time compared to the interval in which it was emitted. Thus, there is always some accumulation of sound pulses, provided that the sound source is moving towards the observer. However, if the sound source moves at a speed close to the speed of sound, then the accumulation intensifies indefinitely. This becomes obvious if we consider that all the sound emitted by a source moving exactly at the speed of sound directly towards the observer will reach the latter in one short moment of time, namely, when the sound source approaches the location of the observer. The reason is that the sound and the source of the sound will travel at the same speed. If sound were moving at supersonic speed during this period of time, then the sequence of perceived and emitted sound pulses would be reversed; the observer will distinguish signals emitted later before he perceives signals emitted earlier.
The process of double hum, in accordance with this theory, can be illustrated by the diagram in Fig. 58. Suppose that an airplane is moving straight towards the observer, but at a variable speed. The AB curve shows the movement of the aircraft as a function of time. The angle of the tangent to the curve indicates the instantaneous speed of the aircraft. The parallel lines shown in the diagram indicate the propagation of sound; the angle of inclination in these straight lines corresponds to the speed of sound. First, on the segment the aircraft speed is subsonic, then on the segment it is supersonic, and finally, on the segment it is subsonic again. If the observer is at the initial distance D, then the points shown in horizontal line correspond to the sequence of perceived
Rice. 58. Distance-time diagram of an airplane flying at variable speed. Parallel lines with an angle of inclination show the propagation of sound.
sound impulses. We see that the sound produced by the aircraft during the second passage of the sound barrier (point ) reaches the observer earlier than the sound produced during the first passage (point). During these two moments, the observer perceives, through an infinitesimal interval of time, impulses emitted during a limited period of time. Consequently, he hears a boom like an explosion. Between two humming sounds, he simultaneously perceives three impulses emitted in different time by plane.
In Fig. Figure 59 schematically shows the noise intensity that can be expected in this simplified case. It should be noted that the accumulation of sound pulses in the case of an approaching sound source is the same process known as the Doppler effect; however, the characteristic of the latter effect is usually limited to the change in pitch associated with the accumulation process. The intensity of perceived noise is difficult to calculate because it depends on the sound production mechanism, which is not very well known. In addition, the process is complicated by the shape of the trajectory, a possible echo, as well as shock waves that are observed in various parts aircraft during flight and whose energy is converted into sound waves after the aircraft reduces speed. In some
Rice. 59. Schematic representation of noise intensity perceived by an observer.
Recent articles on this topic have attributed the phenomenon of double hum, sometimes triple, observed in high-speed dives to these shock waves.
The problem of "breaking the sound barrier" or "sound wall" seems to capture the public's imagination (an English movie called "Breaking the Sound Barrier" gives some idea of the challenges associated with Mach 1 flight); pilots and engineers discuss the problem both seriously and jokingly. The following "scientific report" of transonic flight demonstrates a fine combination of technical knowledge and poetic license:
We glided smoothly through the air at 540 miles per hour. I've always liked the little XP-AZ5601-NG for its simple controls and the fact that the Prandtl-Reynolds indicator is tucked away in the right corner at the top of the panel. I checked the instruments. Water, fuel, revolutions per minute, Carnot efficiency, ground speed, enthalpy. All OK. Course 270°. The combustion efficiency is normal - 23 percent. The old turbojet engine purred calmly as always, and Tony's teeth barely clicked from his 17 doors, thrown over Schenectady. Only a thin trickle of oil leaked from the engine. This is life!
I knew the airplane engine was good for speeds higher than we had ever attempted. The weather was so clear, the sky so blue, the air so calm that I couldn’t resist and increased my speed. I slowly moved the lever forward one position. The regulator only moved slightly, and after five minutes or so everything was calm. 590 mph. I pressed the lever again. Only two nozzles are clogged. I pressed the narrow hole cleaner. Open again. 640 mph. Quiet. The exhaust pipe was almost completely bent, with a few square inches still exposed on one side. My hands were itching for the lever, so I pressed it again. The plane accelerated to 690 miles per hour, passing through the critical segment without breaking a single window. The cabin was getting warm, so I added some more air to the vortex cooler. Mach 0.9! I've never flown faster. I could see a slight shake outside the porthole so I adjusted the wing shape and it went away.
Tony was dozing now, and I blew smoke from his pipe. I couldn't resist and turned up the speed one more level. In exactly ten minutes we reached Mach 0.95. At the rear, in the combustion chambers, the overall pressure dropped like hell. This was life! The Pocket indicator showed red, but I didn't care. Tony's candle was still burning. I knew the gamma was at zero, but I didn't care.
I was dizzy from excitement. A bit more! I put my hand on the lever, but just at that moment Tony reached over and his knee hit my hand. The lever jumped up ten levels! Fuck! The small plane shuddered along its entire length, and a colossal loss of speed threw Tony and me onto the panel. It felt like we had hit a solid brick wall! I could see that the nose of the plane was crushed. I looked at the speedometer and froze! 1.00! God, in an instant I thought, we are at the maximum! If I don't get him to slow down before he slips, we'll end up in diminishing drag! Too late! Mach 1.01! 1.02! 1.03! 1.04! 1.06! 1.09! 1.13! 1.18! I was desperate, but Tony knew what to do. In the blink of an eye he backed up
move! Hot air rushed into the exhaust pipe, it was compressed in the turbine, again broke into the chambers, and expanded the compressor. Fuel began to flow into the tanks. The entropy meter swung to zero. Mach 1.20! 1.19! 1.18! 1.17! We are saved. It slid back, it slid back while Tony and I prayed that the flow divider wouldn't stick. 1.10! 1.08! 1.05!
Fuck! We hit the other side of the wall! We're trapped! There is not enough negative thrust to break back!
As we cowered in fear of the wall, the tail of the small plane fell apart and Tony shouted, “Light up the rocket boosters!” But they turned in the wrong direction!
Tony reached out and nudged them forward, Mach lines flowing from his fingers. I set them on fire! The blow was stunning. We lost consciousness.
When I came to my senses, our small plane, all mangled, was just passing through zero Mach! I pulled Tony out and we fell hard to the ground. The plane was slowing down to the east. A few seconds later we heard a crash, as if he had hit another wall.
Not a single screw was found. Tony started weaving netting and I wandered off to MIT.