When describing the speed of flight, people often use Mach (Mach number) as the unit, what does this Mach mean? In simple terms, Mach is actually the ratio of speed to the speed of sound.
Since the speed of sound is not a fixed value, the speed of sound under different conditions (such as medium, temperature, pressure, etc.) has different values, so if we want to ask how fast Mach 1 is, we do not directly say how many kilometers per hour Mach 1 is equivalent to, but should add the determined conditions before that.
For example, in the Earth's atmosphere, when the air pressure is one standard atmosphere and the temperature is 15 degrees Celsius, the speed of sound in air is about 340 meters per second, and the speed of flight in air under such conditions, Mach 1 is equivalent to about 1224 kilometers per hour.
As you can see, it is not intuitive to use Mach to describe the speed of flight, so the question arises, if so, then why is the speed of flight commonly used in Mach? Wouldn't it be more convenient to just use kilometers per hour? Here we will talk about it.
When an object flying in the air, the air located in front of it will be squeezed, if the object is relatively slow, then the group of air located in front of it will undergo "compression", "recovery" and then "pop off" such a process.
Because the above process takes a certain amount of time, so when an object in the air to reach a specific value of the flight speed, located in front of it, the group of air in the "compressed" just can not "recover", and can not "pop off", so the group of "compressed" air will always stay in front of the object, and as the object continues to fly, the more front of the air will also go through the same process, resulting in more and more "compressed air" piled up in front of the object.
Then the question arises, what is this "specific value" in the end? The answer is the speed of sound.
In essence, sound is actually a wave generated by the vibration of an object, and the sound wave in the air is actually a "longitudinal wave".
The above figure shows the way the sound waves propagate in the air, you can see that the sound waves are actually through the air molecules "compression" and "recovery" to spread, in other words, the air "compression" and "recovery" speed, in fact, is the speed of sound propagation in the air, that is, the speed of sound.
That is, when an object flying through the air at the speed of sound, its front will quickly accumulate a large amount of "compressed air", in this case, the object will encounter a sudden increase in air resistance, this phenomenon is actually the "sound barrier" that we often hear.
On the other hand, when the speed of flight in the air exceeds the speed of sound, although the air in front of it will also be "compressed", but in this case, the speed of these "compressed air" is slower than the speed of flight, so they can not be in front of the object a large number of accumulation, and will not form a strong drag on the object.
So for human aircraft, want to supersonic flight, it is necessary to break the "sound barrier", and the emergence of the "sound barrier" means that the handling performance and stability of the aircraft will suddenly become worse, so the pilot is necessary to understand the current speed and the speed of sound in a timely manner, so that you can prepare in advance, and thus significantly improve the safety of flight.
Since the speed of sound under different conditions has different values, the pilot only knows "how many kilometers per hour", is unable to intuitively grasp the current speed of the aircraft and the speed of sound of the immediate relationship, so those faster aircraft, usually equipped with a special display Mach number of the instrument, in short, this instrument can be based on the current area of the various conditions (such as air density, pressure, temperature, etc.), calculate the current area of the speed of sound, and then its current flight speed for comparison, and then calculate the Mach number.
It is important to note that the "sound barrier" is only one of the reasons why "the speed of flight is often measured in Mach", in fact, the use of Mach number has another important meaning, that is, it can be used to accurately describe the aerodynamic characteristics of the aircraft.
Since the air density, temperature and atmospheric pressure of the Earth's atmosphere will vary significantly depending on the altitude, the current state of the vehicle when flying at high speed in the air will vary greatly with the area where it is located, and cannot be calculated simply based on "how many kilometers per hour".
Strictly speaking, when a vehicle is flying in the air, its Mach number is the ratio of its speed in the air to the speed of sound in the air in the current region.
Since there is a definite correspondence between the speed of sound and air density, temperature and atmospheric pressure, the Mach number of a vehicle can be used as an ideal indicator of the degree of air compression, and the Mach number can accurately describe the aerodynamic characteristics of the vehicle in the current situation.
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