Why do airplanes use Mach instead of km to describe speed?

Unlike kilometers or miles, Mach is measured in terms of the speed of sound. So why do planes choose Mach instead of the usual kilometers ?

Definition and origin of Mach speed: Unit of speed of sound

Mach speed is a unit of measurement for the speed of an object, defined as a dimensionless value obtained by comparing the speed of an object with the speed of sound in a medium. The origin of Mach speed can be traced back to the Austrian scientist Ernst Mach.

In the late 19th century, Ernst Mach became interested in the study of the behavior of sound. At that time, it was believed that sound was the transmission of "sound waves" , and the speed of moving objects in the air attracted Ernst Mach's attention. He observed in his experiments that when an object moves faster than the speed of sound in the medium, a special effect - the "Mach cone" - appears.

The so-called "Mach cone " is a phenomenon similar to a cone-shaped cloud, the faster an object moves, the more obvious the Mach cone becomes. When observing this phenomenon, Ernst Mach came to an important conclusion, that is, when the speed of an object reaches the speed of sound, the pressure waves it causes will no longer be able to continue to propagate but will overlap to form a Mach cone.

There is a close relationship between Mach speed and the speed of sound . Sound is a mechanical wave that propagates in a medium, and its speed depends on the density and elastic modulus of that medium. Under normal temperature and pressure conditions, the speed of sound in air is about 343 meters per second. (Photo: Zhihu)

Based on this conclusion, Ernst Mach proposed a new theory that when an object moves, it not only produces sound but also causes a change in pressure. He continued to verify this theory experimentally and proposed a method for calculating the speed of an object - Ernst Mach divided the speed of the object by the speed of sound in the medium and the resulting dimensionless value was the Mach number.

The introduction of the Mach number is of great significance. In the aerospace industry, the Mach number is a common unit for measuring the speed of aircraft. In air under normal conditions, speeds greater than or equal to Mach 1 (343 m/s or 1235 km/h in air at sea level at 20 degrees Celsius) are supersonic speeds. Speeds greater than Mach 5 are called hypersonic.

The difference between the speed of sound and the Mach speed is that the speed of sound waves in a medium , while the Mach speed measures the speed of a moving object relative to the medium. The advent of the Mach speed not only had great significance for aerospace engineering but also had an impact on research in other fields. For example, in physics and engineering, the Mach speed is widely used to study problems such as high-speed flight, aerodynamics, and explosions.

Mach speed refers to the ratio of the speed of a moving object to the speed of sound. Simply put, when an object's speed exceeds the speed of sound, it reaches a state of hypersonic speed, also known as Mach speed. (Photo: Zhihu).

Why Airplanes Use Mach to Describe Speed

Air density refers to the volume of air contained in a unit of space under specific conditions. It is affected by factors such as temperature, pressure and humidity. As altitude increases, air density gradually decreases, just as breathing becomes difficult at high altitudes. During flight, changes in air density will directly affect the flight performance and flight characteristics of the aircraft.

In the definition of Mach number, the speed of sound is an important reference value. In air, the speed of sound depends on the physical properties of the gas, especially the density of the gas. When the speed of the aircraft reaches or exceeds the speed of sound, the compression force of the air becomes significant and the air density changes due to the high-speed movement of the aircraft. This means that high- speed aircraft need to face a series of special physical problems and challenges in compressed air.

Aircraft design and performance must take these challenges into account. For example, aerodynamic properties change significantly as Mach number increases.

Pneumatics is the science that studies the motion of objects in gases, involving air forces, drag, lift, and other factors. When aircraft travel at supersonic speeds, aerodynamic effects increase and changes in pressure and temperature in the air become more significant. Therefore, it is important to understand and consider Mach number in the design and operation of high-speed aircraft.

Ultrasonic objects will generate shock waves during propagation, which is also the reason why supersonic flight produces a 'rumbling' sound. When an object moves at supersonic speeds, the sound cannot escape the air flow around the object in time, forming a compressed shock wave, the propagation of the shock wave will cause sudden changes in pressure and temperature, resulting in a deafening sound. (Photo: Zhihu)

Changes in air density also have an impact on the aircraft's propulsion system and fuel efficiency . As altitude increases and air density decreases, engine performance may decrease, requiring more thrust to maintain flight. This will have a significant impact on the aircraft's fuel consumption and flight range. Therefore, understanding the impact of air density on aircraft energy efficiency is important for efficient aircraft design and improved aviation technology.

The development of hypersonic aircraft has brought about many breakthrough technologies and applications. For example, the research on hypersonic vehicles has great significance for military, aerospace and other fields. Hypersonic aircraft have higher speeds and can reach their destinations faster, reducing travel time. In addition, hypersonic aircraft can also avoid interception by ground defense systems, increasing tactical unpredictability.

In recent years, with the continuous progress of science and technology, the research and application of human supersonic flight have also made continuous progress. For example, the concept of supersonic trains has been introduced into the field of high-speed rail to improve transportation efficiency. In addition, the design of supersonic aircraft also pursues higher speeds and lower drag to reduce energy consumption and environmental pollution. (Photo: Zhihu).

However, hypersonic flight also faces a number of challenges and limitations in practice. First , the shock waves generated by hypersonic flight can cause damage and disruption to the aircraft itself and its surroundings. In addition , hypersonic aircraft will encounter increased drag when passing through the atmosphere and will need to expend more energy to maintain flight. This is one reason why hypersonic travel is still rarely used in commercial aviation.