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What is Mach Number and What Does it Mean for Aircraft Speed?

# What is Mach Number and What Does it Mean for Aircraft Speed?

Jun 02, 2023

When it comes to aircraft performance and speed, Mach numbers provide an accurate measure of the speed of an aircraft in relation to the speed of sound. Learn more about what these measurements are and why they are important for measuring aircraft performance and speed.

## Introduction to Mach Number

Mach number, or Mach, is the ratio of a vehicle's speed to the speed of sound in the surrounding air. It is named after Ernst Mach, an Austrian physicist and philosopher who studied supersonic shock waves and developed the theory on which this method of measuring aircraft performance was based. Aircraft that travel at speeds faster than the speed of sound are said to be traveling at “Mach 1”, and as speeds reach higher than this value, it indicates more and more supersonic flight.

As aircraft accelerate, the air molecules that surge around it have to travel faster than their own speed of sound. This creates compression ripples called Mach cones in the layer of near-stationary air that is left behind; similar to the pattern created by a stone dropped into a pond. The Mach number of an aircraft can be used as a measure of its performance and as an indicator for when supersonic flight begins. Mach numbers are also used to calculate air density, altitude and temperature which are important considerations for pilots during flight. When flying at a Mach number higher than 1, additional drag forces act on the aircraft due to phenomena such as shock waves and boundary layer effects which will affect how it flies.

## How Do You Calculate Mach Number? What is a knot?

Mach number can be expressed using several different metrics, such as miles per hour (mph) or knots. Knots are a scale of speed measurement that is equal to one nautical mile (1.15 statute miles) per hour. Additionally, the International Standard Atmosphere (ISA) model is used for calculating Mach numbers at higher altitudes because air pressure and temperature decrease with height, which affects the speed of sound.

Mach number is a metric used to measure the speed of an aircraft compared to the speed of sound in the same atmospheric conditions. A Mach number of 1 means that an aircraft is traveling at the speed of sound, and any Mach number greater than 1 means that an aircraft is traveling faster than the speed of sound. For example, if an aircraft has a Mach number of 2, it is travelling twice the speed of sound. The calculation for Mach number involves multiplying knots by the ratio between Mach 1 and total air temperature, which can range from -60 degrees Celsius to 20+ degrees Celsius depending on altitude.

### What Are the Benefits of Mach Number for Aircraft Performance?

Mach number is a helpful tool for gauging aircraft performance. It provides a consistent way to measure aircraft speed no matter the altitude, and offers important insights into the dynamic nature of airflow around an airplane. Additionally, it allows aviators to accurately assess their airspeed in relation to the speed of sound at various altitudes. As an example, when an aircraft passes Mach 1, or the speed of sound, at any given altitude then greater amounts of drag start to occur as shockwaves form around the craft — this increase in drag can affect flight performance significantly.

### What Are Some Examples of Aircraft Speeds in Mach Numbers? How fast do other popular private jets fly?

Private jets typically fly somewhere in the range of Mach 0.6 to Mach 0.8, or 600 to 800 mph at cruising speed. Many modern private jets can even exceed speeds of Mach 0.9, and some older models may only reach around Mach 0.4. Some fast military aircrafts like the SR-71 Blackbird operate at speeds of up to Mach 3.5, making it one of the fastest air-breathing airplanes ever built.

Mach, or often written as Ma, is a measure of the speed of an aircraft relative to the speed of sound. When describing aircraft speeds in Mach numbers, 1.0 Mach equals the speed of sound in air, which for sea level (standard day) is about 1225 km/h (761 mph). Mach 2.0 would indicate twice the speed of sound and so on. As the altitude increases, so does the speed of sound; at 35 000 feet it can reach approximately 1400 km/h (870 mph). This means that higher flying aircraft will achieve much faster speeds than those flying at lower altitudes before reaching a Mach number higher than 1.0.

### What is the fastest private jet aircraft – and how fast does it fly?

The fastest private jet in the world is the Cessna Citation X+. It can fly at a top speed of Mach 0.935 and has a range of 3,460 nautical miles (6,419 km). Other notable fast private jets include the Gulfstream G650ER, which can reach speeds up to Mach 0.925, and the Bombardier Global 6000, which flies at a maximum speed of Mach 0.89.

So what does Mach speed actually mean? The term ‘Mach’ is named after the Austrian physicist and philosopher Ernst Mach, and is used to measure the speed of an object relative to the speed of sound. Mach 1 means that a jet or airplane is traveling at the same speed as sound – that's about 700 mph or 1,126 km/h. Anytime you hear someone refer to an aircraft’s ‘Mach number’, they are referring to how fast it can travel compared to the speed of sound.