Above and Below the Mach

Above and Below the Mach
Most pilots are familiar with the term Mach, and have a basic understanding of the concept of the value as it relates to the speed of sound. However, there are several aspects of Mach and supersonic flight that are not well understood. This article seeks to explain Mach and supersonic flight, while also dispelling some of the common misunderstandings associated with the concept.
First, it is necessary to examine the idea of Mach number. The simplest definition of Mach number is just a ratio of an object’s speed as compared to the speed of sound. For example, on a standard day at sea level sound travels approximately 761 miles per hour (661 Knots). If an object were to be travelling at 571 MPH (496 KTAS), it would be travelling three quarters of the speed of sound, or 0.75 Mach. You may be questioning my numbers since many business aircraft regularly travel around .75 Mach, but nowhere near 496 knots. Well, this brings up the first Mach myth, specifically that the speed of sound is constant. This is incorrect. The speed of sound is actually constantly changing. Why is this? Well, we must look at what is meant by “speed of sound.”
Simply put, the speed of sound is the rate at which a sound wave propagates through a given medium. It is most significantly determined by the elasticity of the medium, which is a measurement of the substance’s tendency to return to its original state after being strained. The higher the elasticity, the faster sound will travel. Because of this, in a solid substance or liquids like water, the speed of sound is relatively high as compared to gasses like atmospheric air. In fact, the speed of sound in a pure diamond is a blazing 26,843 MPH! That is nearly 10,000MPH faster than a satellite in orbit.
The speed of sound, too, changes within a given medium as a function of temperature. The lower the temperature, the lower the speed of sound. This is why a business jet flying at FL350 can reach .75 Mach. Three-quarters the speed of sound at this altitude is only 432 KTAS, or 240 KIAS. Likewise, at higher altitudes, Mach number translates to true airspeed more precisely than indicated airspeed, which is why controllers will start giving airspeed calls in Mach number at higher altitudes.
More myths revolve around the sonic boom that accompanies an aircraft when travelling above the speed of sound. Let’s look at this in a little more detail. Any object’s motion through a medium displaces the molecules of said medium, creating differences in pressure known as sound waves. Specifically speaking, the speed of sound is really how fast these molecules can move, thus how quickly they can propagate sound. However, what happens if an object travels faster than the molecules can be displaced? In this case the object will start pushing air molecules forward, instead of slicing through. The resultant is pockets of compression that instantly expand as the object passes. This expansion is very quick and creates an explosive effect: the sonic boom.
Having said this, the sonic boom is not a single instance of sound accompanying an object as it surpasses the speed of sound, as some believe. Instead, the boom is being continuously generated as the aircraft travels faster than sound, but is heard as an instantaneous single boom on the ground. Furthermore, it is not technically accurate to say that an aircraft creates one sonic boom. Most aircraft produce two pressure cones, one at the nose and one at the tail. However they generally sound like a single boom as they all arrive at the same time to the listener. There are instances where rather large aircraft can generate more than one perceived sonic boom. The Space Shuttle was famous for its two sonic booms.
Now that we have an understanding of Mach and how it relates to the speed of sound, let’s look at travelling at or above the Mach. One of perhaps the most perpetuated myths surrounding the Mach is what happens to an aircraft travelling at these speeds. Many believe that it becomes very rough and turbulent above the speed of sound. However, the reality is much less dramatic. In the end, nothing really happens in an aircraft designed for supersonic flight when above the Mach. In fact, it is not unusual for fighter pilots to exceed the Mach during tactical maneuvers without even noticing due to the lack of physical feedback from the jet in this regime.
Ultimately, the idea of supersonic travel is often misunderstood. Hopefully this article has helped explain some of the intricacies with Mach number and addressed some of the misconceptions therein.

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