The significance of Mach numbers

This article is for time-pass.

Supersonic and trans-sonic flights are those that are greater than or around the speed of sound. They are an interesting and difficult region of flight because the assumed characteristics of the atmosphere no longer hold water.

When a body moves through the atmosphere, a kind of pressure wave is generated and sends an advance warning to the air ahead that a body is 'coming'. These pressure waves travel at the speed of sound. As long as the body is moving at low speeds through the air, the air ahead adjusts itself accordingly and makes way for the body.

But when the aircraft moves at speeds close to the speed of sound, the air ahead gets no 'advance' warning about the incoming body. Instead, the airflow comes up against the object all of a sudden. This is responsible for the changed behaviour of the airflow at transsonic and supersonic speeds.

At this juncture, we have to discard all our traditional assumptions about airflow. Air at low speeds prefers smooth surfaces; air at high speeds prefers sharp and pointed corners. This is one of the reasons behind the 'Delta' configuration for supersonic aircrafts.

The absolute speed of the aircraft or the absolute speed of sound doesn't matter; what matters is the ratio between the two. Again, the speed of sound is not constant, it decreases with increasing altitude. Thus the relative speed of aircraft (as a fraction of the Mach number) increases with increasing altitude.

On the other side of the sound barrier, we face an entirely new world: the supersonic and hypersonic zones. Aircraft are specially designed for flying in these regions of speed. Our knowledge of these regions is better than our knowledge of the transsonic speed zone. Therefore, the pilot's aim is to get through the sound barrier as soon as possible.

In the olden days, when a speed record had to be set, the pilot waited for a warm day, because the speed of sound would be highest on those days. High-speed flying required the speed of sound to be as high as possible. But today, we soar for new heights and new speeds, and can no longer be bound by the speed of sound.