What does Transonic Mean?

Subsonic, Transonic and Supersonic Speeds

There are three speed ranges that are used for most aircraft – subsonic, transonic and supersonic. The differences between the three speeds are to do with how fast the aircraft is going relative to the speed of sound (Mach 1.0 which is about 1225kph or 765mph at sea level).

Subsonic means the aircraft is travelling less than the speed of sound (slower than Mach 1.0) and no shockwaves are present. Aircraft that fly at these speeds are usually small single engine propeller aircraft.

Supersonic means the aircraft is travelling faster than the speed of sound (faster than Mach 1.0) and a shockwave is present in front of the aircraft. Concorde is the most famous example of a supersonic passenger jet.

Transonic is a term used by aircraft designers to describe those high subsonic speeds – usually above Mach 0.7 – where an aircraft is travelling below the speed of sound but shockwaves are still present. So how is this possible if the aircraft is travelling slower than the speed of sound?

Imagine air flowing past an aircraft. Far upstream of the aircraft, the air flows in a uniform fashion, all at the same speed. As the air approaches the aircraft, some regions of the air speed up – in particular the air going over the top of the wings – this is part of the mechanism of flight. So around the aircraft, some regions of the air have been accelerated and are moving very quickly. At Transonic speeds these regions are moving so quickly that they actually move faster than the speed of sound; and these regions of supersonic airflow always end in a shockwave.

Almost all civil airliners flying today travel at Transonic speeds. They are designed to travel as fast as possible without generating very strong shockwaves which would make them inefficient and much more expensive to fly. Since the 1950’s, the major aircraft companies have continually improved their designs so that their aircraft can travel closer to the speed of sound but still fly economically.

This is why being able to test at transonic speeds is so important. The modern jet age would not have been possible without transonic wind tunnels.