Transsonic

Transsonic is a term used in fluid dynamics and aerodynamics to describe flow regimes where the Mach number is near 1, a transitional zone between subsonic and supersonic flow. In many texts, transsonic is treated as synonymous with transonic, though some authors reserve transsonic for the transitional phenomena that occur as a body moves through near-sonic speeds.

When aircraft operate in the transsonic regime, parts of the flow may be subsonic while others become sonic or supersonic. Local sonic speeds cause shock waves, which can increase wave drag and alter pressure distributions. The onset is characterized by the critical Mach number, the lowest Mach number at which a local flow reaches Mach 1 on the surface.

Design and analysis in this regime focus on mitigating drag and buffet, improving stability, and preserving performance. Techniques include wing and tail sweeps, the use of area ruling to manage pressure distribution, and the development of supercritical airfoils that delay and reduce shock-induced drag. Computational fluid dynamics and wind tunnel testing are essential tools for predicting transsonic effects.

Historically, the transsonic regime was a major concern during the development of jet-powered transport aircraft in the mid-20th century, with efforts led by aeronautics organizations such as NACA and NASA. Today, the term remains part of the vocabulary for subsonic-to-supersonic flight research and is used to describe flow characteristics in the high-subsonic range found on modern airframes and engines.