Definition
The sharp, rapid increase in aerodynamic drag that occurs as an aircraft approaches the speed of sound, caused by the formation of shock waves on the wing as airflow over parts of the wing reaches sonic speed. The Mach number at which this rapid drag rise begins is called the drag divergence Mach number.
Plain English
As the aircraft gets close to the speed of sound, drag suddenly shoots up much faster than before. This happens because shock waves start forming on the wing, disrupting the smooth airflow and creating a big extra penalty in drag.
Context Anchor
Seen in high-speed flight discussions about Mach number limits, critical Mach number, and why an aircraft cannot simply keep accelerating as it nears transonic speed.
Derivation
Drag is the rearward aerodynamic force opposing motion. Divergence comes from the Latin divergere, meaning 'to move apart' or 'to depart sharply.' Together the term describes the point where the drag curve departs sharply upward from its previous gentle rise, rather than continuing smoothly with speed.
Why Pilots Care
It limits safe operating speeds and raises fuel use dramatically in the transonic range, influencing aircraft design choices like wing sweep.
Analogy
Like a boat suddenly meeting far more resistance once it tries to climb its own bow wave.
Grounding Statement
Below the drag divergence Mach number, drag rises gently with speed. Above it, drag rises steeply and quickly, as if the aircraft hit an invisible wall of air resistance.
Intuition Check
Drag divergence does not mean the airplane is turning away from its course. It means drag is rapidly increasing away from its normal smooth increase as the aircraft nears very high speed.
Example Sentence 1
The aircraft's MMO is set comfortably below the drag divergence Mach number to keep it clear of the rapid drag rise.
Example Sentence 2
Wing sweep was added to the design to push drag divergence to a higher Mach number.