Definition
A region in supersonic airflow where the air turns away from itself around a corner or convex surface, causing the air to speed up, drop in pressure, drop in temperature, and decrease in density. Unlike a shock wave, an expansion wave is a smooth, gradual change spread across a fan-shaped area rather than an abrupt jump.
Plain English
When supersonic air flows around a surface that bends away from it, the air spreads out and accelerates. This spreading is called an expansion wave, and it makes the air faster, cooler, and at lower pressure on the far side of the bend.
Context Anchor
Seen in high-speed aerodynamics discussions, especially when studying airflow over wings, inlets, nozzles, and other aircraft surfaces at very high speed.
Derivation
From the Latin 'expandere,' meaning to spread out or stretch open. The name fits: the air literally spreads out as it follows the surface turning away, taking up more volume and dropping in pressure as it does so.
Why Pilots Care
Expansion waves change the aerodynamic forces acting on high-speed aircraft, directly affecting lift, drag, and stability.
Analogy
Think of people leaving a crowded doorway and spreading into a wide room. The crowd becomes less packed together as it spreads out; in an expansion wave, the air does something similar, but with pressure and speed changing too.
Grounding Statement
Picture supersonic air following the upper surface of a wing as it curves downward toward the trailing edge -- the air fans out around that turn, speeding up and cooling as it goes. That fan is the expansion wave.
Intuition Check
Do not read “expansion wave” as a wave that makes the airplane expand. Here, “expansion” means the air itself is spreading out and dropping in pressure.
Example Sentence 1
As the supersonic airflow rounded the shoulder of the wing, an expansion wave formed, accelerating the air and lowering its pressure.
Example Sentence 2
The maintenance manual described how expansion waves affect pressure distribution on the airframe during supersonic flight.