Definition
In aerodynamics, a flow condition in which changes in pressure produce no significant change in the density of the air. Airflow is treated as incompressible at low subsonic speeds (generally below about Mach 0.3), where density variations are small enough to be ignored in calculations of lift, drag, and pressure.
Plain English
At slower speeds, air behaves as if it cannot be squeezed — its density stays roughly the same as it flows around the airplane. This makes the math and the aerodynamic behavior much simpler.
Context Anchor
Used in high-speed flight discussions when comparing ordinary lower-speed airflow with airflow near the speed of sound, where air compression starts to matter.
Derivation
From Latin 'comprimere' — to press together. 'Incompressible' literally means 'cannot be pressed together.' In aerodynamics it doesn't mean air is truly impossible to compress — only that at low speeds the compression is so small it can be ignored.
Why Pilots Care
The assumption lets pilots use simple equations for lift and airflow at low speeds; above roughly Mach 0.3 the density changes and different formulas are required.
Analogy
Water in a hose is often treated as nearly incompressible: when it flows around a bend, you do not think of it as being squeezed much smaller. Low-speed airflow is handled in a similar simplified way.
Grounding Statement
At normal training-aircraft speeds, the air moving over the wing changes direction and pressure, but its amount packed into a given space changes very little.
Intuition Check
Incompressible does not mean air can never be compressed. It means the compression is small enough in this situation that it can be ignored.
Example Sentence 1
At typical training speeds, airflow over the wing is treated as incompressible, which is why simple lift equations work well.
Example Sentence 2
The incompressible-flow equations give accurate results until the airplane approaches the speed where density begins to vary.