Jose wrote:
Most aerodynamic equations dealing with low subsonic speeds treat air as an
incompressible fluid because compressibility doesn't have a significant
effect until you approach sonic speeds.

Isn't compressiblity what causes pressure changes (absent temperature
changes)?


No.

Compressible fluids (commonly called liquids) also experience
pressure changes. THis is used advantageously for hydraulic
power.

The distinction is that a compressible fluid (commonly called gas)
undergoes a volume change proportionate to the pressure change,
while the volume of an incompressible fluid changes little with
pressure. Compressible fluids obey Charles' law,
(or is it Boyle's law?):

P1 * V2 = P2 * V1

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FF