[Ref ACE The Technical Pilot Interview 24/348]

What’s compressibility?

Air being compressed onto a surface (at a right angle to the relative airflow), resulting in an increase in density, and thus dynamic pressure rises above its expected value.

__When will the compressibility become significant?__

At *“force divergence Mach number” **– ***above** 5-10% **Mcrit** (critical Mach number), compressibility effects begin.

What’s the effects of compressibility?

There are two (2) main effects:

- Compressibility error on dynamic pressure reading
- onset of compressibility include: shock wave formation, drag increase (wave drag), buffeting (Mach buffet), stability, and control difficulties

How to delay or alleviate the onset of compressibility?

*Ans:* using sweepback design