Compressibility

[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:

  1. Compressibility error on dynamic pressure reading
  2. 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

 

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