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Roskam (book: Airplane Aerodynamics and Performance, Subparagraph: 2.5.2 HIGH-SPEED AIRSPEED INDICATORS (COMPRESSIBLE FLOW)) affirms:

[...] Airplane with no sweep and thick airfoils become transonic at much lower flight Mach numbers than airplanes with significant sweep and thin airfoils. [...]

Question: How does airfoil thickness affect the Mach number at which it experiences transonic flow?

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The displacement effect causes the flow around a thick body to speed up more that around an equivalent but thinner body. The thicker body pushes the air aside and around itself more, causing the flow to accelerate and to become supersonic relative to the vehicle at a lower flight Mach number.

Wing sweep has a similar effect, as only the orthogonal speed component will be affected by the wing and the effective Mach number of all thickness-, camber- and angle-of-attack-related effects is reduced by the cosine of the sweep angle. Again, the displacement effect will be reduced by sweep.

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