Reynolds number Re = ρVL/μ (or Vc/ν) compares inertial to viscous effects. For airfoils, L is usually the chord.
Why Re changes everything
At low Re (small chord or low speed), viscous effects dominate: thicker boundary layers, different transition, and often lower Cl max with higher Cd. Airliner chords at cruise live at much higher Re.
Typical student ranges
| Mission | Rough Re band |
|---|---|
| Small UAV / RC | ~10⁵ to a few ×10⁵ |
| Light aircraft sections | Often ~10⁶ order |
| Large transport cruise | Higher still |
Image placeholder — same airfoil polars at two Re values
Fair-test rule
Never compare two airfoils at different Re and call one better. Match Re to the mission, then shortlist in HFOIL.