A desktop wind tunnel is not the sky made small. Shrink the model and you shrink the physics with it, and the air stops behaving the way it does around a real wing. The governing parameter is the Reynolds number: Re = ρvL/μ Inertia over viscosity. A Boeing 787 wing at cruise sits near Re = 5 x 10^7. A 50 mm foil in a benchtop tunnel at 10 m/s sits near 3 x 10^4 — three orders of magnitude lower. At that scale air is comparatively sticky. Boundary layers stay laminar far longer, separate earlier and more abruptly, and lift curves collapse in ways a full-scale aerofoil never sees. This is the low-Reynolds regime that insects and micro air vehicles inhabit, not airliners. Matching Re exactly would demand absurd velocities, so real facilities cheat with the fluid instead: - Cryogenic tunnels chill nitrogen to 110 K, raising density and dropping viscosity to buy back Reynolds number. - Pressurised tunnels run several atmospheres, lifting ρ directly. - Water tunnels exploit water’s kine...