The downward pull of gravity on the aircraft's mass.
Inviscid (frictionless) theory predicts zero drag and no flow separation. Real physics argues that the —the microscopic layer of air stuck to the surface—dictates everything. Flow separation, stall, laminar-to-turbulent transition, skin friction drag, and even lift degradation all originate here. understanding aerodynamics arguing from the real physics pdf
Why argue from real physics?