Flight requires overcoming drag, which acts parallel to the relative wind. Drag is split into several distinct physical phenomena. Physical Cause Mitigation Strategy Frictional shearing stress within the boundary layer. Polished surfaces, laminar flow airfoils. Form (Pressure) Drag
The pressure differential and fluid acceleration are mutually dependent. One does not strictly "cause" the other; they coexist. The Role of Streamline Curvature understanding aerodynamics arguing from the real physics pdf
ρ(𝜕u𝜕t+u⋅∇u)=−∇p+μ∇2u+frho open paren the fraction with numerator partial bold u and denominator partial t end-fraction plus bold u center dot nabla bold u close paren equals negative nabla p plus mu nabla squared bold u plus bold f is fluid density. is the velocity vector field. is static pressure. is dynamic viscosity. represents external body forces like gravity. The Kutta-Joukowski Theorem and Circulation Flight requires overcoming drag, which acts parallel to
A wing is not a "suction machine." The low pressure on top is caused by the air following a curved path. For a fluid parcel to move in a curve, there must be a net inward force (centripetal force). That force comes from a pressure difference: lower pressure toward the center of curvature. The top of the wing is the center of curvature, so pressure drops. Polished surfaces, laminar flow airfoils
Lift requires viscosity to set the circulation, but once circulation is set, the magnitude of lift is given by inviscid theory (Bernoulli + Newton). You need both.