Dynamic lift

Dynamic lift is the force on a body such as an aeroplane wing, a hydrofoil or a spinning ball because it moves through a fluid.

In cricket, tennis, baseball and golf a spinning ball swerves away from its parabolic path; Bernoulli's principle explains this in part.

Ball without spin: the streamlines are symmetric, so air moves equally fast above and below at matching points. No pressure difference, no vertical force from the air.

Ball with spin: the spinning ball drags air round with it (more if its surface is rough). Relative to the ball the air above moves faster and the air below slower; streamlines crowd above and spread below.

The slower air below is at higher pressure, giving a net upward force. This lift caused by spin is the Magnus effect.

An aerofoil is a solid shaped to give upward lift when it moves horizontally through air; aircraft wing sections look roughly like one. Its shape and tilt crowd the streamlines above it, so air flows faster on top than underneath and the lift balances the plane's weight.

Loaded aircraft of mass 3.3 × 10⁵ kg, wing area 500 m², level flight at 960 km/h, air density 1.2 kg m⁻³: ΔP = (3.3 × 10⁵ × 9.8)/500 = 6.5 × 10³ N m⁻².

Ignoring the small height difference, ΔP = ½ρ(v₂² − v₁²), so (v₂ − v₁)/v_av = ΔP/(ρv_av²). With v_av = 960 km/h = 267 m s⁻¹ this is ≈ 0.08: air above the wing need move only about 8% faster than below.