Generation of lift

Lift Generation

Lift is the aerodynamic force acting perpendicular to the relative wind that balances the aircraft's weight. Several explanatory models exist — all are complementary, not contradictory.

Model 1: Bernoulli + geometry (flow acceleration)

Explanation: the curved airfoil forces the air on top into a longer/narrower path → acceleration (continuity) → lower static pressure on top (Bernoulli). Pressure difference lower-upper = lift.

→ This model explains HOW the pressure distribution arises.

Model 2: Newton 3 (flow deflection)

Explanation: the airfoil deflects air downward (downwash). By Newton's third law, the air exerts an equal reaction force upward on the wing.

Quantitatively: lift = mass flow × velocity change (momentum equation).

→ This model explains WHY pressure differences exist at all.

Model 3: Circulation and Kutta-Joukowski (academic)

Explanation: around an airfoil a circulation Γ (vortex field) forms — by the Kutta condition (flow must leave smoothly at the sharp trailing edge). Lift per span:

L' = ρ · V · Γ

(Kutta-Joukowski theorem). Source: Joukowski 1906, Anderson Ch. 3 + 4.

→ Mathematically most elegant form, valid for ideal fluid.

What happens physically?

At α > 0°:

1. Air meets leading edge → stagnation point slightly below the nose (LE).
2. Flow around the LE strongly accelerates (small radius).
3. Upper surface: flow accelerates → lower p.
4. Lower surface: flow stagnates → higher p.
5. Pressure difference = lift.
6. Behind, flow leaves the airfoil with downwash (directed downward).
7. Momentum change of air downward = reaction force upward.

Which factors affect lift?

Lift equation:

L = ½ · ρ · V² · S · CL

• ρ (density): higher ρ → more lift.
• V² (velocity squared): doubling v → 4× lift.
• S (wing area): more area → more lift.
• CL (lift coefficient): function of α, airfoil shape, Reynolds number, Mach.

Equilibrium in steady level flight

• L = W (lift equals weight).
• T = D (thrust equals drag).

In banked or accelerated flow, the equilibrium shifts.

Common misconceptions

1. "Equal transit time": assumption that air on top and bottom must arrive at the trailing edge at the same time. Wrong: air on top arrives faster.
2. "Vacuum sucks": vacuum sucks nothing. Lower p on top is pushed up by higher p below → airfoil pushed upward.
3. "Camber alone necessary": even flat plates generate lift at α > 0° (via Newton deflection).