You were probably taught in school that the air on top of a wing has to travel further than the air underneath, so it has to go faster, so by Bernoulli the pressure on top drops and the wing is "sucked up." This is called the equal transit time theory, and it is wrong. The two streams do not meet at the trailing edge — the top stream actually arrives well before the bottom one.
The truth is both simpler and more interesting. Bernoulli is right and Newton is right, and they are not competing theories — they are two correct ways of describing the same physical event.
The lift formula
The working definition every ATPL candidate must know:
Resolve the aerodynamic force into components relative to the oncoming flow: the component across the flow is lift; the component along it is drag. For a chosen wing and flight condition, calculate lift with L = ½ ρ V² S CL. This equation defines the coefficient CL rather than prescribing one universal value for every wing.
In plain terms, the lift you produce depends on:
- ρ — air density (kg/m³). Thinner air → less lift.
- V — true airspeed (m/s). Squared, so doubling V quadruples lift.
- S — wing area (m²). Bigger wing → more lift.
- CL — lift coefficient. A dimensionless number that captures the combined effect of angle of attack and aerofoil shape.
Everything else in this concept — Bernoulli, Newton, downwash, AoA, stall — is just a way of explaining where CL comes from.
Two correct ways to describe the same lift
View 1 — Bernoulli (the pressure view)
The wing's shape, angle and surrounding flow establish a pressure distribution over both surfaces. The lifting flow commonly accelerates over the upper surface, where static pressure falls. Treating a wing as half of a Venturi tube cannot determine that flow correctly:
In the low-speed model, follow one streamline through the flow around the wing. Where that flow gains speed, its pressure falls, provided changes in height and viscous losses can be neglected. To calculate lift you still need the actual flow distribution; the equation alone does not determine it.
Static pressure on top of the wing is now lower than static pressure underneath. The pressure difference, integrated over the wing area, is the lift force. The blue arrows in the widget point outward from the upper surface — that is suction pulling the wing up — and the amber arrows point inward on the lower surface — that is the higher pressure pushing it up.
View 2 — Newton (the momentum view)
Stand behind a wing in flight and you will measure a column of air moving downward. The wing has deflected the airflow downward through an angle (this is downwash, visible as the streamlines curving down past the trailing edge in the widget).
Consider a volume surrounding the wing and compare the momentum carried into and out of it by the air. Producing a downward momentum change requires a force on that air; the reaction on the aircraft is upward. For a simplified steady-flow estimate, neglecting other boundary terms:
L = (mass flow per second) × (downward velocity imparted)
More AoA, more downwash, more lift — until the flow can no longer stay attached to the upper surface, at which point the wing stalls.
These are not two theories
Bernoulli describes the pressure field that does the lifting. Newton describes the momentum exchange with the air. They are mathematically equivalent — pressure imbalance is what causes the flow to be deflected, and the deflected flow is what creates the pressure imbalance. Asking which one is "really" lifting the wing is like asking whether a ball falls because gravity pulls it or because its potential energy is converting to kinetic energy. Both. Always.
The misconception: equal transit time
The schoolbook story claims that air molecules splitting at the leading edge must rendezvous at the trailing edge, so the longer path over the cambered upper surface forces the upper flow to go faster. This sounds plausible and is completely false.
There is no physical law requiring the two airflow streams to meet again at the trailing edge. In wind-tunnel measurements the air over the top arrives at the trailing edge well ahead of the air underneath. The upper flow really does go faster — but for the right reason (the circulation that the wing sets up), not because it is racing the lower flow to a finish line.
The killer counter-example: a flat plate, or an inverted cambered aerofoil at positive AoA, both produce lift. Neither has a "longer upper path." Aerobatic aircraft fly inverted by exactly this mechanism.
Angle of attack — the master variable
Hold airspeed, density, and wing area constant and lift becomes a function of one thing: angle of attack. The linear approximation built into the widget:
Try increasing angle of attack gradually in the widget. Initially the wing develops more lift at the same speed. Near its maximum lift coefficient, separation becomes extensive enough that a further increase in angle no longer produces the previous lift gain. The critical angle depends on the aerofoil, configuration and flow conditions; 16° is an illustration, not a universal aircraft limit.
Thin-aerofoil theory gives a useful small-angle model: CL ≈ 2π × (α − α₀), with angles in radians and α₀ the zero-lift angle. A finite wing generally has a smaller lift-curve slope. The widget uses a simplified curve; its stall angle is not an operating limit. A stalled wing still produces some lift, but the separated flow has changed its behaviour substantially.
What ATPL examiners ask about the lift formula
The exam questions on L = ½ ρ V² S CL are conceptual, not numerical.
The patterns to know:
- Speed doubles → lift quadruples (V is squared).
- Density halves → lift halves (linear in ρ — why hot, high or humid days reduce lift at the same TAS and CL).
- AoA increases → CL increases → lift increases, until stall.
- Wing area is fixed by design (flaps deploying effectively changes CL_max; some flap designs also increase effective wing area).
If a question asks how lift changes when one variable is altered, hold the others constant and apply the proportionality.
Common mistakes
- "Bernoulli OR Newton." They are not alternatives. Pressure distribution and momentum deflection are two coordinates describing one event. Any modern aerodynamics textbook derives one from the other.
- "Equal transit time." Air on top does not have to meet the air from underneath at the trailing edge. The longer-path argument is wrong, and even worse, it gives the right answer for the wrong reason — making it harder to spot real misconceptions later.
- "Symmetric aerofoils can't lift." They can — at any non-zero AoA. A symmetric section produces zero lift only at α = 0°. This is exactly how aerobatic and supersonic wings work, and it is visible in the widget if you set camber-style AoA back to 0° and note CL is near zero, then increase α.
Why it matters
Every airspeed, weight, configuration, and altitude limitation in the flight manual traces back to L = ½ ρ V² S CL. Stall speed is the speed at which CL has nowhere left to grow. Service ceiling is the altitude at which the available climb performance reaches the specified minimum, depending on thrust or power as well as drag. Manoeuvre speed is where, in a simplified positive-load diagram, pulling g would push CL past CLMAX before it pushes load past structural limits. Understanding both the pressure and momentum views gives you a complete mental model — which is the difference between memorising a formula and actually flying the wing.