How Do Planes Stay in the Air?
Lift, weight, thrust, drag — without the myths · Updated May 2026
A plane stays in the air because its wings push air downward, and the air pushes the wing upward by an equal and opposite force. That upward force is called lift, and it balances the aircraft's weight. The engines produce thrust to overcome drag (air resistance). Lift can be described mathematically two ways — using Newton's laws (the wing turns the flow downward, so the flow pushes the wing upward) or using Bernoulli's principle (faster airflow over the upper surface means lower pressure on top). Both descriptions are correct; they're the same physics from different angles. NASA Glenn Research Center explicitly says so.
The four forces of flight
At any moment during steady, level flight, four forces act on the aircraft and balance one another:
An upward aerodynamic force, produced primarily by the wings, that opposes weight. Its magnitude depends on airspeed, air density, wing area, wing shape, and angle of attack.
The downward pull of gravity on the aircraft's total mass (structure, fuel, payload, passengers, crew). For a fully loaded 737-800, that's about 79,000 kg — about 775,000 N of downward force.
A forward force from the engines, produced by accelerating a mass of air rearward (Newton's third law again). Modern high-bypass turbofans accelerate a large mass of air a small amount — efficient at subsonic speeds.
A backward force from air resistance. Has two main components: induced drag (the cost of producing lift, biggest at low speeds), and parasite drag (skin friction, form drag, biggest at high speeds).
In steady, level cruise: lift = weight, thrust = drag. To climb, the aircraft increases thrust and pitches up; lift temporarily exceeds the weight component until a new equilibrium climb attitude is established. To descend, the opposite.
Where does lift actually come from?
NASA Glenn Research Center's beginner's guide is unusually direct on this: two descriptions, same physics.
As the wing moves through the air, it deflects the air downward (a "downwash"). Newton's third law: every action has an equal and opposite reaction. The air pushed downward pushes the wing upward. That upward push is lift. This is true for a flat plate at an angle, a cambered airfoil, an upside-down wing — anything that turns the flow.
Where the flow speeds up, the pressure drops. Over the upper surface of a wing at a positive angle of attack, the flow is accelerated; the pressure there is lower than below the wing. The pressure difference, integrated over the wing area, gives an upward net force — lift.
The pressure difference (Bernoulli) is what physically pushes on the wing. The flow turning (Newton) is the integrated result of those pressures acting around the airfoil. You can't have one without the other. The flow is turned because of the pressure field; the pressure field exists because the flow is turning. They're two sides of the same momentum and energy bookkeeping.
Common misconceptions
- ×"Equal transit time" — the idea that air molecules that split at the leading edge must rejoin at the trailing edge "at the same time," so the longer upper path forces faster flow on top. This is wrong. Measurements show the air over the top reaches the trailing edge well before the air going underneath. The longer-path / equal-transit-time explanation is taught in many textbooks but contradicts measurement. NASA Glenn explicitly debunks it.
- ×"A symmetric wing can't make lift" — wrong. A symmetric airfoil at a positive angle of attack produces lift the same way. Aerobatic aircraft (which fly upside-down) typically use symmetric wings.
- ×"The plane is held up by the engines" — no. The engines provide forward thrust, not upward force. If the engines fail, the aircraft becomes a glider: it descends, but it does not fall. A modern airliner has a glide ratio around 17:1 — for every 1 km of altitude lost, it glides about 17 km forward. The 2001 Air Transat Flight 236 glided around 120 km to a safe landing in the Azores after fuel exhaustion.
- ×"Air is too thin at altitude to support a plane" — the air is indeed thinner, but lift increases with speed squared. Aircraft fly faster at altitude to maintain enough lift. The product of density and speed-squared (dynamic pressure) is what counts.
The lift equation
For engineering purposes, lift is calculated with:
where ρ is air density, V is airspeed, S is wing reference area, and CL is the lift coefficient — a dimensionless number that captures the airfoil shape and angle of attack. The pilot's job, when controlling lift, is essentially to manage V (speed) and the angle of attack (which sets CL). Flaps and slats deployed for takeoff and landing temporarily change S and CL — that's why flaps are extended when slow and retracted in cruise.
Takeoff, cruise, landing — same equation, different settings
- →Takeoff: low speed, so the aircraft needs a high lift coefficient (high angle of attack + flaps extended) to produce the lift the weight requires.
- →Cruise: high speed and dynamic pressure, so a small angle of attack and clean configuration are enough. Flaps fully retracted.
- →Landing: slow again, so flaps extend further than at takeoff; the angle of attack increases close to touchdown.
Sources
- • NASA Glenn Research Center — Beginner's Guide to Aeronautics: "Four Forces on an Airplane" and "Bernoulli and Newton."
- • FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) — Chapter 5 (Aerodynamics of Flight).
- • Anderson & Eberhardt — "Understanding Flight" (peer-reviewed treatment of lift theory).
- • AIAA Aviation Safety Magazine — "Theories of Lift."