Lift
L = ½ · ρ · v² · S · CL
The wing's reaction to the air it deflects downward. Double the speed and you get four times the lift, which is why an airliner can fly at 250 t and land at 130 knots with the same wing.
Interactive teardown
How an airplane
actually works
A 70-tonne machine leaves the ground because a wing throws several tonnes of air downward every second, and a fan the size of a doorway throws even more air backward. Everything else — the flaps, the fin, the fuel burn, the whole flight — follows from those two sentences. Pull the levers below and watch.
- Cruise altitude
- 11 km
- Cruise speed
- 0.79 Mach
- Air moved by one engine
- 350 kg/s
- Lift at takeoff
- 690 kN
01 — Anatomy
Every part, and the one job it does
A modern airliner is about four million parts flying in loose formation. These are the twenty that decide where it goes. Tap any part of the aircraft — or any name in the list — to see what it is for.
Swipe the drawing sideways to see all of it
Hover to highlight · click to pin · Esc to clear
02 — The four forces
Flight is a tug-of-war between four arrows
Lift against weight, thrust against drag. Steady level flight is the boring case where both pairs are tied. Everything interesting — climbing, accelerating, stalling — is one of them winning. Move the controls and watch the balance break.
Swipe the diagram sideways to see all of it
——
How fast the air is going past the wing. Lift grows with the square of this.
The angle between the wing and the oncoming air — not the angle to the ground.
Thin air means less lift and less drag. That trade is why jets climb.
Fuel burns off at roughly 2.5 t/h, so the aeroplane gets lighter all flight.
Weight
W = m · g
The only force that never changes direction. It always points at the centre of the Earth, which is why a banked turn needs extra lift: part of the lift is now doing the turning.
Thrust
T = ṁ · (vout − vin)
Mass flow times the speed change you give it. A turbofan grabs an enormous amount of air and speeds it up only a little — which is the efficient way to do it.
Drag
D = ½ · ρ · v² · S · CD
Two kinds, pulling opposite ways. Parasite drag grows with speed; induced drag — the price of making lift — shrinks with it. Their crossover is the aeroplane's best-glide speed.
03 — The wing
What a wing really does to the air
This is a live potential-flow solution around a Joukowski aerofoil — the same maths that first predicted lift correctly in 1906, solved in your browser at sixty frames a second. The streamlines and the pressure field are computed, not drawn.
low pressure — suction
high pressure
streamline
How much the aerofoil is arched. Camber makes lift at zero angle of attack.
Potential flow is a frictionless model, so it happily produces enormous suction peaks and never separates on its own. Past about 14° the panel adds the separation that real, viscous air would produce — and the lift coefficient is knocked down to match. Everything below that angle is the raw solution.
The explanation you were probably taught is wrong
The "equal transit time" myth
You may have been told that air splitting at the leading edge must meet again at the trailing edge, so the longer path over the top forces the air to speed up, and Bernoulli does the rest. Every part of that is false. Nothing requires the two halves to meet, and in fact they never do — the air over the top arrives at the trailing edge well before the air underneath. Watch the paired particles in the simulation above and you will see them separate and never re-form.
What is actually happening
The wing forces the air to follow a curved path. Curving a flow requires a pressure gradient across it, so pressure falls above the wing and rises below. The same curvature means air leaves the trailing edge moving downward: several hundred tonnes of air per minute, pushed down. The wing gets pushed up by exactly the same amount. Bernoulli and Newton are two accountancies of one event, and they agree.
A stall is an angle, not a speed
Past about 15° the air can no longer stay attached to the upper surface. It separates, the suction peak collapses, and lift falls off a cliff — at any speed. A wing can be stalled at 300 knots in a hard pull-up, and perfectly happy at 120 knots on approach.
Flaps and slats buy you slow
Flaps swing down at the back, adding camber and area — more lift at the same speed, so the aeroplane can approach 30% slower. Slats open a slot at the front that re-energises the upper-surface flow and pushes the stall angle from about 15° out past 22°.
The wingtip leaks, and it costs fuel
High pressure under the wing spills around the tip into the low pressure on top, rolling into a vortex that trails for miles. That leak is induced drag — around 40% of the total in the climb. A winglet makes the leak take a longer, more expensive route.
04 — Control
Three axes, three surfaces, three hinges
An aeroplane can rotate about three lines through its centre of gravity, and there is exactly one control for each. Drag the sliders — every surface deflects the way the real one does.
They are not independent
Roll makes yaw. The down-going aileron makes more lift and more drag than the up-going one, so the nose swings away from the turn. Pilots counter it with rudder; airliners do it automatically with a yaw damper and differential aileron rigging.
Turning needs pitch. Bank the wings and lift tilts sideways — the vertical component that was holding you up is now only cos(bank) of it. At 30° of bank you need 15% more lift just to stay level, which is why the nose comes up in a turn.
The rudder does not turn the aeroplane. It points the nose. Aeroplanes turn by banking, the way a bicycle does — the rudder's real job is keeping the tail behind the nose, and holding the centreline when an engine fails on takeoff.
05 — The engine
Inside a high-bypass turbofan
Suck, squeeze, burn, blow. The core is a furnace that exists almost entirely to turn one enormous fan — and that fan, not the jet, is where four fifths of the thrust comes from. Push the throttle and follow the air.
Swipe the cutaway sideways to see all of it
N1 is the speed of the big fan, and it is what a pilot actually sets.
Bypass air per unit of core air. The 1950s flew at 0:1. The newest engines run 12:1.
Hover a stage
Eight sections, one stream of air
Point at any part of the cutaway to see what it does to the air passing through it.
Why bypass wins
Thrust is mass flow times speed change, but the energy you have to pay for goes as the speed change squared. So it is always cheaper to shove a lot of air a little than a little air a lot. Hence the huge fan and the tiny, hard-working core.
Nothing spins together
The fan and low-pressure turbine share one shaft; the high-pressure compressor and turbine share another, hollow, shaft turning around it — three times faster and the other way on some engines. Each spool can then run near its own best speed.
Hotter than the metal can survive
Gas leaves the combustor at around 1700 °C. The turbine blades it hits melt at about 1400 °C. They survive because each single-crystal blade is riddled with cooling passages that bleed compressor air out through its skin as a film.
Losing one is not an emergency
Every twin is certified to take off, climb and fly on one engine, and crews practise it constantly. Under ETOPS rules, a twin-engine airliner may be up to five and a half hours from the nearest runway on a single engine.
06 — The flight
Brake release to rollout, in eleven moves
Everything above, in sequence. Scrub the timeline — or press play — and watch the configuration, the attitude and the numbers change through a complete flight.
Swipe the chart sideways to see all of it
Phase 1 of 11
Taxi
07 — Loose ends
Four things people get wrong
Turbulence is uncomfortable, not dangerous
Wings are tested to bend about 25% beyond anything they will ever meet — in the certification test they are pulled up until they break, typically at more than 150% of the highest load allowed in service. Turbulence is a ride-quality problem. The seatbelt sign is about you hitting the ceiling, not the aeroplane failing.
Nothing falls out of the sky when the engines quit
An airliner at cruise glides about 17 km forward for every 1 km it descends. From 11 km that is roughly 190 km of range and twenty minutes of gliding — an Airbus A330 once glided 120 km to a runway in the Azores with both engines out and everyone on board walked off.
Holes in the wings are supposed to be there
Those little rows of tubes on the trailing edge are static wicks, bleeding off the static charge the aeroplane builds up flying through air. The finger-sized slot on the underside is a drain. The sharp white spike on the nose is the pitot tube, and it is how the aeroplane knows how fast it is going.
The pilots are usually not flying
The autopilot flies most of the cruise, and often the approach too. What the crew are actually doing is managing energy, fuel, weather, routing and failures — the flying is the easy part, and handing it to a machine is what frees them up to do the hard part.