The Physics of Flight

A 400-tonne machine in the sky

A fully loaded jumbo jet can weigh as much as 400 tonnes — heavier than 50 elephants. Yet it rises off the runway, climbs to 11 kilometres up, and cruises there for hours. How? Flight is not magic. It is the result of four forces working together, and you can understand all of them.

The four forces of flight

At any moment in the air, a plane has exactly four forces acting on it. Think of them as two tug-of-war contests happening at once.

The up-and-down contest:

• Lift acts upward. It is the force the wings create to hold the plane up.
• Weight acts downward. It is the pull of gravity on the plane's mass.

The forward-and-backward contest:

• Thrust acts forward. It is the push from the engines that drives the plane through the air.
• Drag acts backward. It is air resistance — the air pushing against the plane as it moves.

When the forces in each pair are balanced, the plane flies straight and level at a steady speed. When they are unbalanced, the motion changes:

• Lift greater than weight → the plane climbs.
• Weight greater than lift → the plane descends.
• Thrust greater than drag → the plane speeds up.
• Drag greater than thrust → the plane slows down.

This is really just Newton's laws of motion in action — an unbalanced force changes how an object moves.

Where does lift come from?

Lift is the most interesting force, because the wing has to make it out of nothing but moving air. The shape of a wing is called an aerofoil: usually curved on top and flatter underneath, and tilted slightly nose-up into the oncoming air. This small upward tilt is called the angle of attack.

Here is the key idea, explained the way modern physicists prefer because it is both simple and correct:

A wing pushes the air downward. By Newton's third law, the air pushes the wing upward by an equal amount. That upward push is lift.

As the wing slices through the air, its curved top surface and tilted angle force the passing air to bend and stream downward behind the wing. The wing has given the air a downward shove. And every action has an equal and opposite reaction — so the air shoves the wing upward in return. Multiply that over the whole huge wing and the upward force is enormous.

You can feel this yourself. Hold your flat hand out of a moving car window (carefully!) and tilt it slightly upward at the front. Your hand pushes air down, and instantly your arm is shoved up. Your hand has become a tiny wing.

More speed and bigger wings mean more lift

Two things increase the lift a wing produces:

1. Speed. The faster the air flows over the wing, the more air gets deflected downward each second, so the more lift is made. This is why a plane must race down a long runway before it can take off — it needs to reach a high speed before the wings make enough lift to beat the plane's weight.
2. Wing size and angle. A bigger wing, or one tilted to a steeper angle of attack, deflects more air and makes more lift. Pilots lower flaps to enlarge the wing for slow take-offs and landings.

But there is a limit. If the wing is tilted too steeply, the smooth airflow breaks away from the top of the wing and becomes turbulent. Lift collapses suddenly — this is called a stall, and pilots are trained carefully to avoid it.

Thrust and drag: the forward battle

To keep air flowing fast over the wings, the plane must keep moving forward, and that needs thrust. Jet engines and propellers both create thrust by pushing a large amount of air backward; in reaction, the air pushes the plane forward — Newton's third law again.

Opposing thrust is drag, the resistance of the air. The faster a plane goes, the more drag it meets, which is why aircraft are built with smooth, streamlined shapes to slip through the air with as little drag as possible. The energy the engines burn to overcome drag links to ideas you can explore in energy, work, and power.

Putting it all together: a flight from start to finish

1. Take-off: Engines give maximum thrust. The plane accelerates down the runway until the wings move fast enough that lift beats weight, and it leaves the ground.
2. Climb: Lift stays greater than weight, so the plane rises.
3. Cruise: The pilot eases back the engines. Now all four forces balance — lift equals weight, thrust equals drag — and the plane flies level at a constant speed.
4. Landing: Thrust is reduced and flaps are lowered. Drag rises and lift falls in a controlled way, so the plane slows and gently descends onto the runway.

Try it yourself! 🧪

Experiment 1 — Make a wing lift. Hold a strip of paper just below your bottom lip so it droops down over your fingers. Now blow steadily across the top of the paper. The strip rises and flutters upward! The fast-moving air you blew helped lift it, just as fast air over a wing creates lift.

Experiment 2 — Build and test paper planes. Fold two paper aeroplanes: one with wide, broad wings and one with narrow wings. Throw each with the same gentle push. The broad-winged plane makes more lift and glides slowly and far; the narrow one needs a faster throw to stay up. Now bend the back edges of the wings slightly up — the nose lifts. Bend them down — it dives. You are changing the angle of attack and steering the airflow, exactly like a real pilot.