How planes fly: lift, thrust, drag and weight explained

Every aircraft in flight is balancing four forces. Weight pulls it down towards the Earth. Lift, generated by the wings, pushes it up. Thrust from the engines drives it forwards, and drag — air resistance — holds it back. In steady cruise all four are in equilibrium: lift equals weight, thrust equals drag. To climb, lift exceeds weight; to accelerate, thrust exceeds drag.

Lift is the force passengers are most curious about. A wing moving through air deflects the airflow downwards. Pushing a large mass of air down produces an equal and opposite reaction pushing the wing up — Newton's third law. The wing's curved upper surface and its slight nose-up angle to the airflow (the angle of attack) are both shaped to maximise that downward deflection efficiently.

Drag comes in two main forms: parasite drag from the airframe pushing through the air, which grows with speed, and induced drag, the by-product of making lift, which is worst at low speeds. Aircraft designers spend careers shaving both — winglets, for example, exist almost entirely to reduce the induced drag caused by high-pressure air spilling around the wingtip.

A wing optimised for efficient high-speed cruise is too small and too flat to lift the aircraft at low speeds. That's a problem, because takeoff and landing are precisely when the aircraft is slowest. The solution is a wing that changes shape on demand.

Flaps are hinged panels on the trailing edge of the wing; slats are their counterparts on the leading edge. Extended together, they increase the wing's area and curvature, letting it generate the same lift at much lower speeds — at the cost of extra drag, which is fine when you're deliberately slowing down or climbing away from a runway.

This is why, from a window seat over the wing, you'll see panels slide out before takeoff and again before landing, with a whirring hydraulic soundtrack. After takeoff the crew retracts them in stages as speed builds; before landing they extend in stages as the aircraft slows. A typical landing configuration uses far more flap than a typical takeoff, because landing speeds are lower.

Every departure is flown against three calculated speeds, called out aloud by the pilots. V1 is the decision speed: below it, a serious problem means braking to a stop on the runway; above it, the aircraft must take the problem into the air because there isn't enough runway left to stop. VR is rotation speed, when the pilot pulls back and the nose rises. V2 is the safe climb speed that keeps the aircraft controllable even on one engine.

Those speeds are computed fresh for every flight from the aircraft's weight, runway length, temperature, wind and flap setting. It's why two takeoffs on the same aircraft type can feel completely different — a light A320 off a long runway leaps off the ground, while a fully loaded 777 on a hot day uses most of the tarmac before a gentle, deliberate rotation.

Landing is takeoff backwards: the crew must shed both speed and altitude in a controlled way, arriving over the runway threshold at the right height and a target approach speed, with full flaps and the gear down. Spoilers — panels on the upper wing — pop up on touchdown to dump the remaining lift and press the wheels firmly onto the runway so the brakes and thrust reversers can work.

If anything isn't right — too fast, too high, unstable in the last few hundred feet — the correct answer is a go-around: full power, climb away, and try again. It's a routine trained manoeuvre, not an emergency, and it's covered in our pilot announcements decoder.