How Do Aeroplanes Fly? What Actually Holds a Wing Up
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A fully loaded airliner weighs around 350 tonnes and stays in the air on wings that look far too small for the job. The explanation most people were taught, that air travelling over the curved top of the wing has further to go and so speeds up and lowers the pressure, is not right, and it cannot be, because the air over the top does not arrive at the trailing edge at the same time as the air underneath. The real answer is simpler and older: the wing throws air downwards, and the air pushes back.
Lift is air deflected downward
Hold a flat hand out of a car window and tilt it slightly upward at the front, and it lifts. The air hitting the underside is deflected down, and by Newton's third law the air pushes the hand up. A wing does the same thing more efficiently. It meets the oncoming air at a slight angle, called the angle of attack, and turns the flow so that a large mass of air leaves the trailing edge moving downward. The upward force on the wing is exactly the rate at which downward momentum is given to the air.
The curved shape helps but is not the source. A curved upper surface lets the air follow the wing round and down smoothly, so the wing can turn more air with less drag, and it means a wing produces some lift even at zero angle. But flat wings fly, paper aeroplanes fly, and an aerobatic pilot flies upside down by pointing the nose up enough that the inverted wing still meets the air at a positive angle. Shape refines the process; angle drives it.
Pressure, and why both explanations are half right
The pressure explanation is not wrong about the pressure. Over the top of a wing the air does move faster and the pressure is lower, and beneath it the pressure is higher, and adding up the pressure over the whole surface gives the lift. What is wrong is the reason given for the speed difference. The air over the top is not racing to meet its neighbour from below; in fact it arrives well ahead. The air speeds up because the wing's shape and angle bend the flow, and bending a flow requires a pressure difference across it, low on the inside of the curve, which is the top of the wing.
The pressure picture and the deflection picture are two descriptions of the same event. The pressure difference is what pushes the air downward, and the air being pushed downward is why the pressure difference exists. Engineers use whichever is more convenient for the calculation in hand.
Angle, speed and the stall
Lift grows with the angle of attack and with the square of the airspeed, so a plane can fly slowly at a steep angle or fast at a shallow one, which is why airliners point their noses up during a slow approach to land. There is a limit. Beyond an angle of about fifteen degrees the air can no longer follow the top surface, breaks away into turbulence, and the lift collapses. That is a stall, the cause of most small-plane accidents, and it has nothing to do with the engine; a glider can stall. Recovery is to lower the nose and let the air reattach.
Because lift depends on speed, a plane needs a minimum speed to fly, and flaps exist to lower it. Extending the flaps on landing increases the wing's curvature and area so that it can make enough lift at a speed slow enough to stop on the runway.
The four forces
Lift is one of four forces, and level flight is their balance:
- •Lift: upward, from the wings, balancing weight
- •Weight: downward, from gravity on the aircraft and its load
- •Thrust: forward, from the engines, balancing drag
- •Drag: backward, from air resistance and from the act of making lift
What the engines are for
Engines do not hold the plane up. They push it forward fast enough for the wings to do that, and they overcome the drag that lift creates as a by-product, since turning air downward also stirs up swirling vortices behind the wingtips that cost energy. That induced drag is why long, narrow wings, like a glider's or an albatross's, are more efficient than short broad ones, and why modern airliners have upturned wingtips. A jet engine makes thrust the same way a wing makes lift, by throwing air backward, only faster and in a smaller stream.
In a glider, gravity supplies the forward push: the aircraft descends gently through the air, trading height for speed, and stays up for hours by finding air that is rising faster than it sinks. A bird gliding, a paper dart and a 350-tonne airliner on approach are all doing the same physics.
The takeaway
A wing flies by meeting the air at an angle and deflecting a large mass of it downward, and the equal and opposite push holds the aircraft up; the low pressure over the curved top is the same effect seen from the other side, not a separate cause. Lift depends on angle and on the square of speed, fails in a stall when the angle is too steep, and engines exist to keep the wings moving fast enough to work.