How Do Aeroplanes Fly? What Actually Holds a Wing Up

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.

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.

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, a leading cause of fatal 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.

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

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.

When Wilbur and Orville Wright began building gliders around 1900, they relied on published tables of how much lift a wing of a given shape would produce. Their gliders lifted far less than the tables predicted. Rather than give up, they built a small wind tunnel in their bicycle workshop in Dayton, Ohio, in 1901 and tested a large number of miniature wing shapes on balances made partly from bicycle spokes and hacksaw blades. The measurements showed that the accepted figures were wrong and gave them the data to design wings with the right curvature and proportions.

Their other key insight was control rather than lift. Earlier experimenters had concentrated on keeping a machine stable, but the Wrights realised a pilot needed to roll the aircraft to turn, as a bird tilts its wings. They achieved this by twisting the wingtips in opposite directions with cables, a method called wing warping, which is the ancestor of the ailerons on every modern aircraft. In December 1903 at Kitty Hawk in North Carolina, their powered Flyer made its first flight, lasting about twelve seconds. By the end of that day, a later flight had stayed up for nearly a minute.

To turn, a pilot banks the aircraft, tilting the wings so that part of their lift pulls the plane sideways round the curve. The catch is that the upward part of the lift must still hold up the full weight, so the wings have to produce more lift in total. The arithmetic is set by the bank angle. At 30 degrees the wings must carry about 1.15 times the aircraft's weight, at 45 degrees about 1.4 times, and at 60 degrees exactly twice. Pilots call this ratio the load factor, and in a 60-degree turn everyone on board feels twice as heavy.

Because lift rises with the square of speed, needing more lift raises the speed at which the wing stalls, by the square root of the load factor. In a 60-degree bank that is a rise of about 41 percent, so a light aircraft that stalls at 50 knots in level flight stalls at about 71 knots in the turn. This is why steep turns close to the ground are dangerous: a pilot turning slowly onto the final approach, and pulling back to tighten the turn, can stall a wing at a height too low to recover. Airliners rarely bank beyond about 30 degrees.

Lift depends on the density of the air as well as on speed and angle, and that explains where airliners choose to fly. At about 11,000 metres, a typical cruising height, the air is only around 30 percent as dense as at sea level. To make the same lift at the same angle of attack, a wing must therefore move through it roughly 1.8 times faster, since lift rises with the square of speed. That sounds like a cost, but thin air also means less drag at a given speed, jet engines run efficiently in the cold air up there, and the aircraft flies above most of the weather.

Pilots deal with this through a clever instrument. The airspeed indicator does not measure true speed through the air but the pressure of the oncoming flow, which is what the wing actually feels, so an aircraft stalls at roughly the same indicated speed high up as near the ground, even though its true speed is far greater. At altitude, though, a second limit closes in. The speed of sound is lower in cold air, and approaching it creates shock waves over the wing that disrupt the flow. Near an aircraft's ceiling the gap between stalling too slowly and going too fast becomes narrow, a region pilots nickname coffin corner.

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.