What if a wing tilted too far?

A wing pushes air down. The air pushes the wing up. That push is called lift.

Tilt the wing up a little and the lift gets bigger. What if Olly tilted it up a lot?

When a plane takes off, the pilot raises its nose. NASA explains that this tilts the wings more steeply into the oncoming air and gives the extra lift needed to leave the ground. The tilt between the wing and the air is called the angle of attack.

So is more tilt always better? In this lab a small plane flies at 60 meters per second with its wing tilted at 14°. Then we tilt the wing to 20° and see what happens to the lift.

Angle of attack, the angle between a wing's chord line and the oncoming air, has a large effect on lift. NASA's Glenn Research Center notes that for thin airfoils lift is directly proportional to angle of attack at small angles, but that the dependence becomes complex at higher angles.

This lab uses the airfoil model (a symmetric wing of 16.2 m², aspect ratio 7.5, a 1,100 kg plane, sea-level air at 1.225 kg/m³) and raises the angle from 14° to 20°, past the model's stall angle of 15°.

At first the wing is tilted up quite a lot. The air flows smoothly over it.

The wing makes lots of lift. The plane flies easily.

In the model, at 60 meters per second with the wing tilted at 14°, the wing makes about 43 kilonewtons of lift. The plane weighs about 11 kilonewtons, so the lift is about four times what is needed.

The air still follows the curved top of the wing smoothly. Tilt a little more, to 15°, and the lift rises to its highest: about 46 kilonewtons.

At 14° the model's lift coefficient is C_L = 2π/(1 + 2/7.5) × 0.244 rad = 1.21, giving L = ½ρv²SC_L = ½ × 1.225 × 60² × 16.2 × 1.21 = 43.3 kN, 401% of the 10.8 kN weight. The speed needed to support the plane at this tilt is 30 m/s.

Lift peaks at the model's stall angle, 15°, with C_L,max = 1.30 and L = 46.4 kN; the slowest the plane can fly, using that peak, is 29 m/s.

Now Olly tilts the wing even more. Uh-oh! The air can't follow the top of the wing anymore.

The lift gets smaller, not bigger. The wing has stalled.

Now tilt the wing to 20°. The air can no longer follow the top of the wing, so it breaks away and swirls. The model says the wing has stalled, and the lift drops to about 35 kilonewtons, about a fifth less than at 14°, even though the wing is tilted more.

At 60 meters per second there is still plenty of lift. But slow down to 30 meters per second, about as slow as this plane can fly. At 14° the lift just matches the weight. At 20° it falls to about 81 percent of the weight: not enough to stay up.

At 20° the model's post-stall C_L = 1.30 × (1 − 0.5 × 5/10) = 0.97, so L = 34.8 kN (322% of weight) and the speed needed rises to 33 m/s. At 25° C_L drops to 0.65 and L to 23.2 kN.

At v = 30 m/s the margin vanishes: 14° gives 100% of weight and the plane flies; 20° gives 81% and it cannot. At that speed only tilts from 14° to 16° hold it up (13.5° gives 97%, 16.5° gives 99%). Above 15°, more tilt means less lift, so pulling the nose up further makes things worse.

Air likes to follow the shape of the wing. That helps make lift.

If the wing tilts too much, the air peels away from the top. Then the wing makes less lift.

Air sticks to a wing's surface in a thin layer called the boundary layer. As long as that layer follows the curved top of the wing, the wing turns the air downward and gets pushed up.

NASA explains that at high angles the boundary layer can lift off, or separate, from the wing, making the wing act like a very different shape. Then lift drops suddenly: that is a stall. Our model's wing stalls at exactly 15°, and its lift falls in a simple straight line after that, a rough stand-in for real stalls.

NASA Glenn attributes stall to separation of the boundary layer, which creates an effective shape very different from the physical one and makes wings abruptly lose lift at high angles. Predicting the stall point is very difficult mathematically, so engineers rely on careful wind tunnel tests; beyond stall the flow is highly unsteady.

The model's limits: thin-airfoil lift slope 2π per radian reduced by 1/(1 + 2/AR); a fixed 15° stall with C_L falling linearly to half its peak by 25°; no flaps, no fuselage lift, no drag or thrust. It compares lift with weight only, and the streamlines are an illustration, not a computed flow.

Pilots learn all about stalls. They watch how much the wing is tilted.

A stalled wing still makes some lift, just not enough to keep flying level.

The U.S. Federal Aviation Administration teaches pilots that every airplane stalls at its own particular tilt, called the critical angle of attack, from about 16° to 20° depending on the design. Our model's 15° is a little lower.

The FAA also says a stall can happen at any speed, and that many pilots wrongly believe a stalled wing makes no lift at all. In fact it still makes some, just not enough for level flight. Flying slowly needs a steep tilt, so the FAA says pilots need extra care at low speeds.

The FAA's Pilot's Handbook of Aeronautical Knowledge defines a stall as a rapid decrease in lift caused by airflow separating from the wing when the critical angle of attack is exceeded, and says this angle varies from approximately 16° to 20° depending on the design. A stall can occur at any pitch attitude or airspeed, and a given aircraft always stalls at the same angle regardless of airspeed, weight or density altitude.

The handbook stresses that a stalled wing does not stop producing lift; it cannot generate enough to sustain level flight. As speed falls, the angle needed to hold altitude rises toward the critical angle, which is exactly what the model shows at 30 m/s.