LESSONS · AERODYNAMICS · 12 MINUTES · AUSTRALIA AND NZ
Angle of attack and the stall.
Tilt a wing further into the oncoming air and it makes more lift, up to one angle. Past that critical angle of attack the air breaks away from the upper surface and the wing stalls.
PREDICT FIRST
You keep raising the angle of attack, past the wing's critical angle. What happens to the lift?
Angle of attack
Angle of attack is the angle between the wing and the air meeting it. It is measured from the chord line, a straight line from the leading edge to the trailing edge. The air meeting the wing is the relative airflow, which PHAK calls the relative wind. The aircraft's flight path sets its direction.
The AOA is defined as the acute angle between the chord line of the airfoil and the direction of the relative wind.
Because the angle is measured against the relative airflow, not the horizon, a stall can happen with the nose high, level or low.
An aircraft stall results from a rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical AOA. A stall can occur at any pitch attitude or airspeed.
Air flows past the wing from left to right. Over the top it speeds up and follows the curve of the wing. Past the critical angle it breaks away from the upper surface into swirling eddies.
Lift coefficientNeeded for level flight at 70 kt
Lift ÷ weight
1.01
Level flight at this angle
70 kt
Margin to critical
10.0°
10.0° below the critical angle. The air follows the upper surface. Lift equals weight: level flight.
The example aircraft stalls at 50 kt wings level. Its wing is an illustration, not a real aerofoil.
Try this
Raise the angle of attack slowly past 16° and watch the air break away from the upper surface.
Set a higher airspeed and raise the angle again. Where does the wing stall now, and how much lift is it making?
Turn on Hold height and bring the airspeed back toward 50 kt. Watch the angle climb.
Why the wing stalls
Below the critical angle the air flows smoothly over the top of the wing, and lift grows as the angle grows. At the critical angle the lift coefficient reaches its peak. Past it, the air can no longer follow the upper surface.
At low angles of attack, the airflow over the top of the wing flows smoothly and produces lift with a relatively small amount of drag. As the AOA increases, lift as well as drag increases; however, above a wing’s critical AOA, the flow of air separates from the upper surface and backfills, burbles, and eddies, which reduces lift and increases drag.
You fly the example aircraft well above its stall speed and pull back hard. At what angle of attack does the wing stall now?
For a given configuration, each aircraft has one critical angle. PHAK gives a range for where it falls:
This critical AOA varies from approximately 16° to 20° depending on the aircraft’s design. But each aircraft has only one specific AOA where the stall occurs.
The wing in the simulation stalls at 16°, at the low end of PHAK's range. Its outline, its curves and the airflow are illustrations, not data for a real aerofoil, and the swirling wake is drawn, not calculated.
Flying slower walks you toward the critical angle
In level flight, lift must equal weight. Fly slower and each degree of angle makes less lift, so the angle has to grow to hold height. At the stall speed, the angle needed reaches the critical angle.
As thrust is reduced and airspeed decreases, the AOA must increase in order to maintain altitude. If speed decreases enough, the required AOA will increase to the critical AOA. Any further increase in the AOA will result in the wing stalling.
Turn on Hold height as speed changes in the simulation and bring the airspeed back. The dashed line on the chart shows the lift coefficient level flight needs. It rises as you slow, and the dot climbs the curve to meet it. At 50 kt they meet at the peak, 16°. Below that, no angle makes enough lift and the aircraft starts to sink.
Recovery needs less angle
The stall comes from the angle, so recovery needs the angle reduced. AFH calls reducing the angle of attack crucial for all stall recoveries, and says to follow the manufacturer's procedure where one is published. As the angle of attack comes down, smooth airflow over the wing returns and lift comes back.
Reducing the AOA is crucial for all stall recoveries.
During this nose-down attitude, the AOA decreases and the airspeed again increases. The smooth flow of air over the wing begins again, lift returns, and the aircraft begins to fly again.
Speed by itself is not a reliable parameter to avoid a stall. An airplane can stall at any speed. Angle of attack is a better parameter to use to avoid a stall.