We’ve all had the experience of sticking a hand out of the car window. When we turn our hand at an angle to the wind, the wind pushes up on our hand, but if we turn our hand at too great an angle, the pushing up disappears.
That’s like how an airplane wing generates lift. The amount of lift depends on both the speed of the air and also on the angle at which the air flow hits the wing. The pilot controls that angle with the control wheel or stick: pull back for a steeper angle, forward for a shallower angle.
The engineers needed a name for that angle to the wind, and they called it “angle of attack.” It’s really a measure of how hard the pilot is commanding the wing to work with the air flowing past. If the pilot is not asking the wing to work very hard (low angle of attack) the wing will generate some lift. If the pilot is asking the wing to work harder (higher angle of attack) the wing will generate more lift.
Similarly, for a fixed angle of attack, more speed means more lift.
For the wing to generate lift, all it needs is speed and angle of attack. The wing can generate lift when the nose of the plane is pointed up or down, or even when the plane is upside down. All that’s needed is speed and angle of attack.
You may have encountered more sophisticated theories on how wings work, such as Bernoulli or Newton. Those are pretty good explanations, but the professional aerodynamicists will tell you that each of those explanations has shortcomings…but don’t worry about it. (There are at least two really good YouTube videos on the subject.)
Like everything else, there are limits to how hard the pilot can command the wing to work. If that angle gets too big, the air no longer flows smoothly along the top of the wing and the wing loses a bunch of its lift. How much it loses and how quickly depends on how the wing was designed. What’s interesting, though, is that the wing always loses that bunch of lift at the same angle of attack, regardless of airspeed. This is called a stall and has nothing to do with the engine. Gliders (with no engine) can stall, political campaigns can stall, you get the idea.
And just like the wing can generate lift with the nose up or down, the wing can stall any time the angle of attack is too big – nose up, nose down, or in a turn. Many pilots who are just learning aerobatics will stall the airplane at the top of a loop, completely upside down. (It’s embarrassing.)
So now for some geek-speak to more precisely describe the situation. The angle at which the wing loses that bunch of lift is called the “critical angle of attack,” and for small airplanes, that’s usually about 16°.
Airplane designers want the airplane wing to lose lift gradually as the angle of attack exceeds the critical angle of attack (stall). If the wing lost that bunch of lift abruptly, this could be startling to the pilot and startled pilots don’t fly as well as calm pilots.
But in addition to making the wing stall gradually, there are ways of alerting the pilot that the airplane is approaching a stall. But first, we need a quick background discussion.
When air molecules hit the leading edge of the wing (the very front of the wing), some air molecules will go up over the wing, and some will go down under the wing. That point where the airflow splits is called the “stagnation point.”
What’s interesting is that, as the angle of attack increases, meaning that the air is hitting the wing at a steeper angle, the stagnation point moves down towards the bottom of the wing.
If a tab is placed at just the right point on the leading edge of the wing, sticking out of the wing, it will normally be below the stagnation point and the air flow will push the tab down. But, if the angle of attack increases and the stagnation point moves down below the tab, then the air flow will push the tab up. If the tab is at just the right place, it will detect that change in airflow detection just before the wing – or at least, that section of the wing – is going to stall. That tab is connected to a buzzer in the cockpit, alerting the pilot that the wing is about to lose that bunch of lift because of a stall. (This was patented in 1948). Many planes have this kind of stall warning system.
Another interesting stall warning system has a small hole in the leading edge of the wing. In normal flight, this hole is below the stagnation point, and the air flows right over the hole. But, when the angle of attack gets bigger and the stagnation point moves down over the hole, air flows into the hole and causes reeds to vibrate, alerting the pilot the wing is about to lose that bunch of lift because of a stall. (This was patented in 1944). Many Cessnas had this kind of stall warning system.
More recently, angle of attack sensors have been devised to give the pilot angle of attack measurements, not just stall warnings. A common kind of sensor is L-shaped and usually mounted under the wing, near the leading edge, with one hole at the forward tip of the L, pointed straight forward. (This is the Pitot tube used in measuring airspeed.) Another hole is near it, pointed down at an angle. As the angle of attack changes, the air flow onto the probe changes, and the pressure difference between the two holes will change. Electronics measure this difference in pressure and indicate angle of attack in a light bar display or with tones that beep ever faster as the plane approaches critical angle of attack, or both.
Not only can the stall warning system give the pilot warning, the airplane itself can give warning. Some planes start to shake and buffet as they approach the stall. This buffeting signals aerodynamically that the plane is approaching the stall and is a highly desirable trait. Aircraft designers will often do things to the airplane design to make that buffeting occur. There can be other, useful aerodynamic cues as well.
So what does all this mean to you, the pilot? The more you pull back on the wheel, the harder you’re asking the wing to work as you increase the angle of attack. There is a limit as to how hard the wing can work, and at some point (the critical angle of attack), the wing will lose a bunch of its lift.
For planes to fly slowly, like when they’re taking off or landing, there’s not as much airspeed so the wing has to work harder (higher angle of attack) to generate enough lift to make the plane fly at those low speeds. That means that at those low speeds, the wing is flying at a higher angle of attack and closer to the stall. Normal procedures are designed so that the plane doesn’t inadvertently stall, but pilots do make mistakes and stall accidents do occur at low speeds. Again, it’s not the low speed per se that is causal, it’s the higher angle of attack that is more common at low speeds that is significant.
So if angle of attack measures how close the wing is to stall, why isn’t it used for guidance? Turns out there are lots of reasons, not all of them obvious.
First is that airspeed will always be needed by the pilot. The airplane has limits on how much airspeed various pieces can handle, like flap extension or even the whole airplane itself. And on takeoff, the pilot uses airspeed to know when to pull back on the wheel for liftoff. So if airspeed is always required, and is flown successfully every day, does angle of attack guidance add enough value to be worth installing and training?
On landing, there’s another phenomenon – not all landings are made at the same speed, and not just because airplanes need to land faster when they are heavier. “Normal” approaches to landing are made at 30% above stall speed (1.3 Vs0) with a corresponding angle of attack, but for short fields, that 30% safety margin is reduced to 20% (1.2 Vs0, with a higher angle of attack than for 1.3 Vs0) for a shorter roll out. And if there are gusts reported, or crosswinds, pilots will often increase their landing speed for safety, according to various rules and formulae we won’t go into here. It’s easy to make adjust speeds for different kinds of landings and gusts, but it’s very, very difficult adjust angle of attack for different conditions. (That’s a long discussion and involves some math.)
There’s more to gusts. Every time you feel a bump in the plane from a gust, that’s because the wing generated more lift, mostly from more angle of attack in the gust. If it’s a bumpy day, the angle of attack indicator readings will jump all over the place and be difficult to read.
As if that wasn’t enough, when pilots get startled, or preoccupied, or busy, sensory inputs (visual and sound) can get ignored. Just because there’s a warning system or a guidance system doesn’t mean it will always be noticed. (This was pointed out in a patent application in 1925).
Lastly, there’s basic flight mechanics. To make a very long story short, pilots almost always fly the airplane by setting the pitch (nose up or nose down) of the airplane and double checking to see if that pitch attitude is giving the desired flight parameters. Pilots are taught not to “chase the airspeed” but rather to fly pitch and let the airspeed settle down. Angle of attack is even harder to chase than airspeed, especially when maneuvering the plane – but that’s another story.
* Angle of attack is the angle at which the air stream hits the wing.
* A wing needs both airspeed and angle of attack to generate lift.
* Angle of attack is a numerical measure of how hard the wing is working.
* The wing can only work so hard before it loses a bunch of its lift, and this is called a stall.
* This loss of lift (stall) always occurs at the same angle of attack, regardless of speed.
* Stall warning systems can warn the pilot of an impending stall. This is required in new airplanes.
* Although angle of attack is an important concept – it describes how hard the wing is working, and how much harder the wing can work before it loses some lift – there are reasons why angle of attack is a poor choice for guidance.