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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.
These two are at the same higher angle of attack. Doesn’t matter which way the wing is pointed toward.
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°.
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.
The basic stall warning horn is really a single-angle of attack device.
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.
Angle of attack can be shown 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.
Aircraft designers will often do things to the airplane design to make that buffeting occurs prior to the stall.
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.
Bottom line:
* 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.
- What NTSB Reports Say About Impossible Turns and AOA (Part II) - September 13, 2024
- What NTSB Reports Say About Impossible Turns and AOA (Part I) - September 9, 2024
- Survival gear after the crash…hmm - July 5, 2024
How about those YouTube links? :-)
This was really helpful – thank you!
Fantastic article, Ed!! Keep up the great work!
You write using pilot language for controlling the plane, not for designing the plane. Pilots don’t need to understand Bernouli, Newton, or Coanda, they just need to know how to control lift and stay out of trouble. As you said, lift comes from AoA and airspeed (squared). My formula for lift is L = AoA * Speed * Speed. I also use your hand out the window to show relation to angle, drag, and speed when I instruct. (I also use the cable tension on a kiddie carnival ride to teach lift direction and amount of lift required). Like you, also preach that fore/aft yoke position commands a specific AoA (with caveats for CG and propwash), just like Langewiesche wrote 80 years ago.
I plan to provide a copy of your article to all of my students.
Now, I present a challenge to you. Could you write a similar article to put the Airplane Flying Handbook’s chapter on Energy Management into Pilot Language. I’m an engineer and former fighter pilot, but it still took me a couple years to wade through chapter, which is written like a PhD thesis.
Thanks again!
Thanks for the kind words! I have published an AirFacts article on energy management (https://airfactsjournal.com/2023/01/energy-management-cliche-or-exactitude/), but to be fair, that article is more of a critique of how energy management is overused than a proper discussion of the topic. Here’s a link to a fun video that satirizes overuse of energy management: https://www.youtube.com/watch?v=y7vGaI47SsI&ab_channel=EdCew
Agree totally with this article and with “Energy Management”—Cliché or Exactitude? The fun video is a perfect example of using terminology that’s meaningless to the learner – a clear disregard of the Fundamentals of Instruction 101.
Related to this subject is another one that I think causes great damage in flight instruction – The Regions of Command. I didn’t realize how big of a problem it was until I was seeing one video after another getting it wrong. I.e. in a Skyhawk where the maximum thrust is approximately 25% of the weight, it is taught that the power controls the altitude in the Reversed Region, which isn’t close to what reverses. I had a flight with someone in a Cherokee Six who was low (red over red) and added power. The pitch didn’t change and we gained 10kts knots without increasing altitude one inch. Post flight as he was scratching his head, he said he didn’t understand why the altitude wasn’t corrected when he added power.
Just wondering if you will be covering the Regions of Command some day or if you have already written on that subject.
Thanks for the kind words.
Region of reversed control? Thanks for the compliment, I think, but couldn’t you give me an easier topic?
But it’s a worthy challenge. I’ll think about it.
Hi Ed,I have designed a small experimental seaplane using composites ,during high speed taxi testing I have experienced,uneven lift ,my wings are at a fairly high AOI to allow for high lift at around 26 knots,my airplane is light and my wings are thick and thin out from the root to tip surface area is 128 square feet and wing loading is around 6 pounds per square foot ,I am wonder if you have any ideas on why I may be getting much more lift from one wing than the other ,thanks
Get to know your airplane’s cues by doing lots of stalls and slow flight…what warnings do you feel…loss of roll response, burble at the elevator felt back thru stick/yoke dancing? Learn these well and your response will be as subconscious as keeping your bicycle upright. Indicators are useful, but require conscious thought to interpret and may not break thru the other distractions that put you near a stall in the first place.
Accelerated stalls, as stated, occur at same AOA, but cues may be more abrupt than 1G flight.
AOA indicators are at their best at speeds on the left side of the flight envelop, their development was critical as jets transitioned to fully hydraulic flight controls with no control feel, no or minimal airframe feedback and swept wings with “sportier” stall behavior…for those of us flying simple GA learn to feel what your plane is telling you.
…one final caution… all of the above goes out the window with frost or icing…then you become an involuntary test pilot because you’ve had your wing shape altered and critical AOA becomes a moving target and anyone’s guess.
Good comments, Rich, but I was only addressing the theoretical side of AOA, a different topic from what your email addresses.
No issue Ed, call it a mild thread diversion adding some AOA thoughts on a Sat am for the others that show up here..
Great article! I have been licensed since 1972 but now, at age 81, I am no longer flying planes (although I do still jump out of them solo now and then). Very helpful to be reminded by you that the critical AOA can be exceeded when a plane is in ANY orientation in the sky. Also, nice to hear why AOA indicator gadgets are perhaps required in many military jets as opposed to some Cessna 206, etc. I am wondering, however, if an AOA “meter” would not be highly useful during basic introductions of students to flight in their early lesson stages at about the third hour?
That suggestion comes up from time to time, but an AOA gauge has more theoretical benefit than actual for almost all GA operations. For a pre-solo student, it would encourage more heads down flying and not learning the cues that are always present, i.e., it would hinder the pilot getting a feel for the airplane. And while you and I may be well versed in AOA, you would not believe how difficult the AOA concept is for some students. As the article implies, you *can* teach a person to fly without AOA as a concept, but it would be wobbly instruction.
Nicely done! I enjoyed your article. I fly airshows in my TS-11 Iskra Jet. We are installing an Alpha Systems AOA HUD in my jet so that I can show the effects of AOA during various phases of my airshow routine! Our goal is to educate on the value of having AOA information for better SA and safety! Thanks for doing this article. With your permission, I will share it!
Some military jet flies loops with a fixed G pull-up, fixed AOA over the top, then back to G.
For GA, as far as SA and safety go, my feeling is that an AOA indicator does not earn its way into the cockpit. For example, most Cirrus don’t have an AOA indicator (the exception is FIKI-equipped Cirrus). Aural stall warning, especially with progressive warnings, you bet! I’ve owned five homebuilts and a Cessna 175, and none of the homebuilts had either AOA indicator or stall warning. Weren’t missed…
…and to add one other reason jets/military rely on AOA…huge weight change within same flt changes stall speeds with fuel burn of 1/4-1/3 of takeoff weight every flt, and that doesn’t include the tons of ordnance that may be expended…all not typical of GA
Excellent article! I guess it takes a PhD from MIT to be able to simplify a complex subject like that so that the rest of us can understand it. I wish the FAA publications described it this way.
Been a pilot for over 35 years and this is the best explanation I’ve seen. Thank you.
Ed,
Please explain the final sentence in your essay. “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.”
I am a CFII with experience with AOA in Citations and with the Alpha System AOA I installed in my C182. I used this instrument for 10+ years and 1,000 hours in this Cessna 182 to manage airspeed at 1.3 and 1.2 Stall Speed into 1,800 foot airstrip. I contend that it is an excellent choice for airspeed guidnace.
That’s a reasonable question, but outside the scope of this article.
I’ve given several presentations on the topic over the years, but unfortunately, none of those were recorded.
Here are some partial answers:
* AOA unusable in gusts: https://www.youtube.com/watch?v=bWSzptdQyFk
* https://www.abbottaerospace.com/downloads/nasa-tn-d-6210-flight-evaluation-of-angle-of-attack-as-a-control-parameter-in-general-aviation-aircraft/
To avoid scope creep, I will not answer further questions on the topic… but that topic might show up here in the future. And, yes, I have 600+ hours with AOA in my RV-9A.
At the NAFI tent at Oshkosh 2023, I presented on this topic. As you might guess, the answer is not for “dummies,” as it involves flight dynamics and other topics.
But here are some partial answers:
* AOA unusable in gusts: https://www.youtube.com/watch?v=bWSzptdQyFk
* https://www.abbottaerospace.com/downloads/nasa-tn-d-6210-flight-evaluation-of-angle-of-attack-as-a-control-parameter-in-general-aviation-aircraft/
In an attempt to stop topic creep, I will not address this topic further in this thread. But who knows, maybe this topic deserves its own post.
Excellent treatise. Goes along with the classic Stick and Rudder.
I will disagree a bit though with the comment ‘ there are reasons why angle of attack is a poor choice for guidance.’.
During the 90’s I had the good fortune of working for Safe Flight Instrument, which designed both stall warning and angle of attack sensors and displays. I was involved in the ‘reboot’ of an approach speed AoA display for light aircraft (Safe Flight had produced the SC100 back in the 60’s and 70’s). Utilizing a lift transducer to indicate relative AoA, I could easily see where my angle of attack was relative to where we had calibrated critical AoA. Interestingly, in gusty conditions, even with higher airspeed, I could see the dips towards critical angle of attack during approach. This direct display helped to inform me how close I was getting to the edge, no matter gross weight or atmospherics.
After I left company this was developed into the SCx system. I’ll go along with what Dr Greene and the company engineers said that a lift transducer was less prone to induced errors of both the pneumatic and derived AoA competitors.
If the concepts of AoA and proper training of flying AoA were taught, I think we’d fine pilots better to handle their planes. AoA displays for light aircraft can show this
The problem with this article is similar to how most general aviation knowledge treats angle-of-attack (AOA). General aviation treats AOA like it is just some incidental parameter that controls stalls but is not important in any other respect. This is fundamentally not true. AOA is everything in flying craft. It determines how they handle and respond. The control stick directly controls AOA. Wolfgang Langewiesche laid all this out in his phenomenal book “Stick and Rudder.” You cannot gain an intuitive “feel” for flying unless you grasp what AOA is and how it determines all of flying.
Previously, I commented that to prevent thread creep, I would not respond to comments outside the scope of the article. Breaking that promise, https://www.sportys.com/blog/episode-62-why-angle-of-attack-is-overrated-with-ed-wischmeyer/