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Stalls, Spins, and Misunderstandings

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The general public understands that, when a car’s engine stops running in an unplanned sort of way, that engine has “stalled.”

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But “stall” means something completely different in the context of aviation.  A stall in an airplane usually has nothing to do with the engine. Sure, an airplane’s engine can stall, but aviators usually use some other word, such as “quit” or “stop.”

Let’s talk about how an airplane stalls. Airfoils develop lift by moving through the air. Airfoils include the wings on airplanes, the rotor blades on helicopters, and lots of other things. The control surfaces on airplanes and even the propeller blades themselves are also airfoils. Heck, a barn door can be an airfoil under the right circumstances.

We’re going to talk about some specific kinds of airfoils, namely the wings on airplanes. Generally, the aircraft engine rotates the propeller, pushing or pulling the airplane through the air and creating airflow over the wings. The wings develop lift when they interrupt the air, sending some over the top and some over the bottom. The air over the wings develops something called “laminar flow,” which is a fancy way of saying that the air on both the top and the bottom of the wing moves quickly and uniformly in the area very close to the wing.

The angle of a wing as it meets the airflow is called the “angle of attack.” When you tip a wing up into the airflow – when you increase the angle of attack - more air hits the bottom of the wing and there’s a greater pressure differential. Low angles of attack are good for cruising and that’s what you see when you see an airplane overhead that’s pretty much level and is on its way somewhere. High angles of attack are good for climbing. You can see an airplane with its wings at a high angle of attack every time you go to the airport and see them taking off.

With me so far?  Good! 

Imagine what would happen if you increased the angle of attack a lot. Thirty or forty degrees or something like that. At some point for every wing, the airflow is simply smacking the bottom of the wing and not enough air goes over the top of the wing to keep that laminar flow. Eddies and turbulence build up on the top of the wing and the laminar flow just dissolves.

At that point, the wing won’t fly anymore. It’s not developing lift. That angle of attack for any given wing is the “critical angle of attack.” When a wing exceeds its critical angle of attack, the wing is “stalled.” When aviators talk about an airplane being stalled, they mean that the airplane’s wings have exceeded the critical angle of attack and that the wings aren’t developing lift like they otherwise might. What does that look like? The airplane’s nose is usually very high and its forward speed is very low.

Technically speaking, stalls are entirely dependent on the angle of attack of the wing.  But airspeed (the speed of the airplane through the air) is a pretty good proxy for that angle of attack.  The slower the airplane is moving through the air, the less air is moving over the wing to create lift.  And the greater the necessary angle of attack if the airplane is to keep flying at the same altitude.  So sometimes pilots talk about stalls in terms of airspeed, specifically “stall speed,” below which the airplane will stall.  The slower the airspeed, the more likely it is that an airplane will stall.

Stalls can be bad if they occur when the pilot isn’t expecting it, so student pilots and experienced pilots alike practice stalling their aircraft so that they know how to recover from stalls. The private pilot practical test standards require that an applicant for a private pilot’s certificate for airplanes be able to stall an airplane – and recover – with a lot of power or with little or no power, and in turns either with or without power at bank angles of up to 20 degrees.
Stalls are bad at low altitude, such as when you’re taking off or landing. It generally takes some altitude in order to recover from a stall – about 100 feet in many aircraft in the case of a power-off stall. That’s altitude you might not have.

Stalls can also lead to other bad things. One of them is a spin. A spin happens when the airplane is stalled and “uncoordinated.” An airplane is uncoordinated with the tail is not where it’s supposed to be – when the pilot doesn’t use the rudder to keep the stalled airplane from rolling in the direction of the wing that is the most stalled. Too much rudder produces a “skid” and too little rudder produced s “slip.” 

If you stall and you’re sufficiently uncoordinated, one wing or the other will drop and the airplane will start falling in a lazy spiral. The spiral will be in the direction of the wing that is the most stalled. The other wing, the one that’s less stalled, will be flying just enough to keep the rotation going. It’s called “autorotation.” Being in a spin is very unpleasant if you’re not use to it. There’s a lot of green in the windshield and the airplane is turning at an increasing rate..

Stall and spin recovery isn’t particularly difficult. The pilot pushes on the yoke or stick to decrease the angle of attack and get laminar airflow over the wings.  That’s usually enough to recover from a stall that hasn’t developed into a spin.  If the aircraft has begun to spin, the pilot must usually use the rudder to stop the autorotation as well.

Aerobatic pilots go up and have fun with stalls and spins.  You might have seen aerobatic pilots at airshows performing maneuvers called “snap rolls,” “falling leaves,” “avalanches,” and other maneuvers with equally exciting names.  These maneuvers have stalls and spins as essential elements.  They look dramatic from the ground and they’re fun to do in the airplane once you’re received enough training and as long as you perform them at altitudes high enough to recover if you goof it up.

The way stalls get into the news – and the way most members of the media get the terminology wrong – is when a stall results in an accident that gets reported. As you can imagine, an accident could easily occur if you stalled an aircraft so close to the ground that you didn’t have enough altitude to recover. That’s doubly true for spins, because spins usually take something like a thousand feet in which to recover.

If a stall or spin results in an accident, it’s most often in the traffic pattern of an airport.  In the pattern, aircraft are moving more slowly and are turning and otherwise maneuvering to take off from, or to land on, a runway.  The most common stall or spin accident in the pattern is a spin on the turn from the base leg to the final leg.  That’s a 90-degree turn that begins when the pilot is flying perpendicular to the end of the runway and the pilot turns to point the airplane at the runway in order to land.  Sometimes wind or distractions cause the pilot to be further away from the final approach course than the pilot planned to be, so the pilot banks further than the pilot should or tries to increase turn rate using too much rudder (a “skid”).   If the pilot allows the airplane to get too slow at this point and the airplane stalls, the uncoordinated state of the airplane can lead to a spin at low altitude.

The aviation community knows a lot about stalls and spins in the pattern.  We pay a lot of attention to the accident reports so that we can learn from them.  Flight instructors work hard with student pilots so that they know how important airspeed and coordination are in the pattern.

Aerodynamic stalls are very rare in everyday flight operations. Unless you’re a pilot who’s training or performing aerobatics, the odds are very small that you’re ever experience one – even if you fly commercially every day of the week and on weekends, too, your whole life. They just don’t happen much.

Stalls are a natural result of the behaviors of airfoils under certain parts of the flight envelope. Aerobatic pilots put them to use in graceful and energetic performances around the world at airshows and other events. Student pilots train to recognize them and recover from them so that they can fly safely for decades to come.

 

4 Responses to “Stalls, Spins, and Misunderstandings”

  1. Pete Washburn CFIG Says:
    May 14th, 2012 at 9:28 am

    Nice article, but a couple of notes with a couple of statements made in this article...

    - In the discussion of critical angle of attack..."At that point, the wing won’t fly anymore. It’s not developing lift." This statement is not completely true. Beyond the critical angle of attack, the wing is still generating lift, just not as much as at or less than the critical angle of attack. Drag is also much higher beyond the critical angle of attack.

    - "If you stall and you’re sufficiently uncoordinated, one wing or the other will drop and the airplane will start falling in a lazy spiral". The use of the word "spiral" implies a coordinated manuever. If you stall and are uncoordinated, a wing drops, and rotation starts, you just entered an incipient spin not a "spiral". A "spiral" is a coordinated manuever with neither wing stalled.

  2. I'd add another point, re: airspeed vs. angle of attack. Many stalls occur when an airplane is asked to make an abrupt change of direction, especially the quick pull up out of a dive such as when doing a low pass over an airport or something. Although the a/c has plenty of speed, a sudden change in the AOA can cause a stall. It's similar to suddenly turning the wheel in a car - the car turns but continues to slide sideways in the original direction of travel. When a pilot pulls up sharply, the wing is pointed up, but the plane's direction of travel continues to "slide" in the direction it was going, so instead of the air flowing over the wing, it is, as described above, simply slapping the bottom of the wing, causing a stall. The same thing happens when a turn is tightened abruptly such as trying to avoid overshooting the runway on final. This is very important for all pilots to understand and think about.

  3. Steve, thank you for this excellent post, and I hope you don't think I'm piling on with this small criticism.

    I think you are conflating laminar/turbulent with separated/attached. The boundary layer can be laminar, which has lower drag but is prone to separation, or turbulent, which has more drag, but tends to stay attached.

    The difference between attached flow and separated flow is what determines whether a wing is stalled. The nature of the boundary layer (laminar or turbulent) is not part of that distinction.

  4. Great article and great comments.

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