Tuesday, February 28, 2006

Understanding Stalls

It’s vocabulary day here at airspeed and today we’re going to make you smarter than 90% of all television, newspaper, and other reporters about one aspect of aviation. That aspect is the “stall.”

Now, the average non-pilot could be forgiven for not understanding that an aerodynamic stall is different from a stall that has to do with an engine.

I stalled an airplane four times earlier this week. I did it intentionally. The guy in the right seat, who was administering my biannual flight review, had no problem with it. In fact, he told me to do it. Had I not done it, he would have failed me. And we landed safely and regarded the whole thing as a great way to spend a Thursday morning.

The general public understands that, when an engine stops running in an unplanned sort of way, that engine has “stalled.” But it’s a completely different thing when an airplane stalls.

When I stalled the airplane, the engine was running each time. In fact, on two of the stalls, the engine was running at full power. 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 spins the propeller, 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.

Nut high angles of attack are only good up to a point.

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 list like they otherwise might. What does that look like? The airplane’s nose is usually very high and its forward speed is very low.

Stalls can happen at any airspeed, at any altitude, and for any wing or other airfoil. They’re generally bad when you’re not 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 twenty degrees.

Stalls are particularly 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 banking or rolling in the direction of the wing that is the most stalled. 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 it’s turning pretty fast.

Stall recovery isn’t particularly difficult. I’ll bet you know how to recover from one right now. If increasing the angle of attack caused the wing to stall, what do you think will break the stall? That’s right. Decrease the angle of attack. Push the nose over and get some laminar airflow over the wings. It also helps to go to full power if you’re not already at full power. For that matter, spin recovery isn’t that tough in most airplanes. Pull the power out so you don’t descend any faster than is necessary. Stop the rotation using the rudder. The wings may be stalled, but the rudder should be pretty effective. Once the rotation stops, push the yoke forward (decreasing the angle of attack), and then, once the wings are flying again, pull out of the dive.

And it’s not like stalls don’t have constructive uses. Aerobatic pilots use them all the time as part of graceful and energetic maneuvers. A stall is what your Frisbee does when it drops to your Labrador retriever’s waiting jaws just as the Frisbee runs out of forward momentum. It causes those pleasant swooping arcs that your paper airplane flies if you release it from a high enough spot.

The way stalls get into the news – and the way most members of the media screw up the references – 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.

Reporters sometime report on an accident saying that the engine stalled when, in fact, the airplane was stalled all right, but the engine was going full tilt when the airplane interfaced with the planet. The actual event that the reporter misreported was probably an aerodynamic stall and/or spin in the pattern or on takeoff or landing. Reporters who get this wrong do aviation a disservice because each time they do it, they cause a few hundred more non-pilots to believe that general aviation aircraft are mechanically unreliable. When it was actually pilot error of some kind.

Hey, I know that pilot error doesn’t help pilot’s reputations much more, but I’d always rather that the cause be something that’s under a pilot’s control because those are things that other pilots can do something to avoid.

So if you’re a member of the media and someone tells you before you go on the air that an aviation accident involved a stall, inquire further and find out whether it was an aerodynamic stall. If your source is a pilot or an aviation official, chances are good that they won’t use the word “stall” unless it was an aerodynamic stall. If the engine quit, they’ll usually say that the engine quit. And if you ask them to clarify, you’ll be an instant Eninstein to them because you’ll have clued them in that you recognize the difference.

If you’re a pilot or aviation official with the solemn job of briefing reporters on an accident or incident involving an aerodynamic stall, please take the time to explain what an aerodynamic stall is and point out that it has little or nothing to do with the engine.

And if you’re a member of the non-flying public, recognize what aerodynamic stalls are – that they generally have nothing to do with a powerplant or any other function of an aircraft. And that pilots train long and hard to avoid situations in which stalls occur, recognize their onset, and be always ready to recover from the rare unexpected aerodynamic stall.

Aerodynamic stalls are vanishingly rare in everyday flight operations. Unless you’re a pilot who’s training or performing aerobatics, the odds are vanishingly 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 extreme conditions. 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.

So now you know the difference and you’re a better consumer of news and information. And you know more than 95% of the population about aerodynamics. Congratulate yourself!

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