What exactly is a stall? To answer that, let's first talk about what a stall is not. Unlike a car, a stall in an airplane has nothing to do with the engine, but everything to do with the wing. When you stall an airplane, the propeller doesn't stop turning, and the engine doesn't cough and quit. A stall is an aerodynamic condition whereby the smooth airflow over the top of the wing is disrupted, resulting in a loss of lift.
Airflow over a wing contributes to lift at normal angles of attack (left); exceed the critical angle of attack, however, and airflow separates from the upper wing surface--rapidly decreasing lift and stalling the wing (right). | |
The magic of lift
To understand how a wing stalls, we must first understand how a wing produces lift. A wing is an airfoil, which means its shape, or section, is designed to interact with a fluid, in this case air. This airfoil shape is commonly seen in nature. For example, a bird's wing is an airfoil, and although less apparent, a fish's body is also shaped like an airfoil. When a wing moves through the air, it produces lift, which is one of the four forces of flight. This lift counteracts the weight of the airplane, enabling the airplane to fly. In simple terms, a wing generates lift primarily by using two proven principles.
One way in which a wing produces lift is by deflecting air downward. This downward flow of air, or downwash, causes the wing to move upward in response. This is simply action and reaction, and follows Sir Isaac Newton's third law of motion, which states that "for every action, there is an equal and opposite reaction." This law can be demonstrated by extending your arm out the window of a fast-moving car. If you tilt the flattened palm of your hand upward, at a positive angle to the airstream, your arm swings upward. This law is also responsible for holding up water skiers.
Another way in which a wing produces lift is by accelerating the airflow over its top surface, creating a low-pressure area on top of the wing. Long before airplanes, Daniel Bernoulli, a Swiss mathematician, showed that the sum of a fluid's static (or still) pressure and dynamic (or moving) pressure remains constant. This is known as Bernoulli's principle. Simply stated, it says that as the velocity of a fluid (air) increases, its internal pressure decreases. Thus, the faster a fluid is moving, the lower its pressure. Think of it this way: Air molecules need energy in order to speed up. The air molecules essentially "borrow" this energy from the static pressure.
Bernoulli's principle can be demonstrated by taking a small sheet of paper, and while holding the corners of one end horizontally and curled downward slightly below your mouth, blowing over the top of the sheet. The sheet will rise as the air pressure below "pushes" or "lifts" the paper into the low-pressure area above created by the fast moving airflow.
The curved shape of the airfoil, combined with its angle relative to the airstream, essentially reduces the area available to the airflow over its top surface. By way of the "Law of Continuity," the airflow must accelerate to pass through this smaller area. In effect, the airflow over the top of wing is constricted or "sandwiched" between the wing and the layers of air above. The greater the angle of attack, the more the airflow is sandwiched over the top of the wing, and the more the airflow is deflected from the bottom of the wing. This results in more lift being generated. But intuitively it is apparent that lift cannot increase indefinitely.
The critical angle of attack
Lift acts perpendicular to both the relative wind and the wingspan. |
Eventually a point is reached whereby any further increase in angle of attack does not produce an increase in lift, and this point is called the stalling, or critical, angle of attack. Each airfoil has a different critical angle of attack, but with typical general aviation airfoils it is roughly 16 to 20 degrees. Beyond this angle, the airflow over the top of the wing becomes turbulent and separates from the upper surface, lift decreases rapidly, and the wing becomes "stalled."
Because a stall is dependent only upon angle of attack, an airplane can stall at any airspeed, attitude, configuration, power setting, or weight. However, an airplane's stall speed is affected by many factors, including weight, center-of-gravity location, load factor, and power setting. Since a stall is dependent upon the wing's angle of attack, and the elevator is the primary control for angle of attack, it could be said that the elevator stalls or "unstalls" the airplane.
Friction
We know that a stall occurs because the airflow cannot continue smoothly over the upper surface of the airfoil. This smooth airflow is required to generate the pressure differences and downwash that produce lift. When this airflow becomes turbulent and separates from the upper surface, downwash is reduced, the pressure on top of the wing increases, and lift is destroyed. But why does the airflow become separated and turbulent as the critical angle of attack is reached? In basic terms, the airflow simply doesn't have enough energy to overcome the skin friction of the upper surface of the airfoil.
Every surface has some roughness. Although the surface of a wing is not as rough as, say, a piece of 100-grit sandpaper, on a microscopic level even its seemingly smooth surface is coarse and generates friction. Combine that with the fact that air, which is a fluid, has some "stickiness" to it (called viscosity), and it's no wonder the airflow slows down as it travels farther back along the airfoil.
Eventually, at some point on the upper surface of the airfoil, the energy in the airflow cannot overcome this surface friction. This is where the airflow separates and turbulent eddies form. As the angle of attack is increased, the point on the top of the wing where the airflow separates (called the stagnation point) moves forward, toward the leading edge.
Recovery from a stall is a three-step process. First, push forward on the yoke (left). Second, apply full power (center). Third, retract flaps (and landing gear if applicable) and return to straight-and-level flight (right). | ||
Stall defense
Stalls are not inherently unsafe, but a stall at low altitude might leave a pilot with insufficient altitude to recover--or, worse, if the airplane is not in coordinated flight and yaw is present, a spin could result. That's why familiarity with stalls and spins is one of the requirements for you to earn a private pilot certificate. Your instructor will make sure that you are fully aware of stalls and spins and that you can successfully demonstrate stall entries and recoveries from various configurations and power settings. Remember, the real objective of practicing stalls is not to learn how to stall an airplane, but to learn how to recognize an approaching stall and take prompt corrective action.
As we said earlier, the elevator stalls or "unstalls" the airplane. Thus, to demonstrate a stall you'll use elevator back-pressure to increase the pitch attitude to the critical angle of attack. You'll release elevator back-pressure to decrease the angle of attack and get the wing flying again. Releasing elevator back-pressure--pushing forward on the yoke--is the first step in stall recovery.
But you will also learn a second step, which is to add maximum available engine power. Although engine power is not required to recover from a stall, you'll learn this integrated recovery technique because engine power does reduce the amount of altitude lost during the stall recovery and helps the airplane to regain airspeed.
The third step in stall recovery is to reduce drag (by retracting flaps and gear, if applicable) and return to straight-and-level flight. For more information on stall entry and recovery procedures, consult the pilot's operating handbook for the airplane you fly, and discuss the topic with your instructor.
What to expect
Before you take to the air for your stall training, you should have some idea of what to expect.
To understand the mechanics of this, we need to understand how weight is distributed in an airplane. In conventional airplanes, the center of gravity (a point on the fuselage where all the weight is concentrated) is located ahead of the center of lift (a point on the wing where all the lifting force is concentrated). The result of this arrangement is that the nose always tends to pitch down. To counter this, the nose is "held up" in flight by the force generated by the horizontal stabilizer and elevator. This force, known as tail-down force, pulls the tail downward, thus pivoting or "lifting" the nose of the airplane up. The horizontal stabilizer acts as an upside-down wing, generating downward or negative lift. Moving the elevator creates more or less negative lift, and the nose pitches up or down in response.
When the wing fully stalls often there is a drastic loss of lift, and the weight of the airplane causes the nose to drop. This is actually beneficial; the nose-down pitch attitude reduces the wing's angle of attack and helps to get the wing flying again. This arrangement is naturally stable. During the stall break, you may experience a slight falling sensation as the nose pitches over. (Depending on aircraft type and pilot technique, airplanes can stall in a nose-high attitude without the break and pitch down.) Although performing a stall in a general aviation trainer may be a slightly unsettling maneuver, the sensation is nothing close to that of a typical amusement park roller coaster.
Using your senses
You can expect the airplane to give you cues that it is approaching a stalled condition. One of the senses you'll rely on to detect these cues is feel. The airplane will feel different as it approaches a stall. For example, during the set-up for an approach-to-landing stall, the airplane will let you know it's slowing down by the feel of the flight controls, especially the ailerons. As the airspeed slows, the controls will become less responsive and will probably feel sluggish or mushy. Since less air is flowing over them, there will be less resistance to the pressures you apply to them. You may notice a lag between your control inputs and the airplane's response. You might also feel the airplane start to buffet or shake just before the stall. You may feel this in the control yoke or through the airframe. This is a result of the turbulent airflow tumbling over the top of the wing.
Another sense you'll rely on to warn you of an approaching stall is sound. As the airplane slows down during an approach-to-landing stall, you'll notice the sound of the slipstream noise around the airframe becoming quieter. One downside of today's wonderful noise-canceling headsets is that students are becoming more and more insulated from what the airplane is trying to tell them. You should always try to listen, because as anyone who's flown an open-cockpit biplane will tell you, airplanes do talk.
Of course, the most noticeable sound you'll hear is that of the stall warning device, generally a buzzer or a horn. This device, found in all modern airplanes, warns the pilot of an impending stall. When you practice stalls and slow flight, you'll probably give your airplane's stall warning device a good workout. But keep in mind this device is designed to activate at least 5 knots before the actual aerodynamic stall occurs. So, just because the stall warning is blaring, it doesn't mean the wing is fully stalled; it may be just approaching the stall. It's hard to do, but don't let the stall warning horn or buzzer distract or alarm you.
Finally, there's one more sense you'll learn to rely on to let you know what your airplane is doing. That is your sense of kinesthesia, or your body's sensing of changes in direction or speed of motion. Commonly known as seat of the pants feel, it may be the best indicator to the trained and experienced pilot. If this sensitivity is properly developed, it will alert you to a decrease in speed or the beginning of a settling or "mushing" of the airplane long before the other cues will.
Stalls are nothing new, and nothing to worry about. Airplanes have come a long way in the past 100 years. Thanks to high-speed computers running complex computational fluid dynamics programs, advanced wind tunnels and testing, and stringent certification criteria, modern airplanes have superb handling characteristics and exemplary safety records. They are highly controllable throughout the flight envelope, with mild stall characteristics and ample stall warning cues. Rest assured, their handling qualities would make any World War I pilot highly envious.
Christopher L. Parker is a CFI and an aviation author, speaker, and FAA remedial training specialist. He flies internationally as a contract captain on a Bombardier Challenger business jet and lives in Los Angeles.
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