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02/08 - Understanding Stalls

By Steve Krog

Steve Krog is a flight instructor and EAA member.

If you mention an airplane stall to someone who does not understand airplane flight, they immediately assume the engine has quit and the airplane falls from the sky. Unfortunately, the naïve media enhances this fallacy whenever reporting airplane incidents. Mention stalls to a beginning student, or even some low-time pilots and they get glassy-eyed and feel a pit growing in their stomach. However, stalls need not be feared. They are nothing more than a set of maneuvers we practice to understand the flight characteristics of the airplane we are flying.

What is a stall?

To understand a stall, we first need to review several aviation terms: angle of attack, relative wind, and chord line of the airfoil (wing).

The angle of attack is always based on the flight path, not the ground.

The forward part of an airfoil is rounded and is called the leading edge. The aft part is narrow, tapered and called the trailing edge. A reference line often used in discussing airfoils is the chord or chord line, an imaginary straight line joining the leading and trailing edges.

The motion of the airplane through the air creates relative wind. During flight only the forward motion of the airplane produces relative wind—the direction and speed of the wind have no effect on the relative wind. We’ll discuss this further in a future newsletter.

The angle of attack is the angle between the wing chord line and the flight path.

Lift is generated by the airflow (relative wind) over and under the wing or airfoil. When the angle of attack is increased to the point where the relative wind can no longer flow smoothly over the wing, the wing is not able to generate enough lift to keep the airplane at a constant altitude and a stall occurs.

A stall in an airplane is not at all like a car stalling, in which case the engine stops. The airplane stall is a situation where the lift has broken down and the wing is no longer able to support the airplane. The pilot has forced the airplane away from the balanced situation he or she desires to maintain.

For the airplane to recover from a stall, the pilot must decrease the angle of attack so that the smooth flow of air over the wing again occurs.

Looking at the illustrations above, we see the airflow over the wing at 8 degrees, 14 degrees and 19 degrees angle of attack. Most all airplanes used in primary flight training are designed to stall at an 18-20 degrees angle of attack. Note the smooth airflow over the wing when positioned at an 8 degrees angle of attack. In this situation the airplane is comfortably generating adequate lift and is climbing to a desired altitude. When increasing the angle of attack to approximately 14 degrees, the airflow over the wing is no longer flowing smoothly. Rather, it’s beginning to break up or “burble” as it passes over the aft portion of the wing. When demonstrating this angle of attack during our flight, it is sometimes referred to as an “imminent stall.” If we continue to increase the angle of attack to approximately 19 degrees, the airflow over the wing is no longer smooth and flowing. The air is “burbling” over the top of the majority of the wing and is not producing enough lift to maintain a desired altitude. A stall occurs. Now that we know what a stall is we can begin today’s flight lesson.

Flow of air over a wing at various angles of attack.

Why do we learn and practice stalls?

Since the early flight days of the Wright Brothers, stalls in airplanes have occurred. And where do they most likely occur? On take offs and landings when the aircraft is operated at a slower airspeed. They need not be feared but rather thoroughly understood.

Once we know what a stall is and how they occur, we can prevent them. We want to be able to recognize and feel impending stalls so that should we become distracted and encounter the approach of a stall, we can instantly recognize it and take corrective action before the stall is allowed to occur.

Types of stalls

There are a number of different stall configurations. However, the stalls we focus on in primary flight training fall into two categories: power off stalls and power on stalls. We can further break the stalls down to power off imminent stalls, power off full stalls, power on imminent stalls and power on full stalls.

Learning and practicing power off stalls

After taking off and climbing to a safe altitude of approximately 2,500 feet above the ground (AGL), and away from the airport, we can begin our stall practice.

After establishing level flight at the normal cruise flight power setting, we will make our clearing turns. Remember doing them prior to practicing slow flight? It is important that we clear the practice area of other aircraft around us, as we will be significantly slowing the airplane as well as descending during our practice of stalls.

Now that we have cleared the area, we will begin by doing a power off imminent stall. NOTE: I like to have students align the airplane on one of the four cardinal compass headings, north, south, east or west before the first stall.

Step one: apply carburetor heat. Then reduce the power to near idle. Note that as the power is reduced, the nose of the airplane will want to drop or “pitch” downward. Begin pulling the control yoke or stick back increasing the angle of attack to maintain altitude. The airspeed bleeds off and the controls become somewhat sluggish. As we approach the stall speed of the aircraft, (let’s say for this discussion the stall speed is 40 mph), our angle of attack in relation to relative wind is very near 14 degrees and our nose attitude is at or just above the horizon. The airflow starts to burble over the aft portion of the wings and we feel the burble in the airplane. With a bit of practice you will feel this burble in the seat of your pants. The nose of the airplane will be at or slightly above the horizon. When we see this nose attitude, feel the burble, and hear the wind noise dissipate as the airplane slows, we will have just entered into a power off imminent stall.

The stall recovery is quite easy. Relax the backpressure you are holding on the yoke or stick and then push it slightly forward. The nose of the airplane now drops below the horizon line. Initiating this action helps the airplane wing generate lift again. Apply full power and push the carburetor heat off to help the airplane generate even more lift as the airspeed increases. Ease the yoke back and position the nose of the airplane on the horizon for level flight and adjust power for cruise flight.

It may seem at this point that you have a lot of things to do in the stall recovery, but after practicing a few stalls the procedure will come to you naturally and simultaneously. Remember the stall recovery procedure, as it will be nearly the same for all stalls.

After practicing several power off imminent stalls with the wings level to the horizon, your instructor will demonstrate and then have you try power off full stalls. The set up is exactly the same as when doing the imminent stall. However, rather than initiating recovery from the stall at the burble point, we will continue applying backpressure until we reach the approximate 19° angle of attack. (Remember the earlier illustration.) When reaching this angle of attack the airflow over the wing breaks up or burbles over most of the wing surface creating a significant loss of lift. The wing is no longer able to maintain altitude and the nose will break or pitch downward on its own accord. As the nose drops below the horizon, initiate the stall recovery procedure as described earlier. Recovery from a full stall will take several seconds before we have enough smooth airflow over the wing to generate the lift needed to re-establish level flight.

After practicing a number of power off imminent and full stalls, your instructor will give you the added challenge of practicing these stalls with a shallow bank or turn of about 15 degrees to the left and right.

When practicing power off full stalls with a shallow bank, the high wing will usually stall before the lower wing causing the airplane to not only pitch nose downward but also turn or roll in the direction of the stalled high wing. Experiencing this for the first time will give you a new sensation but it is not anything that should bring on anxiety, especially after practicing a few full stalls with shallow bank.

Let’s go through a power off full stall with a shallow bank to the left. Set up for the stall just as we did before. However, while increasing backpressure on the yoke, gently apply left aileron and rudder establishing a shallow turn to the left. Once the bank is established return the ailerons and rudder to neutral but continue adding backpressure. As the burble begins followed by the break, the high wing will stall first causing the airplane to roll or turn to the right while the nose simultaneously pitches downward.

Recovery from this stall is the same as all others with one exception. It is important that we stop the turn or roll and get the wings back to a level configuration. While relaxing the yoke backpressure apply left rudder to stop the roll or turn. As soon as the wings are level take your foot pressure off the left rudder and continue with the normal stall recovery. It really isn’t difficult and you’ll soon be applying correct rudder automatically when recovering from full stalls with shallow bank, either left or right.

You’ve now satisfactorily completed the first of two or three flight lessons focusing on stalls!

NOTE: After practicing stalls until they become comfortable for the student, I then like to demonstrate the positive stability of the airplane to all students. This really helps students understand that the airplane will not “fall” out of the sky. To do this I first set up the airplane for a power off full stall. When the burble and break occurs, I take my hands and feet off the controls. The airplane will make its own recovery after a few upward and downward oscillations. You may need to apply very slight backpressure on the yoke to prevent excessive airspeed, however, but it will return to level flight.

In the next issue of "Reach For The Sky," we'll continue with "Understanding Stalls, Part Two - Power-On Stalls."

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