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03/08 - Understanding Stalls, Part 2

By Steve Krog
Steve Krog is a flight instructor, EAA member and president of the Cub Association.

Students often ask, “Why do you include stalls in primary flight training?” A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly, caused by the wing exceeding its critical angle of attack. We learn what stalls are, how to understand and recognize them, and how to take corrective action to prevent accidents or incidents. The simplest answer to that question, though, is that we actually “stall” the airplane at the point of touchdown on every landing! (More on that in another issue.)

The power on stall

In the last issue of Reach For The Sky, we explained and demonstrated the power off stall. This month’s lesson will cover the power on stall. You may be thinking, “Why practice power on stalls? I’m never going to accidentally do a power on stall when flying in the future.”

You may never intentionally do or even approach a power on stall in your future flights, but what if you inadvertently did because of a distraction? Wouldn’t you agree that it would be wise to be able to instantly recognize an approaching power on stall and take corrective action to prevent its occurrence?

For those of you who took a driver’s training course to obtain an automobile or motorcycle driver’s license, you had to perform emergency stops. You never intended to have to make emergency stops because, after all, you are a great driver. Right? But your instructor made you do them anyway so that you could see how the car or cycle acted in an emergency stop situation. You practiced them just in case you were some day faced with the necessity for an emergency stop. You would know what to do without even thinking about it! For the same reason we practice power on stalls. We strive to be safe pilots and practice those “what if” situations to be prepared to recognize and handle them.

Aeronautical terms—A quick review

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

Relative wind is the motion of the airplane through the air. 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.

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.

Practicing the power on stall

After our take off, we will fly from the airport to our practice area. While doing so we’ll climb to a safe altitude of approximately 2,500 feet above the ground (AGL). After reaching our safe altitude, we’ll establish our normal straight and level cruise flight power setting.

Now that we have the airplane properly trimmed and our cruise power setting made, we’re ready to begin our demonstration and practice of power on stalls. The first power on stall will be the “imminent” stall where we take corrective action to recover from the stall when we feel the buffet generated at the onset of the stall.

Step one: Clear the practice area of other aircraft by making two medium bank 90° turns, one to the left and the other to the right. Be very conscientious when making these clearing turns and thoroughly scan the area looking for any other aircraft. We don’t want to interfere with other aircraft nor do we want them to interfere with us. Clear the area not only left and right but also up and down because our altitude will fluctuate by several hundred feet during our stall practice.

NOTE: For practice purposes we will assume the aircraft we are flying today cruises at a speed of 100 mph, has a climb speed of 60 mph, and stalls at 40 mph.

Step two: Apply carburetor heat and reduce power to approximately a fast idle. Remember, the nose will want to pitch downward as the power is reduced, so begin applying backpressure to the control stick or yoke. Continue applying backpressure, raising the nose of the aircraft approximately 30° above the horizon. As the nose is raised, our airspeed begins to slow or dissipate. We want to simulate a take off and climb out situation so as the airspeed approaches 60 mph, begin applying power to approximately cruise power and continue applying backpressure holding the nose 30° above the horizon.

Even with the addition of cruise power, the airplane will continue to slow. Take a quick glance left and right along the wings and you will see that your attitude looks and feels steeper as compared to the power off stall configuration. The airspeed continues to dissipate and as it approaches 40 mph, our angle of attack begins exceeding the angle where smooth airflow over the wing can be maintained. At this point we begin feeling the buffet or the interruption of air flow over the wing. In most airplanes the buffet will be prominent enough to “feel” it in the seat of your pants and in the control stick or yoke. Only the inboard portion (about one-third) of the wing is stalled. The smooth relative wind flow over the wing is breaking up causing the buffet or shaking that you are feeling. We are now in the imminent power on stall configuration and at this point we will initiate the recovery procedure.

Recovery from the imminent power on stall is quite easy. Simply relax the backpressure you are holding on the yoke allowing the nose to lower to just below the horizon. Level the wings, if needed, with rudder input, apply full power and push the carb heat off. Remember, we entered the imminent power on stall with cruise power. As the nose is lowered the airplane’s wings will almost immediately begin generating enough lift to maintain altitude. As the airspeed increases to our 60 mph climb speed, raise the nose slightly to re-establish level flight and re-adjust your power to the cruise power setting, if required. You’ve just performed your first imminent power on stall.

After practicing the imminent power on stall about a half-dozen times, the set up and the recovery procedure will begin to come naturally. And the 30° nose high attitude will become more comfortable to you. Now it’s time to move on to the full power on stall.

The set up for the full power on stall is identical to the imminent stall. After clearing the practice area via our clearing turns, apply carb heat, reduce the power to a fast idle and begin applying backpressure raising the nose approximately 30°. As the airspeed approaches 60 mph, begin applying power to about the cruise power position while continuing to apply backpressure. The buffet indicating the imminent stall will be felt but continue applying backpressure. Note the left and right wing attitude in relation to the horizon. As the stall progresses the buffeting will increase until the wings are in a near full stall configuration and can no longer provide enough lift to maintain altitude. At this point the nose of the airplane will pitch downward. This is known as the stall break.

When the break occurs, simply relax the backpressure you are holding on the yoke allowing the nose to lower to just below the horizon. Level the wings with rudder input, apply full power, and push the carb heat off. Remember, we entered the full power on stall with cruise power so as the nose is lowered the airplane wings will almost immediately begin generating enough lift to maintain altitude. Raise the nose slightly to re-establish level flight and re-adjust your power to the cruise power setting, if needed. You’ve just performed your first full power on stall.

After practicing the full power on stall a few times they, too, will become more comfortable.
Your instructor will review the imminent and full power on stalls for a few minutes in each of the next two or three flight lessons until you have a full understanding of the procedures and become comfortable in performing them.

Just as with the power off stalls, after mastering the power on stalls straight ahead your instructor will have you perform a shallow bank turn left and right while demonstrating the power on stalls. Again, the high or outside 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. Stall recovery requires the input of rudder while simultaneously relaxing the backpressure. For example, if we are performing a full power on stall with a shallow bank to the left, the right or high wing will stall first. This loss of lift will cause the high wing to drop causing the airplane to not only pitch nose downward but also turn or roll to the right. Again, relax the backpressure on the yoke allowing the nose to pitch downward and apply opposite or left rudder stopping the turn to the right. When the wings return to level, relax the left rudder pressure and continue with your recovery by applying full power, pushing the carb heat off and returning the airplane to straight and level flight. Once level re-set the power for cruise flight.

After practicing the imminent and full power on stalls using cruise power, your instructor will then have you perform them using full power. This will feel the same but your nose attitude will be several degrees higher on the horizon.

Doing power on stalls with shallow turns to the left and right will give you a sensation or feel similar to some fun carnival rides. But remember we are performing the stalls at a safe altitude allowing plenty of time to understand, feel, and perform the recovery. After several good practice sessions you’ll find power on stalls to even be fun to perform!

In the next issue of “Reach For The Sky,” we’ll discuss ground reference maneuvers in preparation for beginning to fly the traffic pattern and performing take offs and landings.

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