Slow flight isn't just for training. You'll fly it briefly every time you take off or land. Here's what you should understand about the aerodynamics...
Flying On The "Back Side" Of The Power Curve
When you demonstrate slow flight on a check ride, you are required to "establish and maintain an airspeed at which any further increase in angle of attack, increase in load factor, or reduction in power, would result in a stall warning" (e.g., aircraft buffet, stall warning, etc.). according to the new Airman Certification Standards.
This is a little different than the old FAA PTS standards for slow flight. Regardless, there's a lot to understand about the aerodynamics affecting your airplane, which we'll cover below.
Flying at this speed means that you are on the back side of the power curve, also known as the "region of reversed command." In normal cruise flight, you pitch the aircraft to maintain altitude, and power the aircraft to maintain airspeed. However, on the back side of the power curve, the inputs are the opposite. You pitch the aircraft to maintain airspeed and use power to maintain altitude. Why is this? The answer lies in induced drag, which dramatically increases as the aircraft's angle-of-attack (AOA) increases to maintain sufficient lift at low airspeeds.
Here's what the FAA has to say in Chapter 4 of the Airplane Flying Handbook:
"Since slow flight will be performed well below L/D MAX, the pilot must be aware that large power inputs or a reduction in AOA will be required to prevent the aircraft from decelerating. It is important to note that when flying on the backside of the power curve, as the AOA increases toward the critical AOA and the airplane's speed continues to decrease, small changes in the pitch control result in disproportionally large changes in induced drag and therefore changes in airspeed. As a result, pitch becomes a more effective control of airspeed when flying below L/D MAX and power is an effective control of the altitude profile (i.e., climbs, descents, or level flight)."
What Is "Speed Instability"?
When an airplane flies below L/D MAX, it exhibits a characteristic called "speed instability." Airspeed will continually decrease without appropriate pilot action. If the airplane is disturbed by turbulence and the airspeed decreases, the airspeed may continue to decrease without the pilot reducing AOA or adding power. This is another result of flying on the backside of the power curve (FAA).
Visual And Physical Cues
During slow flight, control responsiveness degrades and maintaining altitude becomes difficult because of less airflow over control surfaces. As airspeed is further reduced, larger and larger control movements are required to create the same response from the airplane. Pilots sometimes call this a "mushy" or "sloppy" feeling on the controls.
On the backside of the power curve, you'll typically need a much higher power setting than usual for the associated airspeed, because your induced drag is high. To maintain altitude, you'll increase or decrease your power setting, and you'll fly at a high angle of attack, with a nose-up attitude to maintain slow-flight airspeed.
If you begin to feel a slight buffeting of the control surfaces or you hear the stall warning horn, you've reached one of the first stages of a stall, and you need to pitch down to increase your airspeed.
When's the last time you practiced slow flight? Tell us in the comments below.
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