Figure 2-28. Wings stall due to an excessive angle of attack. Figure 2-27. Flare on landing typically increases the angle  of attack.  Porpoising Creates Variations in AOA Stalls: Exceeding the Critical Angle  of Attack Another slight variation in the angle of attack is the  swinging pendulum action of the PPC when high  thrust engines provide strong and immediate full  thrust of the propeller. This extra thrust swings the  cart through the pendulum arc relative to its position  under the wing. This is why many times you will see  the PPC take off and porpoise until it stabilizes. This  is a good example of the dynamic pendulum effect.  As the propeller thrust swings the cart out front, the  cart peaks then swings back to center. The cart suc- cessively swings back and forth, continuing to reduce  oscillations until it stabilizes in a climb. This porpois- ing is most common with a high power engine. This  can be eliminated by using gradual throttle increases  so as not to create a dynamic pendulum effect enter- ing a climb. The critical angle of attack is the angle of attack at  which a wing  stalls regardless of airspeed, flight at - titude, or weight. The drawings in Figure 2-28 show  airflow over a typical rectangular PPC wing. The first  shows a laminar, smooth, lift-generating airflow—one  that is typical when the angle of attack is within the  flight range. The second depicts an exceeded angle  of attack, turbulence and loss of the lifting force. [Figure 2-28] To prevent a stall, do not go to full-throttle while hold- ing a full-flare, or as specified in the POH. Note: For  explanation of a stall recovery, see Chapter 12: Night,  Abnormal, and Emergency Procedures. Unlike a fixed-wing aircraft that takes constant aware - ness of angle of attack to prevent a stall, the powered  parachute wing is designed by the manufacturers to  maintain a specified range of angle of attack and air - speeds. It is resistant to stalls because for all practical  purposes, it is designed to fly at a constant normal op - erating range. This range is maintained if the operator  flies within the operating limitations specified in the  POH. Flying the PPC within the limitations specified  in the POH and avoiding turbulence means you will  not exceed the critical angle of attack and stall the  wing. However, situations that could contribute to a stall are: • A large increase in wing drag (full-flare)  — which the PPC pilot controls by pulling the  wing back, thus increasing the AOA. (Note: A  full-flare is normally used and recommended  only for landings.) • A quick full RPM throttle input, creating a  climbing dynamic pendulum effect loading the  wing. • A quick reduction of throttle during a high pitch  angle climb. This quickly turns a high pitch  climb into a high angle of attack. The wing is  initially pitched high, climbing the inclined  plane under full power, then quickly changes  to a gliding flight path when the throttle is  reduced, just like an airplane. • A wind gust from flying in turbulent air. 2-14