As the AOA increases to large values on the wing chord, the air separates starting at the back of the airfoil. As the AOA increases, the separated air moves forward towards the leading edge. The critical AOA is the point at which the wing is totally stalled, producing no lift—regardless of airspeed, flight attitude, or weight. [Figure 2-36]
Because the AOA of the WSC wing root chord/nose is so much higher than the AOA of the tips, the nose stalls before the tips. It is similar to stalling with the airplane canard in which the nose stalls first, the main wing (or tips for the WSC aircraft) continues to fly, and the nose drops due to lack of lift.
In most normal situations, the root chord/nose stalls first because it is at a much higher AOA. The tips continue to fly, making the WSC wing resistant to a complete wing stall. A pilot can even bring the aircraft into a high pitch angle stall attitude and keep the nose high. The nose stalls and rotates down because of the loss of lift, while the tips keep flying and maintain control of the aircraft.
If flying within the operating limitations of the aircraft and the WSC reaches a high AOA, the nose stalls, but the tips continue flying. However, it must be understood that there are many wing designs with many types of stall characteristics for each unique design. For example, high-performance wings could have less twist to gain performance, which could cause the wing to stall more abruptly than a training wing with more twist.
A WSC aircraft can get to a high pitch attitude by flying outside of its limitations or flying in extreme/severe turbulence. If the wing gets to such a high pitch attitude and the AOA is high enough that the tips stall, a whip stall occurs. [Figure 2-37]
In a WSC wing, most of the area of the wing is behind the CG (about three-quarters). With the tips and aft part of the wing having the greatest drag, and the weight being forward, an immediate and strong nose-down moment is created and the WSC nose starts to drop. Since both the relative wind and the wing are rapidly changing direction, there is no opportunity to reestablish laminar airflow across the wing.
This rotational momentum can pull the nose down into a number of increasingly worse situations, depending on the severity of the whip stall. Figure 2-37 shows a whip stall and the phases that can result, depending on the severity.
Phase 1—Minor whip stall results in a nose-down pitch attitude at which the nose is at a positive AOA and the positive stability raises the nose to normal flight, as described in Figure 2-25C.
Phase 2—If the rotational movement is enough to produce a vertical dive, as illustrated in Figure 2-29, the aerodynamic dive recovery might raise the nose to an attitude to recover from the dive and resume normal flight condition.
Phase 3—The rotational momentum is enough to bring the nose significantly past vertical (the nose has tucked under vertical), but could still recover to a vertical dive and eventually resume a normal flight condition.
Phase 4—The rotational momentum is severe enough to continue rotation, bringing the WSC wing into a tumble from which there is no recovery to normal flight, and structural damage is probable.
Avoidance and emergency procedures are covered in Chapter 6, Basic Flight Maneuverers, and Chapter 13, Abnormal and Emergency Procedures.