Pitch-up

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In aerodynamics, pitch-up is a severe form of stall.

The phenomenon of pitch-up is directly related to inherent properties of all swept wings. Wingtips of a swept wing carry a higher load relative to the lift they generate compared to more inboard sections of the wing. In addition, swept wings tend to generate span-wise (along the length of the wing from root to tip rather than across the wing from front to back) flow of stagnant air. The combination of these factors means that at a high angle of attack the wingtips stall before the rest of the wing. Since the wingtips of a swept wing are by definition farther back than the wing roots, wingtip stall results is a considerable forward shift of the center of lift relative to the center of mass. The result is a rapid pitch up of the aircraft nose. If the tailplanes of an aircraft are caught in the resultant wake coming off the wings, they may not have enough authority to recover horizontal flight and the aircraft will continue to nose up higher and higher. In some circumstances, the aircraft can actually tumble end over end, sometimes at supersonic speeds, and the recovery may be difficult or impossible. Sometimes an intentional rapid pitch-up of the nose by the pilot, such as in a high speed hairpin turn in fighter jets such as the F-16, it becomes uncontrollable and the aircraft may begin to tumble.

Before the pitch-up phenomenon was well understood, it plagued all early swept-wing aircraft. In the F-86 Sabre it even got its own name, the Sabre dance. In aircraft with high-mounted tailplanes, like the F-101 Voodoo, recovery was especially difficult because the tailplane was placed directly in the wing wake during the pitch-up, causing deep stall. Deployment of the braking parachute and a considerable altitude above the ground were essential for a chance at recovery.

The pilot thus has ample time to react and the aircraft behavior is far more predictable. Other solutions include wing fences which disrupt spanwise flow and create a buildup of stangant airflow over the inboard portions of the wing, again causing them to stall before the tips. Angle of attack sensors on the aircraft can also detect when the AOA approaches the attitude known to result in pitch up and activate devices like the stick shaker to warn the pilot and the stick pusher which overpowers the pilot and forces the nose of the aircraft down to a safer AOA. Twist or camber built into the wingtips can also alleviate pitch-up. In effect, the angle of attack at the wingtip becomes smaller than elsewhere on the wing, meaning that the inboard portions of the wing will stall first. An unusual solution tried on the XF-91 Thunderceptor prototype fighter was to give the wingtips a wider chord than the wing roots. The idea was to increase wingtip lift and cause the wing roots to stall first.

The most common modern solution to pitch up is the use of slats which increase wing camber and both decrease the effective angle of attack of the wing and increase lift.