Pitch-up

In aerodynamics, pitch-up is a severe form of stall in an aircraft. It is directly related to inherent properties of all swept wings, and seen primarily on those platforms. Unlike conventional low-speed stalls, pitch-up can occur at any speed, and are especially dangerous when they take place in the transonic; at these speeds the aerodynamic loads can become so high as to break up the aircraft, as occurred in 1964 when a F-105 Thunderchief of the USAF Thunderbirds broke up in mid-air. It can also occur at low speeds, in which case it has been called a Sabre dance, a particularly dangerous behaviour of swept wings that became apparent during the development of the USAF F-100 Super Sabre. In aerodynamics, pitch-up is a severe form of stall in an aircraft. It is directly related to inherent properties of all swept wings, and seen primarily on those platforms. Unlike conventional low-speed stalls, pitch-up can occur at any speed, and are especially dangerous when they take place in the transonic; at these speeds the aerodynamic loads can become so high as to break up the aircraft, as occurred in 1964 when a F-105 Thunderchief of the USAF Thunderbirds broke up in mid-air. It can also occur at low speeds, in which case it has been called a Sabre dance, a particularly dangerous behaviour of swept wings that became apparent during the development of the USAF F-100 Super Sabre. Pitch-up problems were first noticed on high-speed test aircraft with swept wings. It was a common problem on the Douglas Skyrocket, which was used extensively to test the problem. Before the pitch-up phenomenon was well understood, it plagued all early swept-wing aircraft. In the F-100 Super 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 (although the T-tail was meant to prevent pitch-up from starting in the first place). Deployment of the braking parachute and a considerable height above the ground were essential for a chance at recovery. Wings generate a relatively complex pattern of forces at different points on their planform. These are usually described as lift and drag components, using vector decomposition. If these vectors are added up for the entire wing, the result is a single force acting at some point on the wing. This point is known as the 'center of pressure', or CoP, and is normally located somewhere between ⅓ and ½ of the way back from the leading edge. The exact location changes with changes in the angle of attack, which leads to the requirement to trim aircraft as they change their speed or power settings. Another major consideration for aircraft design is a similar vector addition of all of the weight terms of the parts of the aircraft, including the wing. This too can be reduced to a single weight term acting at some point along the longitudinal axis of the aircraft, the 'center of gravity', or CoG. If the wing is positioned so its CoP lies near CoG for the aircraft, in level flight the wing will lift the aircraft straight up. This reduces any net forces pitching the aircraft up or down, but for a number of reasons the two points are normally slightly separated and a small amount of force from the flight control surfaces is used to balance this out. The same basic layout is desirable for an aircraft with a swept wing as well. On a conventional rectangular wing, the CoP meets the aircraft at the point on the chord running directly out from the root. While the same analysis will reveal a center of pressure point for a swept wing, its location may be considerably behind the leading edge measured at the root of the wing. For highly swept planforms, the CoP may lie behind the trailing edge of the wing root, requiring the wing to meet the aircraft at a seemingly far-forward location. In this case of a swept wing, changes to the CoP with angle of attack may be magnified. The introduction of swept wings took place during a move to more highly tapered designs as well. Although it had long been known that an elliptical planform is 'perfect' from an induced drag standpoint, it was also noticed that a linear taper of the wing had much the same effect, while being lighter. Research during the war led to widespread use of taper, especially in the post-war era. However, it had been noticed early on that such designs had unfavourable stall characteristics; as the tips were more highly loaded in high angles of attack, they operated closer to their stall point.