In aerodynamics, pitch-up is an uncommanded nose-upwards rotation of an aircraft. It is an undesirable characteristic that has been observed mostly in experimental swept-wing aircraft at high subsonic Mach numbers or high angle of attack. History 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. Description Wings generate pressure distributions on their upper and lower surfaces which produce a single force acting at a point known as the "center of pressure", or CoP, which is normally located between ⅓ and ½ of the way back from the leading edge. This upward and rearward leaning force is replaced by an equivalent pair of forces called lift and drag. The longitudinal position at which these forces act and the magnitude of the forces change with angle of attack. In addition a varying pitching moment exists for any force location other than the CoP. These changes lead to a requirement to trim aircraft as they change their speed or power settings. Another major consideration for aircraft design is a 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.