page_title: Pitch-up
section_title: Mitigation
hierarchical_section_title: Pitch-up / Mitigation
context_page_description: 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.
context_section_description: As the primary causes of the pitch-up problem are due to spanwise flow and more loading at the tips, measures to address these issues can eliminate the problem. In early designs these were typically "add-ons" to an otherwise conventional wing planform, but in modern designs this is part of the overall wing design and normally controlled via the existing high-lift devices.
  The first known attempt to address these problems took place on the platform where they were first noticed, the Douglas Skyrocket. This took the form of a series of vortex generators added to the outboard portions of the wing, breaking up the boundary layer. However, this was found to have almost no effect in practice. Nevertheless, a similar solution was attempted on the Boeing B-47 Stratojet where it proved considerably more effective. This may have been helped by the presence of the podded engines, whose vertical mountings acted as barriers to span wise flow.
  More common solutions to the problem of spanwise flow is the use of a wing fence or the related dogtooth notch on the leading edge of the wing. This disrupts the flow and re-directs it rearward, while also causing the buildup of stagnant air inboard to lower the stall point. This does have an effect on overall airflow on the wing, and is generally not used where the sweep is mild.
  To address the problems with spanwise loading, a wider variety of techniques have been used, including dedicated slats or flaps, the use of washout or automated control of the ailerons. 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 efficiency and cause the wing roots to stall first.
  Angle of attack sensors on the aircraft can also detect when the angle of attack 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 angle of attack. Twist or washout 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.
  A commonly used solution to pitch-up in modern combat aircraft is to use a control-canard. Another modern solution to pitch-up is the use of slats. When slats are extended they increase wing camber and increase maximum lift coefficient.
  Pitch-up is also possible in aircraft with forward-swept wings as used on the Grumman X-29. With forward-swept wings the span wise flow is inboard, causing the wing root to stall before the wingtip. Although at first glance it would appear that this would cause pitch-down problems, the extreme rear mounting of the wing means that when the root stalls the lift moves forward, towards the tips.
caption: Washout is clearly visible in this image of a CF-18 Hornet. Note the angle of the Sidewinder missile on the wingtip rail as compared to the angle of attack of the fuselage.
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