In aerodynamics

Aerodynamics

Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

, pitch-up is a severe form of stall

Stall (flight)

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

 in an aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

. It is directly related to inherent properties of all swept wing

Swept wing

A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...

s.

History

Pitch-up problems were first noticed on high-speed test aircraft with swept wings. It was a common problem on the Douglas Skyrocket

Douglas Skyrocket

The Douglas D-558-2 Skyrocket was a rocket and jet-powered supersonic research aircraft built by the Douglas Aircraft Company for the United States Navy...

, 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

F-100 Super Sabre

The North American F-100 Super Sabre was a supersonic jet fighter aircraft that served with the United States Air Force from 1954 to 1971 and with the Air National Guard until 1979. The first of the Century Series collection of USAF jet fighters, it was the first USAF fighter capable of...

 it even got its own name, the Sabre dance. In aircraft with high-mounted tailplanes, like the F-101 Voodoo

F-101 Voodoo

The McDonnell F-101 Voodoo was a supersonic military jet fighter which served the United States Air Force and the Royal Canadian Air Force...

, 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 height above the ground were essential for a chance at recovery.

Description

The wingtips of a swept wing aircraft operate at a higher local lift coefficient

Lift coefficient

The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...

 than inboard sections of the wing so the wingtips are more heavily loaded than inboard sections. In addition, swept wings tend to generate spanwise flow of the boundary layer

Boundary layer

In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...

 (along the length of the wing from root to tip rather than across the wing from front to back). This problem is primarily seen at low speeds. The combination of these factors means that at a high angle of attack

Angle of attack

Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

, especially at low speed, 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 in a considerable forward shift of the center of pressure

Center of pressure

The center of pressure is the point on a body where the total sum of a pressure field acts, causing a force and no moment about that point. The total force vector acting at the center of pressure is the value of the integrated vectorial pressure field. The resultant force and center of pressure...

 relative to the center of mass

Center of gravity of an aircraft

The center-of-gravity is the point at which an aircraft would balance if it were possible to suspend it at that point. It is the mass center of the aircraft, or the theoretical point at which the entire weight of the aircraft is assumed to be concentrated. Its distance from the reference datum is...

. The result is a rapid upwards pitch of the aircraft nose. If the tailplane

Tailplane

A tailplane, also known as horizontal stabilizer , is a small lifting surface located on the tail behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed wing aircraft such as helicopters and gyroplanes...

s 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 a fighter jet, will cause the aircraft to become uncontrollable and it may begin to tumble.

Mitigation

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 device

High-lift device

In aircraft design, high-lift devices are moving surfaces or stationary components intended to increase lift during certain flight conditions. They include common devices such as flaps and slats, as well as less common features such as leading edge extensions and blown flaps.-Purpose:Aircraft...

s.

A common solution to the problem of spanwise flow is the use of a wing fence

Wing fence

Wing fences, also known as boundary layer fences and potential fences are fixed aerodynamic devices attached to aircraft wings. Not to be confused with wingtip fences, wing fences are flat plates fixed to the upper surfaces parallel to the airflow. They are often seen on swept-wing aircraft...

 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 loading, a wider variety of techniques have been used, including dedicated slats

Leading edge slats

Slats are aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower...

 or flaps, the use of washout

Washout (aviation)

Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of the wing of an aircraft. The wing is designed so that angle of incidence is higher at the wing roots and decreases across the span, becoming lowest at the wing tip...

 or automated control of the aileron

Aileron

Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...

s. An unusual solution tried on the XF-91 Thunderceptor

XF-91 Thunderceptor

|-See also:-References:NotesBibliography* Jenkins, Dennis R. and Tony R. Landis. Experimental & Prototype U.S. Air Force Jet Fighters. North Branch, Minnesota, USA: Specialty Press, 2008. ISBN 978-1-58007-111-6....

 prototype fighter was to give the wingtips a wider chord

Chord (aircraft)

In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil...

 than the wing roots. The idea was to increase wingtip lift and cause the wing roots to stall first.

Angle of attack

Angle of attack

Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...

 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

Stick shaker

A stick shaker is a mechanical device to rapidly and noisily vibrate the control yoke of an aircraft to warn the pilot of an imminent stall...

 to warn the pilot, and the stick pusher

Stick pusher

A stick pusher is a device installed in some fixed-wing aircraft to prevent the aircraft from entering an aerodynamic stall. Some large fixed-wing aircraft display poor post-stall handling characteristics or are vulnerable to deep stall...

 which overpowers the pilot and forces the nose of the aircraft down to a safer angle of attack. Twist or washout

Washout (aviation)

Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of the wing of an aircraft. The wing is designed so that angle of incidence is higher at the wing roots and decreases across the span, becoming lowest at the wing tip...

 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

Lift coefficient

The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...

.

Pitch-up is impossible in aircraft with forward-swept wings as used on the Grumman X-29

Grumman X-29

|-See also:-References:NotesBibliography* Thruelsen, Richard. The Grumman Story. New York: Praeger Publishers, Inc., 1976. ISBN 0-275-54260-2....

. With forward-swept wings the spanwise flow is inboard, causing the wing root to stall before the wingtip. One might imagine this could lead to the possibility of pitch-down problems, but the onset of this effect would lower angle of attack, and thus eliminate the problem.