Washout (aviation)
Encyclopedia
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 (angle to the fuselage) is higher at the wing roots and decreases across the span, becoming lowest at the wing tip
. This is usually to ensure that, at the stall
, the wing root stalls before the wing tips, providing the aircraft with continued aileron control and some resistance to spinning
. Washout may also be used to modify the spanwise lift distribution to reduce lift-induced drag
.
Washout is commonly achieved by designing the wing with a slight twist
, reducing the angle of incidence
from root to tip, and therefore causing a lower angle of attack
at the tips than at the roots. This feature, pioneered in fighter aircraft like the Spitfire, is sometimes referred to as structural washout, to distinguish it from aerodynamic washout.
Wingtip
stall is unlikely to occur symmetrically, especially if the aircraft is maneuvering. As an aircraft turns, the wing tip on the inside of the turn is moving more slowly and is most likely to stall. As an aircraft rolls, the down-going wing tip is at higher incidence and is most likely to stall. When one wing tip stalls it leads to wing drop, a rapid rolling motion. Also, roll
control may be reduced if the airflow over the aileron
s is disrupted by the stall, reducing their effectiveness. Thus, a pilot
may be unable to maintain control, perhaps leading to an unrecoverable spin
.
On aircraft with swept wing
s, wing tip
stall also produces an undesirable nose-up pitching moment
which hampers recovery from the stall.
Washout may be accomplished by other means e.g. modified aerofoil
section, vortex generator
s, leading edge wing fence
s, notches, or stall strips
. This is referred to as aerodynamic washout. Its purpose is to tailor the spanwise lift distribution or reduce the probability of wing tip
stall.
Winglets have the opposite effect to washout. Winglets promote a greater proportion of lift being generated near the wing tips. (This can be described as aerodynamic wash-in.) Winglets also promote a greater bending moment
at the wing root, possibly necessitating a heavier wing structure. Installation of winglets may necessitate greater aerodynamic washout in order to provide the required resistance to spinning
, or to optimise the spanwise lift distribution.
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
distribution across the span
Wingspan
The wingspan of an airplane or a bird, is the distance from one wingtip to the other wingtip. For example, the Boeing 777 has a wingspan of about ; and a Wandering Albatross caught in 1965 had a wingspan of , the official record for a living bird.The term wingspan, more technically extent, is...
of the wing
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
of 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...
. The wing is designed so that angle of incidence (angle to the fuselage) is higher at the wing roots and decreases across the span, becoming lowest at the wing tip
Wing tip
A wing tip is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft.Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of shapes, including:* Squared-off...
. This is usually to ensure that, at the 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...
, the wing root stalls before the wing tips, providing the aircraft with continued aileron control and some resistance to spinning
Spin (flight)
In aviation, a spin is an aggravated stall resulting in autorotation about the spin axis wherein the aircraft follows a corkscrew downward path. Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw...
. Washout may also be used to modify the spanwise lift distribution to reduce lift-induced drag
Lift-induced drag
In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is a drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or a lifting body redirecting air to cause lift and also in cars...
.
Washout is commonly achieved by designing the wing with a slight twist
Wing twist
Wing twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing.Often, the purpose of lift redistribution is to ensure that the wing tip is the last part of the wing surface to stall, for example when executing a roll or steep climb; it involves twisting the...
, reducing the angle of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
from root to tip, and therefore causing a lower 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...
at the tips than at the roots. This feature, pioneered in fighter aircraft like the Spitfire, is sometimes referred to as structural washout, to distinguish it from aerodynamic washout.
Wingtip
Wing tip
A wing tip is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft.Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of shapes, including:* Squared-off...
stall is unlikely to occur symmetrically, especially if the aircraft is maneuvering. As an aircraft turns, the wing tip on the inside of the turn is moving more slowly and is most likely to stall. As an aircraft rolls, the down-going wing tip is at higher incidence and is most likely to stall. When one wing tip stalls it leads to wing drop, a rapid rolling motion. Also, roll
Flight dynamics
Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...
control may be reduced if the airflow over 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 is disrupted by the stall, reducing their effectiveness. Thus, a pilot
Aviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
may be unable to maintain control, perhaps leading to an unrecoverable spin
Spin (flight)
In aviation, a spin is an aggravated stall resulting in autorotation about the spin axis wherein the aircraft follows a corkscrew downward path. Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw...
.
On aircraft with 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, wing tip
Wing tip
A wing tip is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft.Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of shapes, including:* Squared-off...
stall also produces an undesirable nose-up pitching moment
Pitching moment
In aerodynamics, the pitching moment on an airfoil is the moment produced by the aerodynamic force on the airfoil if that aerodynamic force is considered to be applied, not at the center of pressure, but at the aerodynamic center of the airfoil...
which hampers recovery from the stall.
Washout may be accomplished by other means e.g. modified aerofoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
section, vortex generator
Vortex generator
A vortex generator is an aerodynamic surface, consisting of a small vane or bump that creates a vortex. Vortex generators can be found on many devices, but the term is most often used in aircraft design....
s, leading edge 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...
s, notches, or stall strips
Stall strips
A stall strip is a fixed aerodynamic device employed on fixed-wing aircraft to modify the airfoil used. They are usually factory-installed or, on rarer occasion, an after-market modification. Stall strips are almost always employed in pairs, symmetrically on both wings...
. This is referred to as aerodynamic washout. Its purpose is to tailor the spanwise lift distribution or reduce the probability of wing tip
Wing tip
A wing tip is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft.Because the wing tip shape influences the size and drag of the wingtip vortices, tip design has produced a diversity of shapes, including:* Squared-off...
stall.
Winglets have the opposite effect to washout. Winglets promote a greater proportion of lift being generated near the wing tips. (This can be described as aerodynamic wash-in.) Winglets also promote a greater bending moment
Bending Moment
A bending moment exists in a structural element when a moment is applied to the element so that the element bends. Moments and torques are measured as a force multiplied by a distance so they have as unit newton-metres , or pound-foot or foot-pound...
at the wing root, possibly necessitating a heavier wing structure. Installation of winglets may necessitate greater aerodynamic washout in order to provide the required resistance to spinning
Spin (flight)
In aviation, a spin is an aggravated stall resulting in autorotation about the spin axis wherein the aircraft follows a corkscrew downward path. Spins can be entered intentionally or unintentionally, from any flight attitude and from practically any airspeed—all that is required is sufficient yaw...
, or to optimise the spanwise lift distribution.
External links
- http://www.allstar.fiu.edu/aero/Wing32.htm
- http://www.fly-imaa.org/imaa/hfarticles/const/v1-4-10.html