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Dihedral

 
Dihedral

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Dihedral



 
 


Dihedral is the upward angle from horizontal of the wings or tail pane of a fixed-wing aircraft
Fixed-wing aircraft

A fixed-wing aircraft is an aircraft capable of heavier-than-air flight whose Lift is generated not by wing motion relative to the aircraft, but by forward motion through the air....
 or the wing of a bird
Bird

Birds are wing, Bipedalismal, endothermic , vertebrate animals that lay egg . There are around 10,000 living species, making them the most numerous tetrapod vertebrates....
. Dihedral is also used in some types of kites such as box kites. The aerodynamic stabilizing qualities of the dihedral were first described by Sir George Cayley in 1808/09.






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Dihedral


Dihedral is the upward angle from horizontal of the wings or tail pane of a fixed-wing aircraft
Fixed-wing aircraft

A fixed-wing aircraft is an aircraft capable of heavier-than-air flight whose Lift is generated not by wing motion relative to the aircraft, but by forward motion through the air....
 or the wing of a bird
Bird

Birds are wing, Bipedalismal, endothermic , vertebrate animals that lay egg . There are around 10,000 living species, making them the most numerous tetrapod vertebrates....
. Dihedral is also used in some types of kites such as box kites. The aerodynamic stabilizing qualities of the dihedral were first described by Sir George Cayley in 1808/09. Downward angled wings have negative dihedral, or anhedral. Wings with local dihedral angles that change along the span are polyhedral.

The purpose of positive dihedral is to confer longitudinal static stability
Longitudinal static stability

Longitudinal static stability is important in determining whether an aircraft will be able to fly as intended. ...
 to the roll axis. Most aircraft in the civilian or transport sector use dihedral for roll stability. The dihedral angle is usually greater on low-wing aircraft, compared to an otherwise similar high-wing aircraft.

Simple explanation

Sk Dihedral 2>
Sk Dihedral 3


If a disturbance causes an aircraft to roll away from its normal wings-level position, the aircraft will sideslip in the direction of the down-going wing (see fig. 1). This creates an airflow component along the length of the wing from tip to root called the relative wind. The dihedral angle can be seen as presenting a positive angle of attack
Angle of attack

Angle of attack is a term used in aerodynamics to describe the angle between the chord of an airfoil and the vector representing the relative motion between the airfoil and the air....
 to this lateral flow, hence generating some additional lift. It is this lift which restores the aircraft to its normal attitude (fig. 2).

Anhedral


Military fighter aircraft
Fighter aircraft

A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets by dropping bombs....
 often have near zero dihedral, or anhedral. This reduces inherent stability, but increases maneuverability. Many modern military aircraft have relaxed stability
Relaxed stability

In aviation, relaxed stability is the tendency of an aircraft to change its Aircraft attitude and angle of bank on its own accord. An aircraft with relaxed stability will oscillate in simple harmonic motion around a particular attitude at an increasing amplitude....
, and require continuous corrections made by on-board computers.

Anhedral is also seen on aircraft with a high mounted wing, such as the BAe 146
BAe 146

The BAe 146 is a medium-sized commercial aircraft which was manufactured in the United Kingdom by British Aerospace . Production ran from 1983 until 2002....
 and Lockheed Galaxy. In such designs, the high mounted wing is above the center of mass
Center of mass

The center of mass of a system of wiktionary:Particles is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated....
 which confers roll stability due to the pendulum
Pendulum

A pendulum is a weight suspended from a pivot so it can swing freely.When a pendulum is displaced from its resting Mechanical equilibrium, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position....
 effect also called the Keel effect
Keel effect

In AviationThe result of the Lift being above the center of gravity in high wing aircraft. Results in a tendency for the aircraft to return to level flight when the aircraft is put into a bank....
, so additional dihedral is not required. In fact, such designs can be excessively stable, so the anhedral is added to cancel out some of the roll stability to ensure that the aircraft can be easily maneuvered.

Roll coupling

A side effect of dihedral can be roll coupling, a tendency for an aircraft to dutch roll
Dutch roll

Dutch roll is a type of aircraft motion, consisting of an out-of-Phase combination of "tail-wagging" and rocking from side to side. This yaw-roll coupling is one of the basic flight dynamics modes ....
. This is unpleasant to experience, and can lead to loss of control or can overstress an aircraft. A certain amount of anhedral can combat this effect.

Sweepback

Wing sweepback
Swept wing

A swept-wing is a wing planform common on jet aircraft capable of near-sonic or supersonic speeds. The wings are swept back instead of being set at right angles to the fuselage which was common on propeller.driven aircraft and early jets....
 also increases roll stability. This is another reason for anhedral configuration on military aircraft with high sweep angle, as well as on some airliners, even on low-wing aircraft such as Tu-134 and Tu-154.

Polyhedral


Most aircraft have been designed with planar wings with simple dihedral (or anhedral). Some pre-World War II
World War II

World War II, or the Second World War , was a global military conflict which involved a Participants in World War II, including all of the great powers, organised into two opposing military alliances: the Allies of World War II and the Axis powers....
 aircraft had gull wing
Gull wing

The gull wing is an :Category:Aircraft configurations with a prominent bend in the wing somewhere along the Wingspan, generally near the wing root....
s bent near the root. Modern polyhedral wing designs generally cant upwards near the wingtips, increasing effective dihedral angle. Winglets are a special case of polyhedral.

Polyhedral is seen on glider
Glider

Heavier-than-air unpowered aircraft do not need propulsion once airborne. Gliders, balloons and kites are unpowered aircraft.Gliders such as gliders, hang gliders and paragliders gain their initial flying speed from some launch mechanism, and then gain additional energy from gravity and from updrafts such as thermal currents....
s and some other aircraft. The McDonnell Douglas F-4 Phantom II
F-4 Phantom II

The McDonnell Douglas F-4 Phantom II is a two-seat, twin-engined, all-weather, long-range supersonic interceptor jet fighter/fighter-bomber originally developed for the United States Navy by McDonnell Aircraft....
 is one such example, unique among jet fighters for having dihedral wingtips. This was added after prototype flight testing (the original prototype of the F-4 had a flat wing) showed the need to correct some unanticipated roll instability - angling the wingtips, which were already designed to fold up for carrier operations, was a far more practical solution than re-engineering the entire wing.

History/etymology

Di-hedral, from Latin, from Greek; akin to Old English twi- di- "double" + New Latin -hedron: having (such) a surface or (so many such) surfaces

Dihedral angle – angle between two planes. This is the unsigned angle between two surfaces and makes the most sense when applied to planar surfaces. Note that the prefix “di-“ does not imply up or down; acute or obtuse angles. It is simply the measured angle between two planes. To assign direction the typical prefix of “an-“ from Greek anodos way up, from ana- or “cat-“ from Greek kathodos way down, from kata- can be applied to the root “hedral”. However, in aviation the prefix “di-“ has incorrectly evolved to mean the positive, up angle between the left and right WRP (Wing Reference Planes). While the prefix “an-“ has likewise evolved to mean the negative, down angle between the WRP.