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# Dihedral (aeronautics)

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 Title: Dihedral (aeronautics) Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

### Dihedral (aeronautics)

The upward tilt of the wings and tailplane of an aircraft, as seen on this Boeing 737, is called dihedral angle

Dihedral angle is the upward angle from horizontal of the wings or tailplane of a fixed-wing aircraft. "Anhedral angle" is the name given to negative dihedral angle, that is, when there is a downward angle from horizontal of the wings or tailplane of a fixed-wing aircraft.

Schematic of dihedral and anhedral angle of an aircraft wing.

Dihedral angle (or anhedral angle) has a strong influence on dihedral effect, which is named after it. Dihedral effect is the amount of roll moment produced per degree (or radian) of sideslip. Dihedral effect is a critical factor in the stability of an aircraft about the roll axis (the spiral mode). It is also pertinent to the nature of an aircraft's Dutch roll oscillation and to maneuverability about the roll axis.

Measuring the dihedral angle.

Longitudinal dihedral is a comparatively obscure term related to the pitch axis of an airplane. It is the angle between the zero lift axis of the wing and horizontal tail. Longitudinal dihedral can influence the nature of controllability about the pitch axis and the nature of an aircraft's phugoid-mode oscillation.

When the term "dihedral" (of an aircraft) is used by itself it is usually intended to mean "dihedral angle". However, context may otherwise indicate that "dihedral effect" is the intended meaning.

## Contents

• Dihedral angle and dihedral effect 1
• Longitudinal dihedral 2
• History 3
• Uses of dihedral angle and dihedral effect 4
• Aircraft stability analysis 4.1
• Provision of stability 4.2
• Wing clearance 4.3
• Using dihedral angle to adjust dihedral effect 5
• Common confusions 6
• How dihedral angle creates dihedral effect and stabilizes the spiral mode 7
• How dihedral angle creates rolling moment from sideslip (dihedral effect) 7.1
• How dihedral effect stabilizes the spiral mode 7.2
• Other factors contributing to dihedral effect 8
• Sweepback 8.1
• Vertical position of the center of mass 8.2
• Effects of too much dihedral effect 9
• Anhedral and polyhedral 10
• Anhedral 10.1
• Polyhedral 10.2
• References 11
• Notes 11.1
• Footnotes 11.2

## Dihedral angle and dihedral effect

Dihedral angle is the upward angle from horizontal of the wings of a fixed-wing aircraft, or of any paired nominally-horizontal surfaces on any aircraft. The term can also apply to the wings of a bird. Dihedral angle is also used in some types of kites such as box kites. Wings with more than one angle change along the full span are said to be polyhedral.

Dihedral angle has important stabilizing effects on flying bodies because it has a strong influence on the dihedral effect.

Dihedral effect[1] of an aircraft is a rolling moment resulting from the vehicle having a non-zero angle of sideslip. Increasing the dihedral angle of an aircraft increases the dihedral effect on it. However, many other aircraft parameters also have a strong influence on dihedral effect. Some of these important factors are: wing sweep, vertical center of gravity, and the height and size of anything on an aircraft that changes its sidewards force as sideslip changes.

## Longitudinal dihedral

Dihedral angle on an aircraft almost always implies the angle between two paired surfaces, one on each side of the aircraft. Even then, it is almost always between the left and right wings. However, mathematically dihedral means the angle between any two planes. So, in aeronautics, in one case, the term "dihedral" is applied to mean the difference in angles between two front-to-back surfaces:

Longitudinal dihedral is the difference between the angle of incidence of the wing root chord and angle of incidence of the horizontal tail root chord.

Longitudinal dihedral can also mean the angle between the zero lift axis of the wing and tail instead of between the root chords of the two surfaces. This is the more meaningful usage because the directions of zero-lift are pertinent to longitudinal trim and stability while the directions of the root chords are not.

## History

In geometry, dihedral angle is the angle between two planes. Aviation usage differs slightly from usage in geometry. In aviation, the usage "dihedral" evolved to mean the positive, up angle between the left and right wings, while usage with the prefix "an-" (as in "anhedral") evolved to mean the negative, down angle between the wings.

The aerodynamic stabilizing qualities of a dihedral angle were described in an influential 1810 article by

• http://www.aeroexperiments.org/washoutbillow.shtml