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In aeronautics, dihedral is the angle between the left and right wings (or tail surfaces) of an aircraft. "Dihedral" is also used to describe the effect of sideslip on the rolling of the aircraft. 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. Dihedral angle has a strong influence on dihedral effect, which is named after it. Dihedral effect is the amount of roll moment produced in proportion to the amount 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. 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 the zero-lift axis of the 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.
References
- Roskam, Jan (1979). "4.1.7". Airplane Flight Dynamics and Automatic Flight Controls. 1. Ottawa, Kansas: Roskam Aviation and Engineering Corporation. pp. 139. Library of Congress Catalog Card Number: 78-31382
- "This angular form, with the apex downward, is the chief basis of stability in aerial navigation . . . and this most effectively prevents any rolling of the machine from side to side." George Cayley. On Aerial Navigation. (part II). Journal of Natural Philosophy, Chemistry, and the Arts., vol. 25 (Feb, 1810), pp. 81-87. As reprinted in Gibbs-Smith, Charles H. Sir George Cayley's Aeronautics, 1796-1855. HMSO. 1962. page 223 has the quote. Online at NASA (pdf) http://www.aeronautics.nasa.gov/fap/OnAerialNavigationPt2.pdf
- Pronounced "See-ell-beta".
- A rolling moment coefficient is a "normalization" of the rolling moment. Rolling moment has units of force times length. The rolling moment coefficient is normalized so it has no units. This is done by dividing the moment by wing area and by wing span and by dynamic pressure.
- "Roll Stability" is an ambiguous term requiring context to discern the intended meaning of the user. It usually means "Spiral Mode Stability", but it is also often misused to mean dihedral effect or dihedral angle, both of which are not "stability" themselves, though they contribute to spiral mode stability.
- In the spiral mode, if it is unstable, the aircraft will slowly, then more rapidly, diverge from "nominal wings-level" if the pilot makes no control inputs. If the spiral mode is stable and the pilot makes no inputs, when the aircraft starts from a banked attitude, it will return close to wings-level by itself.
- Etkin, Bernard; Dynamics of Flight ;Section 3.10; 1982; ISBN:0-471-08936-2
- The "pendulum effect" is also less commonly called the "keel effect".
- The downward curve of a paraglider wing could be termed "continuous polyanhedral".
- Donald, David and Jon Lake, eds. McDonnell F-4 Phantom: Spirit in the Skies. London: AIRtime Publishing, 2002. ISBN:1-880588-31-5.
