A definition of anhedral
This page was last modified on September 13, 2006
Here is the definition of anhedral that we'll use in these tutorial pages: a pair of matched aerodynamic surfaces has an anhedral geometry if, in the presence of a sideways component in the relative wind, the "upwind" or "leading" surface experiences a lower (less positive or more negative) angle-of-attack than the "downwind" or "trailing" surface.
To keep things simple, in this particular application we'll usually define the angle-of-attack in relation to the relative wind, which is equal and opposite to the direction of the flight path, rather than defining the angle-of-attack in relation to the more localized direction of the airflow at the particular surface in question.
This definition of anhedral is basically compatible with, but not exactly the same as, a more "conventional" definition of anhedral, which holds that a pair of matched aerodynamic surfaces have an anhedral geometry if the tips of the surfaces are lower than the roots of the surfaces when the aircraft is a positioned in a "level" attitude, e.g. with the mean chord line of the wing parallel to the horizon.
We'll also be cognizant of the fact that a sideways airflow will create much more roll torque when interacting with an anhedral wing panel that is located far from the aircraft's CG, i.e. near the wingtip, than when interacting with an anhedral wing panel that is located much further inboard, i.e. near the wing root. In recognition of this, if we make a modification to a wing that decreases the anhedral of the inboard panels and increases the anhedral of the outboard panels, in a way that doesn't change the position of the wingtips in relation to the wing roots but does result in a net increase in the amount of roll torque created by a sideways airflow, we'll say that we've increased the wing's anhedral geometry.
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