Dihedral

[ClipperLuna]

I could add here that if roll rate is an issue as defined by an aircraft's mission, anhedral can be used which works to increase roll rate by biasing the lift vectors outward instead of inward.
Dudley Henriques

So this would be why, for example, something like an F-18 would have anhedral?

[DHenriques_]

I could add here that if roll rate is an issue as defined by an aircraft's mission, anhedral can be used which works to increase roll rate by biasing the lift vectors outward instead of inward.
Dudley Henriques

So this would be why, for example, something like an F-18 would have anhedral?

There are various engineering considerations concerning anhedral. Generally it is used to "help" the aircraft's roll rate. In a fighter with a design that favors high inertia in roll anhedral will help cancel out the negatives concerning maneuverability. The roll rate improvement is prime on the Hornet.
Sometimes anhedral is used for other reasons as well. The F4 for example needed BOTH dihedral AND anhedral for two entirely different reasons. McDonnell realized late in the production run that some dihedral was needed on the wings of the F4. Rather than redesign the entire wing structure which meant a total redesign of the landing gear, they simply bypassed the main wing and introduced dihedral to the wingtips. Thus the bent wing on the Phantom.
They also noticed a flow issue at specific angles of attack on the tail section on landings. The answer to that was to anhedral the tail taking it out of the turbulent flow. Thus the inverted V stabilizer on the Phantom.
Dudley Henriques

[AKar]

So this would be why, for example, something like an F-18 would have anhedral?

Just to add something, one more specific consideration in case of Hornet is having various underwing stores. That's why the anhedral angle is in fact very minimal. Depending on what you are to hang down there and where you need to do that, it is of some benefit to retain some ground/deck clearance. With any significant anhedral, the outer pylons would get rather close to the ground. Further, you may note the aircraft has rather large trailing edge flaps and also drooping ailerons acting as additional high-lift devices. Now there is a plenty of ground clearance for those too should the aircraft roll a bit on trapping for instance.

One more thing to bring in mind is what I alluded to previously: diherdal effect (not the geometric dihedral angle, but the actual effect caused in part, but not solely by, the dihedral angle) is, by definition, the rolling moment away from the sideslip angle. Hornet, having its mass distributed like it has, is somewhat prone to 'slice it nose' due to inertial coupling of the axes in extended rolling maneuvers. With any significant dihedral/anhedral effect, you'd effectively couple further dynamics into those generated sideslip angles, which is something I understand they wanted not to do. Thus, the wing is rather neutral from dihedral/anhedral point-of-view.

-Esa