Only the Harrier hovers, so only the Harrier needs such pronounced anhedral.
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.
“Dihedral aids stability in the roll axis.
“But military aircraft don’t want much inherent stability, if any. They need fast response to the pilot’s commands, so neutral or slight instability is preferable there.”
Actually, military fighter aircraft often have near zero or even anhedral angle reducing dihedral effect and hence reducing the stability of the spiral mode. This increases maneuverability which is desirable in fighter-type aircraft.
“The Harrier has probably the most pronounced anhedral among military jets. This increases manoeuvrability as before, but it’s also there for two other reasons – it has a high-mounted wing so all the weight is below the centre of roll, conferring inherent stability (the so-called “pendulum” effect), but also, the anhedral helps to contain the downward jet efflux in hover, helping to maintain stability when hovering. Only the Harrier hovers, so only the Harrier needs such pronounced anhedral. This design also forces the unusual ventral undercarriage plus outriggers design.”
Check out this impressive video featuring a US Marine Corps (USMC) AV-8B Harrier II flying backwards, turning, and hovering at Oshkosh 2011.
“However, apart from the Harrier, most military jets only have a small degree of anhedral.
“The other class of aircraft that have pronounced anhedral are large transport planes with a high-mounted wing. These aircraft are very stable in the roll axis anyway, having all the weight suspended below the centre of roll (cargo, fuselage, engines, all hang down). The anhedral cancels most of this excess stability, leaving the aircraft able to turn and bank normally.”
Photo credit: U.S. Marine Corps