Dihedral is used to stabilize the airplane about the _______.

Dihedral refers to the upward angle of an aircraft's wings relative to the horizontal plane. This design feature plays a crucial role in enhancing the aircraft's stability, particularly about the longitudinal axis, which runs from the nose to the tail of the airplane. When an aircraft with dihedral wings experiences a roll due to turbulence or external forces, the wing that is lower will generate more lift than the wing that is higher. This differential lift causes the aircraft to naturally return to level flight, effectively stabilizing it along the longitudinal axis.

Stability about the longitudinal axis is essential for maintaining controlled flight and aiding pilot handling. In contrast, other axes of an aircraft, such as the vertical and lateral axes, relate to different aspects of stability and control, which dihedral design does not directly address.