What produces lateral stability in an airplane?

Answer 1

Lateral stability is the measurement of an aircraft's stability around the roll axis of the aircraft. The roll axis is an imaginary line running from the nose of the aircraft through the tail, and is the axis around which you rotate when you "rock your wings" or when you initiate a roll for a turn. Aircraft dihedral is a measurement of how much the wings are angled upward (positive dihedral) or angled downward (negative dihedral) from where the wing attaches to the fuselage. On wings with zero dihedral in level flight, lift forces on each wing point directly up to oppose gravity. If the aircraft rolls to one side, then both wing lift forces shift to one side of vertical and the lift component reduces equally on both wings, therefore, there is no tendency to roll the aircraft into a steeper bank or back to level flight. This is called neutral stability. While the aircraft is rolled or banked to one side, the resultant components of lift from the wings result in the lift component opposing gravity being reduced and the horizontal component increasing. The aircraft will begin descending and will turn in the direction the aircraft is banked. To assist an aircraft to return to level flight with little pilot input, designers use positive dihedral; both wings angle upward so that the wing tips are "higher" than where the wing attaches to the fuselage. In this case, during level flight, both wing lift forces point slightly inward toward the fuselage. With positive dihedral when the aircraft rolls, as one wing lowers and becomes parallel with the ground, the vertical component of lift points straight up from the ground and fully opposes gravity. As the raised wing tilts further away from being parallel with the ground, the vertical component of lift is further offset from vertical and results in a reduced vertical lift component. With reduced lift on the raised wing, it descends until level and the lift components of both wings are again equal. This is called positive stability. When an aircraft is built with negative dihedral, the lift forces tend to increase the roll. This is called negative stability and can make an aircraft difficult for a pilot to control.