Dihedral  Sweepback and Wing Location  Keel Effect and Weight Distribution  5-18  Figure 5-28.  Dihedral is the upward angle of the wings from a horizontal (front/rear view) axis of the plane as shown in the graphic  depiction and the rear view of a Ryanair Boeing 737.  Figure 5-30.  Sweepback wings.  Figure 5-29.  Sideslip causing different AOA on each blade.  Some aircraft are designed so that the outer tips of the wings  are higher than the wing roots. The upward angle thus formed  by the wings is called dihedral.  [Figure 5-28]  When a gust  causes a roll, a sideslip will result. This sideslip causes the  relative wind affecting the entire airplane to be from the  direction of the slip. When the relative wind comes from the  side, the wing slipping into the wind is subject to an increase  in AOA and develops an increase in lift. The wing away  from the wind is subject to a decrease in angle of attack, and  develops a decrease in lift. The changes in lift effect a rolling  moment tending to raise the windward wing, hence dihedral  contributes to a stable roll due to sideslip.  [Figure 5-29]  Many aspects of an aircraft's configuration can affect its  effective dihedral, but two major components are wing  sweepback and the wing location with respect to the fuselage  (such as a low wing or high wing). As a rough estimation,  10° of sweepback on a wing provides about 1° of effective  dihedral, while a high wing configuration can provide about  5° of effective dihedral over a low wing configuration.  A sweptback wing is one in which the leading edge slopes  backward.  [Figure 5-30]  When a disturbance causes an  aircraft with sweepback to slip or drop a wing, the low  wing presents its leading edge at an angle that is more  perpendicular to the relative airflow. As a result, the low  wing acquires more lift, rises, and the aircraft is restored to  its original flight attitude.  A high wing aircraft always has the tendency to turn the  longitudinal axis of the aircraft into the relative wind, which is  often ref e rred to as the keel effect. These aircraft are laterally  stable simply because the wings are attached in a high  position on the fuselage, making the fuselage behave like a  keel exerting a steadying influence on the aircraft laterally  about the longitudinal axis. When a high-winged aircraft is  disturbed and one wing dips, the fuselage weight acts like a  pendulum returning the aircraft to the horizontal level.  Laterally stable aircraft are constructed so that the greater  portion of the keel area is above the CG.  [Figure 5-31]  Thus,  when the aircraft slips to one side, the combination of the  Dihedral Dihedral  Wing has decreased  AOA, hence reduced lift  due to sideslip.  Wing has increased AOA, hence  increased lift due to sideslip.  Sideslip  Restoring lift 