Certified Flight Instructor - Flight Instructor Airplane Practice Exam

The primary purpose of an aircraft's wing dihedral angle is to increase lateral stability. When wings are inclined upward from the horizontal plane, this design aids in the stabilization of the aircraft during flight.

The dihedral angle creates a situation where, during a roll, the wing that is lower to the ground experiences a greater angle of attack than the wing that is higher. This differential in lift causes the aircraft to naturally roll back toward a level attitude, thereby enhancing the aircraft's lateral stability. The inherent design encourages the aircraft to maintain balanced flight, making it an essential feature in aircraft design for improving handling characteristics and safety.

The other options relate to different aspects of flight characteristics. Increasing longitudinal stability, for example, generally involves design elements such as tail volume and the tail’s moment arm rather than wing dihedral. While reducing drag and increasing lift coefficient are vital factors in overall aircraft performance, these attributes primarily relate to the airfoil design of the wing and its shape rather than the angle itself. Therefore, the correct emphasis on dihedral angle in this context lies in its role in enhancing lateral stability.