What is the main consequence of rivets sticking out above the surface of the airplane?

Rivets that stick out above the surface of an airplane can significantly interfere with the aerodynamics of the aircraft. In aerodynamics, maintaining a smooth surface is essential for minimizing drag, which is the resistance an aircraft encounters while moving through the air. Any protrusions, like sticking rivets, disrupt the smooth airflow around the aircraft, potentially leading to increased drag and reduced fuel efficiency. This disruption can also adversely affect the overall performance of the aircraft during flight, impacting stability and control.

In contrast, while there may be aesthetic considerations, structural integrity, and paint adhesion aspects to consider, these factors do not directly address the important aerodynamic implications of rivets that extend above the surface. Therefore, the principal consequence of such rivets is their detrimental effect on aerodynamic performance.