Final answer:
In straight and level flight, an airplane supports its weight through the lift force generated by the wings. The lift force counteracts the force of gravity and keeps the airplane in the air. Additionally, the airplane may experience drag and thrust forces that are balanced to maintain a constant speed.
Step-by-step explanation:
In straight and level flight, an airplane supports its weight as the main load. The weight of the airplane is balanced by the lift force generated by the wings. The lift force is perpendicular to the wings and acts upward, counteracting the downward force of gravity.
Additionally, in straight and level flight, the airplane may experience drag and thrust forces. Drag is the resistance the airplane faces as it moves through the air, and thrust is the force produced by the engines to propel the airplane forward. These forces are balanced so that the airplane maintains a constant speed in a straight line.