If an unmanned aircraft is in equilibrium, what does it imply about its forces?

When an unmanned aircraft is in equilibrium, it means that the aircraft is in a state where it is not accelerating, which implies that all the forces acting upon it are balanced. In this context, the two primary pairs of forces to consider are thrust and drag, along with lift and weight.

When we say that lift is equal to weight, it implies that the aircraft is not climbing or descending; it is maintaining a steady altitude. This balance ensures that the vertical forces acting on the aircraft are in equilibrium, which is crucial for stable flight.

In addition to this vertical balance, the forces acting in the horizontal plane must also be considered. For an aircraft to be in equilibrium horizontally, thrust must equal drag. This means the forward thrust generated by the aircraft's engines is perfectly balanced by the drag opposing its motion through the air, allowing the aircraft to maintain a constant speed without accelerating.

Therefore, both conditions—lift equaling weight and thrust equaling drag—are necessary for the aircraft to be in total equilibrium, which validates the inclusion of both in the correct answer. Consequently, the choice highlighting that thrust equals drag and lift equals weight more accurately captures the necessary conditions for equilibrium in flight and reflects a comprehensive understanding of how forces interact in aerial dynamics.