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The four forces acting on a UAS (Unmanned Aerial System) are thrust, drag, lift, and weight. These forces are said to be in equilibrium when they balance each other out, resulting in no net change in the aircraft's speed or direction.

Un-accelerated flight is characterized by a steady altitude and consistent velocity, meaning that the lift generated by the wings is equal to the weight of the UAS, and the thrust produced by the propulsion system is equal to the drag acting against it. In this state, the forces are balanced; there is neither an increase nor a decrease in speed or altitude.

In scenarios like steep climbs, descents, or aggressive maneuvers, the UAS experiences acceleration, which means that the forces are not in equilibrium. For instance, during climbs, lift must exceed weight, and during descents, weight exceeds lift. In aggressive maneuvers, thrust may exceed drag, causing changes in speed and direction. Hence, these conditions do not represent a state of equilibrium.