What must occur for a UA to maintain equilibrium in flight?

For an unmanned aircraft (UA) to maintain equilibrium in flight, it is essential that the forces acting on it are balanced. In this case, lift and weight are pivotal forces. When lift is equal to weight, the UA is able to maintain a stable altitude. If lift exceeds weight, the UA will ascend, and if weight exceeds lift, it will descend.

While drag and thrust are also critical components in the overall aerodynamics, they specifically relate to the UA's horizontal motion rather than its vertical stability. For steady horizontal flight, thrust must equal drag to ensure that there is no acceleration in the forward or backward direction. However, to achieve equilibrium in the context of vertical flight and overall stability, the key factor is that lift must equal weight.

Therefore, the correct understanding aligns with the principle that for a UA to maintain equilibrium, lift must equal weight. This ensures that the aircraft can sustain its flight without ascending or descending undesirably. The focus on thrust equaling drag pertains more to maintaining a constant speed or direction rather than vertical flight stability, which is where lift and weight play their crucial roles.