When the UAS is flying straight and level, what must the airfoil's lift equal?

In straight and level flight, the concept of equilibrium is crucial. The lift generated by an airfoil must exactly balance the weight of the UAS to maintain a constant altitude. Therefore, the correct answer indicates that lift must equal the weight plus any downforce acting on the aircraft.

Under certain conditions, such as when the UAS experiences additional forces leading to a downforce – like an angle of attack that creates a downward pressure due to airflow around the wings or external forces – the total lift must reflect not just the weight of the UAS but also counteract this additional downforce. Thus, for equilibrium in a straight and level flight scenario, the lift must equal the combined weights: the actual weight of the UAS and any downforce acting on it.

Options that suggest lift should equal just the UAS's weight overlook the impact of additional aerodynamic forces that may be acting upon the UAS, which can be significant in various flight configurations. Hence, the requirement for lift is to be greater than just the weight alone, ensuring a balance against any downforce.