Even when an aircraft is in straight level flight, it is not horizontal. There is an "angle of attack" between the longitudinal axis (the gangway) of the aircraft and the airflow, so the leading edge of each wing is higher than the trailing edge. This means that the air is deflected downwards -resulting in a reactive force with an upward component (lift) and a backward component (drag). Forward motion is needed to generate lift. The faster the aircraft is travelling, the smaller the angle of attack needed to generate the lift which is required to counter the aircraft's weight. As the aircraft slows down, the required angle of attack must increase to maintain lift. But increasing the angle of attack also increases drag, which slows the aircraft further, reducing lift. The stall speed is the speed at which increasing the angle of attack can no longer be used to generate lift, so the aircraft descends.
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