Moving aircraft and moving air

Before we begin our description of the generation of lift, it is necessary to establish an important fact, which is, that if air is blown at a certain speed past a stationary aircraft, as for example in a wind-tunnel, the aerodynamic forces produced are identical to those obtained when the aircraft flies through stationary air at the same speed. In other words, it is the relative speed between the air and the aircraft that matters. This is fortunate, because it is generally much easier to understand and describe what happens when air blows past a fixed object, than when a moving object flies through still air.