Consider an aeroplane in steady flight. To an observer on the ground the aeroplane is flying into air substantially at rest, assuming no wind, and any movement of the air is caused directly by the motion of the aeroplane through it. The pilot of the aeroplane, on the other hand, could consider that he is stationary, and that a stream of air is flowing past him and that the aeroplane modifies the motion of the air. In fact both
viewpoints are mathematically and physically correct. Both observers may use the same equations to study the mutual effects of the air and the aeroplane and they will both arrive at the same answers for, say, the forces exerted by the air on the aeroplane. However, the pilot will find that certain terms in the equations become, from his viewpoint, zero. He will, therefore, find that his equations are easier to solve than will the ground-based observer. Because of this it is convenient to regard most problems in aerodynamics as cases of air flowing past a body at rest, with consequent simplification of the mathematics.
Types of flow
The flow round a body may be steady or unsteady. A steady flow is one in which the flow parameters, e. g. speed, direction, pressure, may vary from point to point in the flow but at any point are constant with respect to time, i. e. measurements of the flow parameters at a given point in the flow at various times remain the same. In an unsteady flow the flow parameters at any point vary with time.