When air is flowing at subsonic speed, it acts like an incompressible fluid. As discussed earlier in this chapter, when air at subsonic speed flows through a diverging shaped passage, the velocity decreases and the static pressure rises, but the density of the air does not change. In a converging shaped passage, subsonic air speeds up and its static pressure decreases. When supersonic air flows through a converging passage, its velocity decreases and its pressure and density both increase. [Figure 3-74] At supersonic flow, air acts like a compressible fluid. Because air behaves differently when flowing at supersonic velocity, airplanes that fly supersonic must have wings with a different shape.
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