The question really has to be centered on the pressure at the engine inlet face. There is no suction involved in its function in a cruise flight condition. In this way, the inlet performs part of the compression process. In flight, the purpose of the inlet is to slow the freestream. ![]() The reality for an airplane in flight is that nothing could be further from what actually happens. 13.1.įull size image 13.1.1 An Old Wives’ Tale About Inletsĭescriptions of how a jet engine works are sometimes said to involve the notion that the engine “sucks” itself along to propel the airplane. The two stagnation streamlines are shown in Fig. At cruise speed the stagnation line is almost coincident with the foremost highlight of the nacelle, its leading edge if you will. The rounded lip allows the entering air flow entry to be relatively uniform. The inlet must manage inlet flow to be uniform in low-speed flight, specifically at high angle of attack or with a cross wind on the runway. The roundedness on the inlet parallels the design of a wing at its leading edge. Near a nacelle’s maximum width, the curious person will find a red line and words to the effect: “don’t stand forward of this line during engine runups on the ground, or else”! The point is to warn persons not to participate in the suction exercised by the engine when it is running on the ground. ![]() At low speed, the stagnation point (line) on the inlet air flow is located on the outside of the nacelle. That same thinking also applies to the inlet that has to handle air nicely in flight and on the ground where the airplane is almost stationary. Recall that a nice rounded leading edge on the wing is desirable to allow for operation over a wide range of angles of attack. One aspect of an engine installation design is reflective of the comments made on the wing design. We return to vortices to see where they play important roles in the operation of a jet engine.
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