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As I have learnt from the Venturi tube that Pressure is inversely proportional to velocity. When a general aircraft is flying at say Mach 0.8 and at the same time there is a pressure decrease with increasing altitude. How is pressure achieved in the case. Is there stagnation pressure in play here?

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I will answer the question in the body of the post.

The venturi effect is not applicable here. The air intake for a small plane with a piston engine is usually exposed head-on to the incoming air and also to the backwash from the propeller. Both of these add ram air pressure to the carburetor inlet.

In fact, it is possible to magnify this effect and get a few inches of manifold boost by positioning an air scoop on the inlet which is very close to the trailing edge of the propeller in front of it, and then mounting the propeller so that the blade passes across the scoop mouth coincident with the opening of the intake valves on the engine.

Regarding turbine engines, the air inlet to the compressor stage is sized so that at cruise conditions, the velocity of the incoming air is the same as that of the air passing through the first stage of the compressor fan so there is no spillage of excess air nor suction pressure inside the inlet duct.

In this case, the fan and compressor stages of the engine perform the compression work to support the thermodynamic cycle of the engine, and ram air pressure recovery is not important.

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