Actually, the airflow does speed up and "accelerate" in terms of both velocity and direction, in the compressor section. It is in the diffuser section that airflow slows and pressure rises, prepatory to introduction to the burner cans.
Ram air rise begins the moment the aircraft begins moving down the runway, and hits an equal point for most engines about 70 KIAS. Above that speed, ram air increases inlet pressure such that air is being "crammed down it's throat" instead of it "sucking." In reality, both are happening, but forward speed is creating an increase in inlet air pressure well above ambient from about 70 KIAS in most aircraft engine installations, on up.
"Ambient" for most engines isn't the outside pressure, as we would normally think of the term ambient. Turbine engines work by doing one of two (or both) things...producing torque to move things, and/or producing thrust. In either case, many engines utilize pressure probes as part of the fuel metering system or fuel control unit which make comparisons between inlet air pressure and air pressure in other parts of the engine. These are referred to as pressure stations. A few, but not many, aircraft also referece true ambient air pressure, though it's meaningless for most installations. A few use a separate external pitot tube to sample undisturbed (relative) airflow, but these also account for ram air rise, and the "ambient" that the engine sees isn't what we'd see from an independent gauge outside the aircraft somewhere. It's a relative term, and ambient to a jet engine can be considered the pressure inside the inlet prior to introduction to the first stages of the compressor. In some jet engines using EPR, this is the first element of the pressure ratio, or comparison between what the engine is taking in and what it's putting out. In short, it's the basis for the engine knowing how much power it's producing, and how to regulate it.
Some stator vanes are variable, but most are fixed.
Ram air rise begins the moment the aircraft begins moving down the runway, and hits an equal point for most engines about 70 KIAS. Above that speed, ram air increases inlet pressure such that air is being "crammed down it's throat" instead of it "sucking." In reality, both are happening, but forward speed is creating an increase in inlet air pressure well above ambient from about 70 KIAS in most aircraft engine installations, on up.
"Ambient" for most engines isn't the outside pressure, as we would normally think of the term ambient. Turbine engines work by doing one of two (or both) things...producing torque to move things, and/or producing thrust. In either case, many engines utilize pressure probes as part of the fuel metering system or fuel control unit which make comparisons between inlet air pressure and air pressure in other parts of the engine. These are referred to as pressure stations. A few, but not many, aircraft also referece true ambient air pressure, though it's meaningless for most installations. A few use a separate external pitot tube to sample undisturbed (relative) airflow, but these also account for ram air rise, and the "ambient" that the engine sees isn't what we'd see from an independent gauge outside the aircraft somewhere. It's a relative term, and ambient to a jet engine can be considered the pressure inside the inlet prior to introduction to the first stages of the compressor. In some jet engines using EPR, this is the first element of the pressure ratio, or comparison between what the engine is taking in and what it's putting out. In short, it's the basis for the engine knowing how much power it's producing, and how to regulate it.
Some stator vanes are variable, but most are fixed.
