Eli5: Jet engines

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Do a jet engine’s turbines actively pull air into the engine once the turbines begin to spin? (Both during the starter motor phase and also during flight at higher speeds)

And does the front half of the engine produce a substantial amount of drag as it is travelling at high speeds? (not including the thrust coming from the back)

If anyone could help it would be much appreciated!

*I’m thinking about axial and turbofan jet engines, but am curious about all types, *apart from pulse jets*

Cheers!

In: 3

the only jet engine that doesn’t actively pull air through the inlet would be a ramjet.

the others have compressor stages and spin faster when the aircraft attains higher speed. commercial jet engines are turbo fans where the fan acts like a propeller to push the aircraft faster.

Ever seen videos where people get sucked into the turbines?

Generally speaking, jet engines has 6 main parts; fan, compressor, combustor, turbine, mixer and nozzle. The fan pulls air in, a compressor compresses the air. Then fuel is added in the combustor. The fuel air mixture is ignited with an ignition source which forces the mixture of gasses to rapid expand and shoot out the nozzle providing thrust. As the gasses pass through the nozzle it passes over a turbine which is connected to the compressor via a shaft. Spinning the turbine spins the compressor. The nozzle may be preceded by a mixer which combines the hot air coming from the engine with the lower temperature air that was bypassed at the fan. The mixer helps to make the engine quieter.

Check out this link from nasa for more information on each type of jet engine.
https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/engines.html

Yes. There is loads of research being done to minimise the drag effect of jet engines.

Typically a turbofan on modern jets will be a high
-bypass ratio engine, meaning a high ratio of the air sucked into it won’t go through the engine core.

I can’t recall offhand but the CFM-56 is around 80% bypass, meaning only a small fraction of the air sucked in actually gets “used” by the engine. Most of it goes around the core, having been propelled by the intake fan and spat out the back as thrust.

The core of the engine is there to power the large fan. While there is of course some thrust from the expanding heated gasses of the “jet” the majority of the thrust is just the air. Think super efficient, self contained propeller. Not quite but the idea is solid enough.

>Do a jet engine’s turbines actively pull air into the engine once the turbines begin to spin?

Yes, there are alternating stages of rotating compressor blades and fixed stator blades, which increase pressure. This high pressure air is then heated further by burning fuel, expanded through nozzles to turn that heat and pressure back into speed, and then that speed is captured by the spinning blades in the turbine, turning it into mechanical energy that rotates the compressor and fan.

>And does the front half of the engine produce a substantial amount of drag as it is travelling at high speeds? (not including the thrust coming from the back)

Not as much as you’d expect. Most of the thrust comes from the fan blades forcing air through the bypass duct. When you look at an engine from the front you see a wall of blades, but the air flowing through the engine sees those blades edge on. Though it is a lot of drag if the engines aren’t running.