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
Ever seen videos where people get sucked into the turbines?
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.
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.
Aircraft mechanic here:
Drag: The engine sucks air from the front, faster than the air is hitting it, let’s say. So it’s not making drag, it’s actually pulling itself forward like a propeller. What makes drag is the nacelle but you can’t really do otherwise. The nacelle is basically the best way to cover the engine, protect the engine and streamline the airflow to the engine and around it. Technically we can say it’s drag, but it’s drag you can’t do without. If you shut down an engine in flight, the entire thing will make some drag, not little, not the end of the world.
Pulling air: yes, it pulls water, tarmac, rabbits, bolts, shoes, people, and whatever you leave on the floor into it. Definitely sucking stuff.
Turbines: no, the turbine is a windmill that is moved by the hot gases provided by the combustion chamber. The combustion chamber is a glorified kerosene stove. The compressor is the one that sucks the air. The Fan is the first stage of the compressor in a turbofan and does suck a lot of air. A fraction of the fan air will then get sucked into the next compressor stages while a lot of this air is just shot out of the back of the engine and it’s what gives you a big chunk of the engine thrust.
Any combustion engine works on 4 basic principles. Suck, squeeze, bang, blow.
Suck: air is drawn into the engine, In this case the first stages, also called fan stages.
Squeeze: through multiple stages the air is compressed using rotor and stator blades as the passage gets smaller.
Bang: in the combustion chamber air and fuel are mixed in a heat resistant basket looking thing that is coated in a ceramic dip and has a bunch of holes to let the air pass through. The heat of the combustion keeps the reaction going continuously, and a bunch of sensors monitor the air/fuel mixture, temp, load, rps, etc.
Blow: the exhaust goes out the combustion chamber through the turbine stages. Those turbine stages act as the driving force for the fan and compressor stages (the rotor can be hollow and in 2 separate pieces allowing the fan and compressor to spin at different speeds). The turbine uses the high enthalpy (a form of molecular kinetic energy) of the exhaust gasses to make them spin. Then lastly the combustion air is then directed out the exhaust, depending on how the jet engine is used, that can be as thrust, or just let out as most of the energy was converted into the rotational kinetic energy of the rotor.
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