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GmeGoBrrr123 t1_irox752 wrote

I was flabbergasted by this too a few months ago. Just how does a “fan” create this much thrust?

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Aviator506 t1_iroyjdr wrote

The fan essentially functions like a propeller that's enclosed by an engine nacelle. Just instead of the fan being turned by engine cylinders like on a traditional propeller driven piston airplane, the fan is turned by the turbines in the jet engine.

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JJohnston015 t1_irp91sh wrote

It's also huge, so it has an enormous area. That means it's moving a lot of air, and thrust is a function of how much air you move, and how hard you move it.

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Thecna2 t1_irphtnw wrote

This change from Low Bypass Turbofan to High Bypass occurred in the 80s. Engines prior to this point provided all/most of their thrust from the central exhaust core.

The change occurred mainly because of fuel efficiency but it did require changes. The newer type have much larger fans than the old type and thus the engines on the wings will get closer to the ground meaning some design changes (higher wings/mounts).

The fan creates the thrust because it is very big and running very fast, like a prop. No real other reason.

I worked on the old style engines as they changed over to the new style.

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p33k4y t1_irqpdit wrote

>This change from Low Bypass Turbofan to High Bypass occurred in the 80s.

1970s:

The CFM56 also started production in 1974 but did not enter commercial service until 1982.

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[deleted] t1_irr5od3 wrote

[deleted]

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Diligent_Fact2236 t1_irrhccw wrote

Sorry, but this answer is actually the wrong one.

  1. The fan does move the air. The blades are an aerofoil shape, pulling the air in front of it in. Much like a normal propellor on a GA plane, where the propellors create a pressure difference behind itself and push the aircraft along. turbofans do exactly the same, but just with an enclosure around it
  2. Much of the air (upto 80% on some designs) goes around the compressor stages.The fan compresses all of the air (due to it's aerofoil shape pulling air in) evenly. It's only the air that goes through the core that is compressed in relation to the air around it.
  3. The air is not heated at all at this stage, and the bypass air temp is slightly raised by the slight compression at the fan stage, and again when it passes over the combustion chamber. The heat of the air comes from it's compression in the core.
  4. The exhaust velocity of the bypass air is directly controlled by how much thrust is presented to the compressor at the rear of the turbine. Say there are 3 shafts. One shaft connects the fan at the front to the low pressure turbine at the rear of the engine. The faster that the exhaust gasses from the combustion chamber spin this low pressure turbine, the faster the fan at the front spins. The other 2 shafts drive 1st/2nd stage turbines to compress some of the bypass air to direct it to the combustion chamber core

​

No doubt someone else will correct me where I am wrong, but by and large, that's the gist of how high bypass turbofan engines work

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Boomshrooom t1_irp9etq wrote

Massive blades spinning at incredibly high speeds move a shit ton of air, beyond that its just a matter of Newtons third law. If you throw a lot of stuff out of the back at high speed, you're gonna move forward.

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slater_just_slater t1_irq3c05 wrote

The fans don't actually spin very fast (compared to the compressor) fans only spin about 3000 rpm so the tips don't go supersonic. The compressor spins much faster (12,000 rpm is typical however small engines can spin much faster up to 50,000 rpm) they do this by using 2, sometimes 3 shafts in the engine (one shaft inside the other). The fan is powered by 1 or 2 turbine at the very back, the compressor is powered by separate turbines at the front of the turbine section, just ahead of the fan turbines

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Boomshrooom t1_irqiqsl wrote

I'm aware of this, my point is that the fan is still spinning at a very high speed, and moves a ridiculous amount of air. 3000rpm still gives 50 rps which is very high especially given the size of the fan.

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BobbyP27 t1_irsbfwc wrote

Typically the fan tips are supersonic at the design condition. Tip Mach numbers of about 1.3 are typical.

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Wizatek t1_irqxbdr wrote

Compared to a small consumer pc-case fan and similar, a turbofan is running at much higher Reynolds Number, which allows you to have more aggressive aerofoil profiles for generating lift. At small scales, aerofoils do not work better than flat plates (hence insects evolved thin wings), while at medium scales you need aerofoils that can keep a laminar flow attached. At high reynolds numbers, you have a similar effect as the dimpled golf ball keeping the air from detaching and recirculating early when it passes over the aerofoil of the fan blade, so you can give it an aggressive, high thrust shape.

Of course you cannot put a very high lift aerofoil near the blade tip as you would otherwise run into problems with locally supersonic flow creating lots of drag.

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BobbyP27 t1_irsbpb4 wrote

Because density plays a role in Reynolds number, for smaller engines at high altitude (low air density), you can't always count on the flow being turbulent. Laminar flow and transition to turbulence are significant design issues for things like business jet size engine compressors.

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