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View Diary: How Airliners Work - Propulsion (191 comments)

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  •  So is the exhaust end fan pushing air back (2+ / 0-)
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    Simplify, Orinoco

    toward the engine? 'reverse fan'? to maintain pressure?

    •  It's a windmill (3+ / 0-)
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      lazybum, Bisbonian, fisheye

      Another way to look at a turbojet engine is to start a the combustion chamber:
      - Fuel mixed with air burns, heats up, expands, and accelerates.
      - Those hot, fast, high-pressure gasses turn the turbine a the back (effectively a windmill) and rush out.
      - It's the gas moving quickly out the back that makes thrust.
      - The turbine has a shaft that turns the compressor up front.
      - The compressor effectively squeezes the air, preparing it for the combustion chamber.

      Pretty much all of the squeezing comes from the compressor. It turns out that, very early in jet engine development, at least one group tried using a piston engine (and not a turbine) to drive the compressor.

      One might ask: why not just have a combustion chamber and not bother with the rest? That would be a rocket engine, for which the vehicle has to carry its own oxygen in addition to the fuel.

      A turbofan or turboprop adds an extra turbine/windmill at the back with a shaft to a fan/propeller up front. As Kong points out, in that case the energy from the hot exhaust drives the fan to move lots of air and cause thrust that way, not just via a small amount of combusted air moving very fast. Depending on the speed, this is more efficient: accelerating lots of air a little bit rather than accelerating a little bit of air a lot.

      Another way to look at a turbofan/turboprop is to compare it to a piston engine: the compressor is like the piston on the upstroke, and the turbine is like the piston on the downstroke. Note that the downstroke, through the driveshaft, both produces thrust (fan/prop) and drives the next upstroke (compressor).

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      by Simplify on Tue Apr 02, 2013 at 10:44:50 AM PDT

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    •  The exhaust end fan, the turbine (1+ / 0-)
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      is being pushed by the hot air leaving the engine. It provides the power that makes the rest of the fans spin.

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      by Orinoco on Tue Apr 02, 2013 at 10:54:05 AM PDT

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      •  Funny I always assumed different stages of (1+ / 0-)
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        fans and turbines spun at different speeds. But it's all on a solid state shaft? It's actually much simpler than I imagined. I suppose that accounts for it's durability which I could never quite reconcile with a geared system.

        I think there's a multi-stage engine that super-compressed air on that old SR-71. Or like a turbo fan into a hyper turbo fan with maybe even successive ignition stages.  

        And then I read (surfing off this diary) that Pratt & Whitney just developed a gear box for the fan at the front of their turbofan engine and it saves a ton of fuel. Because the fan operates better at lower speed than the turbines and compressor.

        I suppose that's the same effective limitations confronted with a turboprop

        Anyway. Another great diary. I love them all Major.

        •  Different RPMs for Different Stages (1+ / 0-)
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          The first axial flow turbines had a single compressor attached to a shaft connected to the turbine section.  More modern jet turbines/turbofan have two separate compressor and turbine sections.  The low-pressure compressor section is hard physically attached to the low-pressure turbine section while the high-pressure compressor attaches to the high-pressure turbine via a separate shaft.  The one shaft is hollow to allow the other to run through it.   That enable the high-pressure section to run at a higher RPM than the low-pressure sections.  Some engine designers have even built three-section designs to squeeze out more efficiency from this approach.

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          by PrahaPartizan on Wed Apr 03, 2013 at 01:58:44 PM PDT

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        •  ... (0+ / 0-)

          That fact means that jets/turbines are very inefficient off their design speeds. IIRC, 90% N1 is typically 50% thrust or so.

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