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

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  •  It's a windmill (3+ / 0-)
    Recommended by:
    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).

    Government and laws are the agreement we all make to secure everyone's freedom.

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

    [ Parent ]

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