I feel a bit stupid for asking this, but I simply cannot wrap my head around how a jet engine directs its thrust backwards.
A piston engine opens and closes valves, such that air is alternately sucked in and compressed to create force. That makes sense and I understand how it works.
But with a jet engine there are no valves opening and closing, so why doesn’t the air that is sucked in just get pushed right back out from the thrust generated? How can an engine simultaneously suck in air from the front and force it out the back?
It makes no sense to me :-/
In: Engineering
If you look at a cross section of a jet engine, it’s usually really wide at the front, there’s a cone/cylinder-like thing running down the middle from front to back that goes from narrow to wide back to narrow, and the back end is again wide. The part in the middle creates a venturi effect (fancy name for taking air in a wide tube and shoving it quickly through a sudden narrowing of the tube) which squeezes the incoming air and makes it move much faster.
Into this faster moving air you add jet fuel and ignite it. What happens is the jet fuel and compressed air ignites into something very hot, and the fuel and air mix creates a lot of extra stuff – mostly gas – from burning (carbon monoxide, carbon dioxide, water vapor, etc.). Jet fuel takes up little space. Compressed air (venturi effect) takes up little space. But burning jet fuel with compressed air creates hot gases which takes up a LOT of space which has to move out the back end of the engine – enough space to help push the aircraft forward.
So, what’s coming out of a jet engine isn’t just fast moving air, it’s fast moving air mixed with jet fuel and combusted. And what comes out is a LOT of hot gases that’s a lot more than just the air that went in.
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