MonkeyMensch, a gas turbine using the Brayton cycle has a minimum operating speed of typically around 50% of its maximum RPM. The starter motor has a tough job to do to spool it up that fast before introducing fuel. As **Berkut ** points out, most of the energy released in the combustor goes, via the turbine, simply to run the compressor. Most of a gas turbine engine combustor’s power is recycled internally. Below a minimum speed, the process can’t sustain itself. Perhaps you’re thinking of a turbine by itself, which can indeed generate torque at zero speed if you’re forcing a gas or liquid through it. But then where does that gas or liquid come from?
Um, as long as the fraction is less than 1 you have useful energy output. If it were more than 1 the engine would not work without additional power input. That’s my point.
I thought the OP was going to be a different question. Since it’s closely related, I’ll ask.
I’ve been doing a lot of flying lately, and I have also been mystified by jet engines. But my reason for being mystified was because it looked to me like the engine was open in front and just tapers off in the back. Kind of like a cone shape with the large end in front and the closed off point in the back.
Does anyone know what I’m talking abot? Can you explain my misunderstanding to me?
Does anyone know where I can get a good BB gun. There are some hamsters in the Chicago area that need some attention…
ElvisL1ves All honor and glory is yours for doing this stuff for real. I was speaking about airplane speed being zero, not engine rotational speed. Just trying to point out that ramjets just don’t idle very well on the ramp but turbojets, once started, do. No worries. Hey. Do you wanna swap jobs?
The big-front dinky-back look indicates a high bypass turbofan. The cone at the back shows how small the diameter of the pure “jet” portion of the engine is. The full diameter of the turbojet is a smidge bigger, but not much. For the big turbofans more than 80% of that inlet air going in that honking big 8’ diameter Air-Hoover goes right on by the compressor and combustion stages. Hence the “Giant Tube - Dinky Cone” look that is all the rage at airports worldwide. High ratio bypass turbofans are not only more effecient, but quieter too. No wonder dey’s popular.
For reference look at pictures of early air transport engines, say on the 707. They were low bypass, if they were bypass at all, are are nearly cylindrical.
I wasn’t disagreeing with you, I was just supplying a figure since it came up. Most people are shocked to hear that it takes that much power to run the compressor.
Of course, with modern bypass engines, a huge amount of your thrust is coming from your first stage of compression, so it isn’t so surprising that it takes so much power to turn.
It’s interesting that they chose to illustrate a turboprop with a gear drive instead of a free turbine like the PT-6, considering how popular the PT-6 is. I guess they didn’t want to get into the mess of reverse-flow and centrifugal compressors and such.
