I understand how turbine engines work, but I have a few questions about the details of how they’re designed. Bear with me if my terminology isn’t precisely correct.
From what I’ve seen of modern turbine engines, there’s a core, air flowing through compressors and turbines, and some sort of outer casing. My questions have to do with how the core is mounted in the center of the engine while still allowing the necessary airflow all around it:
How much of a design problem is this in modern engines? I’ve seen old radial-flow engines, and since the air gets passed through multiple, discreet channels there’s no problem. But from what I’ve seen in diagrams of axial-flow engines, it looks as though the airflow completely surrounds the core. There are stator vanes between stages of the compressor, but these look like they’re intended more to direct the airflow than to hold the core in place. Are there multiple combustion chamers in an axial-flow engine, like the radial-flow, or is there a single chamber surrounding the core?
It seems to me that the compressor and turbine disks provide the thrust that moves the plane forward, so how much of the thrust load on the aircraft is borne by the core and transmitted to the airframe by the core supports?
Besides the mechanical force, what else has to be exchanged between the core and outside of the engine? Are there channels to supply oil to the bearings? hydraulic lines (supplying pressure to the whole aircraft from a pump in the engine core)? Are there rpm sensors on any of the engine components connected to guages in the cockpit? Anything else?
(This question came to me after seeing two of the prototype aircraft designed by Rene Leduc. He experimented with mostly ramjets, placing the cockpit directly on the front of the core, with the inlet for the engine completely encircling the fuselage. Looking at that, it seemed to me that design would have the problem of routing all the control cables from the inside of the engine to the outside.)
Oh, goody, a jet engine design question. That’s what I do when I’m not too busy reading this board.
The rotor, or rotors (as many as 3), are supported by bearings mounted on structural frames that have struts crossing the airflow passage to the outside. The struts are normally shaped to minimize drag and wake, making them resemble thick airfoils.
Multiple-can combustors haven’t been designed since the early Sixties. Modern combustors are annular, partly for reduced weight, partly for reduced internal friction drag, and partly because they need less cooling air as a percentage of the total (only about a third or so of the air leaving the compressor is actually available for combustion even so).
Even centrifugal-compressor engines have annular combustors now, too - that has nothing to do with compressor design, it was simply the state of the art early on. That in turn was forced by limitations of test equipment - a combustor could be tested as a single can with much less airflow required than a full annular one, which cannot be tested usefully except as a unit.
2. Most of it. The rotor bearings push against static structures and so do the stator vanes, and those loads make their way into the mounts and from there into the airframe. Some load comes from air pressure against the projected area of noncylindrical surfaces.
3. Oil supply and scavenge lines are typically the only services that run through struts into the bearing compartments. Speed pickups typically don’t penetrate oil-wetted walls; they don’t have to - they are normally simply capacitance probes with their tips near castellated lock nuts on the shafts, producing a speed signal for the engine control system (and the cockpit instrument panel, like you guess) by just counting pulses. Engines do typically have hydraulic systems to actuate the variable vanes, but that’s on the outside of the engine. It is also common to have accelerometers mounted on the static bearing supports to measure vibration levels, as a warning of possible impending problems.
You’re probably more familiar with the details of Leduc’s designs than I, but if you look closely I’ll bet you find some struts buried inside to hold the airplane together. How could there not be?
I should have added that at least one strut will house a radial drive shaft, driven via bevel gears from a rotor, that powers the accessory gearbox. The AGB holds the oil pump, electrical generator, hydraulic pump, and so forth.
Some engines have a funky annular combustors (with multiple fuel injectors). I think there are still several (for reasons of accessability if no other).
Looking at a decent poster of a current engine it looks like the stators are structural. (That is just a wag though)
The turbine extracts energy from the flow, not adding energy to the flow, and roughly only enough to run the compressor. The thrust comes from aerodynamic forces that result from expanding/accelerating the hot, high pressure, (relatively slow) flow to cooler, lower pressure, much faster flow. This will operate over the area of the turbines, but in many turbine engines the actual turbine area is a small annular portion of the total cross section of the engine- the loads will be more on the engine nozzle and the nonmoving central portion of the engine, on which the turbines are mounted, etc.
All engines made in recent decades have annular combustors AFAIK, and multiple fuel injectors, evenly spaced in a circle, as well.
Compressor and turbine stators *are * structural in a way; they take reaction loads from the air or exhaust gas whose flow they’re turning and transmit those loads into the casings and frames.
Exhaust gas isn’t under high pressure; most of its pressure goes into driving the turbines. At the actual point of exhaust, it’s at atmospheric pressure. Velocity of core exhaust is normally supersonic, though, and it isn’t all that efficient in producing thrust - much of that energy creates noise and turbulence, not airspeed. The thrust of a high-bypass turbofan like most airliners have comes mostly from the fan discharge and not the core, though - even though its relatively slow, its mass flow is huge and its propulsive efficiency is high because it mostly goes into pushing the airplane.
I love this board. Where else could I ask a question like this and get such good answers? Thanks to everyone, especially ElvisL1ves, who really knocked the OP out of the park.
MMI, I know that the turbines extract energy from the airflow to power the compressors. All I meant was that the net result of all the stages of the engine is to accelerate a mass of air, and Newton’s Third Law says the resultant opposite force has to be picked up somewhere.
Some of what I’ve seen and read about the history of jet aircraft seems to focus on the air intake as a pretty tricky thing to get right, especially in supersonic aircraft. And I’d seen pictures of Leduc’s prototypes. But then I saw a couple of his planes in a museum and it got me thinking about what the sticking points would be with that design, and thus is a General Question born.
(Doing a little web research on Leduc, it looks like most of his planes were ramjets, which simplifies things quite a lot. But there still have to be some struts or something buried inside to hold it all together. Alas, I can’t easily get back to that museum to check it out. And at least one picture on the web shows one of his aircraft with the full annular air intake and two nozzles on the back. There have been a lot of cool planes built over the years, and I haven’t seen them all, yet.)
There’s a cross-section illustration here of the Leduc 021, showing the wing spar doubling as the struts holding the intake centerbody / cockpit section to the barrel fuselage. The text won’t help you much if you don’t read Czech, though.
You’re right about intake design being more and more critical the higher the Mach number involved. Controlling the position of the shock wave where the incoming flow is slowed to subsonic velocity is absolutely critical.
Part of what I was trying to communicate in my reply was that the turbine area being reacted on is only a small portion of the total area (at least in some engines) - between the central non moving portion and the external nozzle, etc. In the limit of a Brayton cycle limit with no compressor or turbine, the outcome is the same.
