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I only ask this 'cos, after doing a lot of flying recently I've become slightly fascinated by how jet airliners work.
Now... when taxi-ing on the tarmac they appear to be able to move (and even partially steer!) using their jet engines, but can only go in a forwards direction to do this - and hence the need for one of those funny little car things to initially 'shunt' them out onto the taxi-way.
But... when walking onto the plane - I'm always aware of some secondary jet engine running somewhere - that is definitely not wing-mounted. I can hear and smell engines running, even though the wing-mounted jet engines are stationary. Infact - an engine somewhere appears to be exhausting from that strange 'anus' 🙂 on the tail of the plane - what is this? It there some sort of secondary [i]internal[/i] jet engine inside the plane, somewhere, that provides power (electricity) for while we're sitting stationary on the tarmac..?
So... is power from this engine used to 'start' the main engines on the wings? ... if so, how? ...And what else is this internal engine used for?
Or have I got this all wrong..?
I could go on and on.. but I'll stop there. I think I may have become a little bit obsessed with all this 😛
That small engine is to power the conveyor belt.
Auxiliary power units, often left running on the ground to provide cooling. I think.
APU, basically a very small gas turbine that provides electrical supply and air bleed primarily to start the main engines but also to keep air con and electrical systems running when the main engines are shut down. Usually shut down in flight but if you have probs in flight with engines/main generators/air/hydraulic supply it's there as a back up.....
Awesome..! Ask an engineering-type question to the STW massive and great answers will come streaming in thick and fast!
Ok... so what's the difference between a gas turbine and a jet engine.. or is that a daft question..?
- so does that mean the main jet engines are started by some sort of pneumatic system driven by the APU..?'Air bleed'
So.. what actually starts the APU - an even smaller APU? 😛
Same as the main engines/turbine but smaller. The majority of the main turbine is not turbine, there's a lot of volume in that "tube" used to compress air at the front to be pushed out the back. The turbine itself is relatively small but very powerful.
We had one at work about the sixe of a red bull can but half the length and it put out about 4-6kg of thrust.
The opeating principle of the APU and the main engines is indeed the same. However, the power of the turbine in the main engines is used to drive the fan (propeller effectively), hence the name Turbofan engine, whilst in the APU the power of the turbine drives a shaft which will drive a generator to produce electricity, and hence drive air compressors, hydraulics, and A/C plant, leading to the name Turboshaft.
Just to confuse things, there are also turbojets, where all the air fron the compressor fan passes through the engine (nasty smoky noisy old things), and turboprops, which have a conventional propeller. Helicopters are also (generally) powered by turboshaft(jet)engines.
A gas turbine and a jet engine are basically the same thing (well they work in exactly the same way using a sequence of turbines to compress incoming air that is then mixed with fuel, combusted and the exhaust gasses produce thrust and turn a final turbine that drives the compressor turbine at the front of the engine) but a gas turbine usually has a driveshaft that can run a generator.
You need to get a jet engine up to speed (ie get the turbine blades spinning) to actually start working properly, you do this by blasting compressed air through the front compressor turbine. You can start the big jet engines with the APU or they can take bleed air from another of the main engines ( if you listen when the aircraft main engines are started they dont usually start at the same time for this reason) As an APU tends to drive a generator they can be started using external electrical power.
if you really wont to know more have a look at wikipedia for starters 🙂
so where does the magic fairy dust come into it then?
The pilots after-flight party?
Air Bleed means air bled from the compression stages of the turbines or APU, usually the first stage. The bled air is used for pneumatics (for cabin pressure), air conditioning, wing leading edge heating, etc.
Depends upon the APU, but on the ground the APU can be started via an electrical hook up to the aircraft. In the air, assuming a total loss of both engines prior to APU start, you may be able to generate anough power from the Ram Air Turbine (RAT) to start the APU.
'Someone' might have told me this week that alot of his colleagues wouldnt fly on certain Airbus models (they just dont have faith in them).
It's the devils work, I tell ye!
[i]usually the first stage[/i]
Usually round about the 8-10th stage of the compressor.
They attach a bloody great rubber band to the front, tow it to the far end of the runway and let go. 😉
@ muddy@rseguy
Close but not quite....
In most engine starts on airliners the air from the APU (or the other engine) starts an engine by driving a starter air turbine which is connected to the HP (high pressure) shaft by a series of shafts and gears (ie the air from the APU is not fed into the core engine itself). Also 'compressor turbine' is not a correct term - a compressor is upstream of the combustor and is there to compress air, a turbine is downstream of a comobuster and is there to extract energy to drive either the compressor, a propeller or a fan.
@ Nobeerinthefridge
8-10 stage is probably about right for a pratt or GE engine. For RR (3 shaft) it is typically IP8 for higher power conditions, and HP3 (via a precooler) for lower power conditions - at flight idle IP8 would be too low a pressure for example....
Exceptions to all of the above are the GEnX and Trent 1000 - both engines can be started electrically via a pair of VFSGs (variable frequency starter generators). These take electrical power from APU. They are also used to power compressors to supply air to the cabin, avoiding the need to do it by bleeding air from the engines....
Col.
Had half written a reply, then checked back and found Col has written my words for me! How did you do that Col?
You sound like another RR bod in the STW massive.
Rob.
An ex-RR bod!
lol.
Yes RR.
I'm currently the assistant chief design engineer for this fella:
[url] http://www.rolls-royce.com/civil/products/largeaircraft/trent_xwb/ [/url]
Guessed so.
I was Config Control Project Manager for Trent 500/700/900 in my previous life. I ran away in 2003 for a quieter life!
XWB looks interesting.
Rob
u got jet aircraft pretty well summed up
here is an intersting fact...i think its accurate...my memory isnt brillient...but when ur taxying to the runway in ur jet aircraft the engines are spinning at just over 80% of thier overall speed...the engines only come alive in the last few % of their max speed...which is probly over 100,000 rpm on modern aircraft.
That's kinda true....
[img] 
[/img]
The pic above is a Trent 1000. The blue system (ie a fan, shaft and multiple turbine stages) is the 'LP' system, the yellow system (8 stage compressor, shaft and single stage turbine) is the 'IP' system and the red system (6 stage compressor, shaft and single stage turbine) is the 'HP' system.
The way to think of it is the 'core' of the engine generating all the power is the HP and IP system. This core is used to generate power which the LP turbine extracts to drive the fan.
At idle the Fan will only rotate at around say 10% of it's full speed (say 3000 rpm), generating only a few thousands pounds of thrust. But as you said, to do this the core shafts, in particular the HP shaft will need to go at 70-80% of its maximum speed (around 13,000 rpm). In going to full thrust (say 75,000lb certified sea level thrust for Trent 1000) the HP shaft will only increase in a bit to get to 100% speed, but the fan will go around 10 times faster!
The eagle eyed among you may have also spotted that in my first post it should be HP6 rather than HP3....
Cheers,
Col.
The eagle eyed among you may have also spotted that in my first post it should be HP6 rather than HP3....
I didn't want to say anything.
LOL @ Higs
"Yo mamma is so fat, she works at Heathrow kick-startin' Jumbos"
The eagle eyed among you may have also spotted that in my first post it should be HP6 rather than HP3....
😀
Well if we're talking cabin bleed (not bleed air into the bypass, as in the IP+HP bleed valves, to prevent compressor surge+stall..) then isn't it IP8 and HP 3 and 6 combined..?
Hp 3 and 6 exit via one common duct (via the venturi) at tdc of the 04 mod, think this is true for T500, 700, 800 and 900. I'm sure you'll put me right tho BigCol as i'm not a tech bod, just a lowly experi fitter 😉
No cabin bleed on T1000 and as you've said electric start only via the vfsg's, as shown on the GA drawing above...
BigCol.
Looking forward to building the xwb, should be a slighty different assy method aswell (from an engine build perspective anyway) by looks of it..?
😀
Edit.
Thinking about it now i'm wrong, as you said its just hp6 for cabin bleed, the venturi prevents the hp3 and hp6 mixing...?
I think i need to get back riding again or find something to occupy my mind 😆
Pah, who needs all that nonsense! I can start one of these:
[img] 
[/img]
With one of these:
[img] 
[/img]
😀
How fail safe are the various bits of the jet engine? Is it totally ****ed if just one small component goes.
I should really check my facts before posting. Still learning 😳
@ Jackal
As you say there are IP and HP bleeds for engine operability (these dump air into the bypass duct as you said). The cabin bleed is just HP6 (low power) and IP8 (high power).
There is an HP3 offtake which is used for inlet anti-icing though...
Not too long to go now and we'll be building the first XWB - I'm really looking forward to it. Nothing beats going down to the build line to see some actual hardware!!
Col.
Not too long to go now and we'll be building the first XWB - I'm really looking forward to it. Nothing beats going down to the build line to see some actual hardware!!Col.
See you down there then 😉
DavidB
depends which bit,
the HP turbine blades (about the same size as an mtb stem for scale) get worn down, as does the ceramic honeycomb the tips seal against, takes about 3 years before they need replacing, and about another 2 before the engine is scrap. So in general you'd expect no other problems withthe engine in that time, they pretty well built.
However if anythign foreign gets past the inlet it pretty much destroys the engine. Hence why a flock seagulls can bring down an airliner.
However if anyting fails catastrophicaly they'r prepared for it, what BigCol's pic doesnt show is the outer part of the engine, essentialy a big carbon/kevlar/epoxy composite wraped arroud the engine to catch any blades that come flying off. They test them by drilling holes in the base of the blades and filling them with explosives!
what BigCol's pic doesnt show is the outer part of the engine, essentialy a big carbon/kevlar/epoxy composite wraped arroud the engine to catch any blades that come flying off.
Not for more recent engines, kevlar wrapped fancases are a thing of the past, susceptible to moisture/water ingress.
See the 'ribs' inline with the LP compressor (fan) blades on the T1000 fancase above, these have now replaced the kevlar wrap preventing any 'high energy debris' exiting the fancase.
🙂
I don't care how a plane works! 😆
Jesus
Guess that I am not the only ex RAF Techie
However if anythign foreign gets past the inlet it pretty much destroys the engine. Hence why a flock seagulls can bring down an airliner.
Thought engines were tested with the frozen turkey cannons?
Seem to remember Stephen Fry saying at the end of an episode of QI that there are several theories about how a plane manages to fly and they all contradict each other. Anyone know how true that is?
Planes fly because I let tell them to.
How would you get a job like that BigCol? I'd love to talk about this all day...
I stand corrected then, when I was doing my work experience (2001 ish) the 800 was the bigest, the 1000 was just a doodle in someones notebook.
They do test them with various sized birds, but they all destroy the engine. Its a bit like crash testing a car, you don't expect it to survive, just making sure it won't kill the passengers.
BigCol - do you know if it's possible to perhaps arrange a visit to the Rolls Royce factory? I'm qualified to fly the things, but would love to get a closer look at the manufacturing and development side.
My dad made many of the patterns for the RB211, so I've always had an interest in Rolls Royce engines. 🙂
Cheers,
Dave.
BigCol - do you know if it's possible to perhaps arrange a visit to the Rolls Royce factory? I'm qualified to fly the things, but would love to get a closer look at the manufacturing and development side.My dad made many of the patterns for the RB211, so I've always had an interest in Rolls Royce engines.
Cheers,
Dave.
If you can get a visit sorted make sure you come to experimental, much more interesting than production 😉
Seem to remember Stephen Fry saying at the end of an episode of QI that there are several theories about how a plane manages to fly and they all contradict each other. Anyone know how true that is?
It might be true that there are various theories. Various theories of people who don't actually understand the things. I'm sure the engineers who design such things understand pretty well how they work (and there isn't much in the way of disagreement between them), given it's not that complicated physics, it's just the general public who tend not to.
How various insects fly is another matter - they tend to use very complex physics to maximise lift, and I'm not totally sure anybody understands completely!
How various insects fly is another matter
Yeah have also heard it's theoretically impossible for a bumble bee to fly - could just be one of those useless incorrect factoids that gets passed around tho.
Flaperon - I'll have an ask as to what the process is for visitors these days.....