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Question Re Turbine Engine Start  
User currently offlineRedFlyer From United States of America, joined Feb 2005, 4335 posts, RR: 28
Posted (8 years 11 months 4 weeks 1 day 6 hours ago) and read 5119 times:

Several questions I've always been curious about:

1. If an APU is utilized to start a main engine via bleed air (or direct exhaust pressure), what is used to start the APU?

2. When the turbine on a main engine is started, what keeps the sudden build-up of pressure in the cumbustion chamber at the moment of fuel/air ignition from blowing back out the front of the engine through the compressor blades? In other words, what keeps the flow of pressure moving to the rear of the engine?

Thanks


My other home is a Piper Cherokee 180C
26 replies: All unread, showing first 25:
 
User currently offlineGoldenshield From United States of America, joined Jan 2001, 6048 posts, RR: 14
Reply 1, posted (8 years 11 months 4 weeks 1 day 5 hours ago) and read 5107 times:

Number 1 is easy. Battery or ground power.

2: Air is like water and will take the path of least resistance. When starting, the turbine is sucking in and compressing air. The starter is the only thing that keeps the turbine from turning backwards from during the slower portion of the start cycle.

The area inside the combustion chamber is is much bigger than the exit point of the compressor, which means that the compressed air will expand, and since it takes the easy road, it goes by the cans, ignites, expands more, and is expelled out the back. That's a simple explaination.

Any mechs want to get more verbose?



Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineRalgha From United States of America, joined Nov 1999, 1614 posts, RR: 6
Reply 2, posted (8 years 11 months 4 weeks 1 day 5 hours ago) and read 5103 times:

Quoting Goldenshield (Reply 1):
Any mechs want to get more verbose?

Since when have you seen a thread here NOT have people getting more verbose?

We'll have 6 people say the exact same thing you just did, 3 will go into more detail, and 17 will tell us how it works but be completely wrong.



09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0
User currently offlineGoldenshield From United States of America, joined Jan 2001, 6048 posts, RR: 14
Reply 3, posted (8 years 11 months 4 weeks 1 day 5 hours ago) and read 5101 times:

Quoting Ralgha (Reply 2):
We'll have 6 people say the exact same thing you just did, 3 will go into more detail, and 17 will tell us how it works but be completely wrong.

Every garden grows weeds.  Wink



Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineAGM114L From , joined Dec 1969, posts, RR:
Reply 4, posted (8 years 11 months 4 weeks 1 day 3 hours ago) and read 5064 times:

Quoting Goldenshield (Reply 1):
Number 1 is easy. Battery or ground power.

You forgot hydraulic accumulators.  Wink


User currently offlineAmtrosie From United States of America, joined Jan 2005, 274 posts, RR: 0
Reply 5, posted (8 years 11 months 4 weeks 1 day 3 hours ago) and read 5062 times:

Quoting Goldenshield (Reply 1):
Number 1 is easy. Battery or ground power.

To be more "verbose" and more technically correct a Starter/generator is used to start a APU. This is also true of most turboprop engines, using a starter/generator, ( I say most, I only KNOW of the engines and airframes I worked) I will not expand on the second answer given.


User currently offlineAGM114L From , joined Dec 1969, posts, RR:
Reply 6, posted (8 years 11 months 4 weeks 1 day 3 hours ago) and read 5053 times:

Quoting Amtrosie (Reply 5):
I will not expand on the second answer given.

You're right, tecnically speaking an ignition source, air, and fuel are required to start an APU, just like any other internal combustion engine. An accumulator provides pressure to the hydraulic start motor which provides ignition for starting the APU.

[Edited 2005-10-08 02:38:43]

User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 7, posted (8 years 11 months 4 weeks 1 day 3 hours ago) and read 5049 times:

I would argue that saying a starter is used to start an engine is less verbose, and while correct, out of context. The intent of the question, in my interpretation, was to ask what power source is used to start the APU. A starter is not a power source. The APU starter requires electrical power (or, in a few corner cases, accumulated hydraulic power).


Position and hold
User currently offline727EMflyer From United States of America, joined Mar 2005, 547 posts, RR: 0
Reply 8, posted (8 years 11 months 4 weeks 1 day 2 hours ago) and read 5037 times:

Quoting Ralgha (Reply 2):
17 will tell us how it works but be completely wrong.

ooo ooo! Can I do that one? There are tiny little elves... Fairies really... the ones in the APU spin the turbine like its the big wheel on "Price is Right" and the ones in the mains sit in front of the combustors and fan their wings like crazy if anything gets moving the wrong way.


User currently offlineMX757 From United States of America, joined Aug 2005, 628 posts, RR: 12
Reply 9, posted (8 years 11 months 4 weeks 1 day ago) and read 5003 times:

Quoting 727EMflyer (Reply 8):
ooo ooo! Can I do that one? There are tiny little elves... Fairies really... the ones in the APU spin the turbine like its the big wheel on "Price is Right" and the ones in the mains sit in front of the combustors and fan their wings like crazy if anything gets moving the wrong way.

We had them working for us at CO but they refused to take a pay cut like the rest of us. So they went out on strike and we are now using Pixies as replacements. Big grin



Is it broke...? Yeah I'll fix it.
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 10, posted (8 years 11 months 4 weeks 1 day ago) and read 4999 times:

Quoting Ralgha (Reply 2):

I just fail to see how that post contributes anything. I guess you forgot to mention how a number of people will get a case of "diarrhea of the hands" and spew garbage into the thread.

Oh, and don't forget the plethora of posters saying "this has been answered before, do a search."

God forbid anyone ask a reasonable question around here. If you don't want to answer, why bother posting? Maybe those people were all born with infinite knowledge and have never had to ask a question.



Position and hold
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 11, posted (8 years 11 months 4 weeks 22 hours ago) and read 4976 times:

Quoting RedFlyer (Thread starter):
what is used to start the APU?

Battery,Battery cart,GPU,Compressed Bottles,Cross start from Adjacent Aircraft via flexible duct  Smile

Quoting RedFlyer (Thread starter):
what keeps the flow of pressure moving to the rear of the engine

Area difference,Pressure Difference,Direction of Blade mounting/Rotation [Compressor/Turbine].

regds
MEL



Think of the brighter side!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 12, posted (8 years 11 months 4 weeks 21 hours ago) and read 4953 times:

Quoting Bri2k1 (Reply 10):
I just fail to see how that post contributes anything. I guess you forgot to mention how a number of people will get a case of "diarrhea of the hands" and spew garbage into the thread.

FWIW I thougt it was a spot-on and rather amusing observation. And did it add to the thread? Of course it did! That statement should be on top of the forum to warn newcomers! Big grin

Quoting Bri2k1 (Reply 10):
God forbid anyone ask a reasonable question around here.

Oh, lots of those. Lots of inane ones too, but that's a different matter. I did not see Ralgha comment on questions asked, however. I did see him comment on the many posters in this forum who muddle every thread up by answering those reasonable questions without having a clue themselves, spewing guesses and assumptions as facts without stating that it is, in fact, guesses and assumptions.

Rather annoying to see good posts based on actual facts (such as those by yourself) disappear in the sheer volume of the noise.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineG4Doc2004 From United States of America, joined Feb 2004, 123 posts, RR: 0
Reply 13, posted (8 years 11 months 4 weeks ago) and read 4857 times:

1. As said in other posts, the APU is started off the batteries with a electric starter/generator, or in other cases, a HM off an accumulator.

2. Think of air as a fluid (as it is) and once it gets going thru the core, it maintains its direction due to the pressure from the pressure rise as the air goes thru the compressor. Also, the compressor rotor/turbine shaft assembly is a heavy piece of machinery...once it begins to rotate and accelerate, it pushes quite a lot air and has a lot of momentum...hence the reason that whichever starting method is used (electric or air) the goal is the get the core speed up to 10-13% RPM quickly to get the engine lit.



"Failure to prepare is preparing to fail"--Benjamin Franklin
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 14, posted (8 years 11 months 3 weeks 6 days 20 hours ago) and read 4837 times:

Quoting AGM114L (Reply 4):
You forgot hydraulic accumulators



Quoting G4Doc2004 (Reply 13):
or in other cases, a HM off an accumulator.

Which type use these.What about Commercial Aircraft Types.
regds
MEL



Think of the brighter side!
User currently offlineAGM114L From , joined Dec 1969, posts, RR:
Reply 15, posted (8 years 11 months 3 weeks 6 days 17 hours ago) and read 4822 times:

Quoting HAWK21M (Reply 14):
Which type use these.What about Commercial Aircraft Types.

That would be the venerable hajji hunter itself.

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Photo © KBMphotography



User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 16, posted (8 years 11 months 3 weeks 5 days 21 hours ago) and read 4752 times:

Quoting AGM114L (Reply 15):
That would be the venerable hajji hunter itself.

Whats the story behind the name.
regds
MEL



Think of the brighter side!
User currently offlineVenus6971 From United States of America, joined Dec 2004, 1443 posts, RR: 0
Reply 17, posted (8 years 11 months 3 weeks 5 days 1 hour ago) and read 4693 times:

From a DC power or hyd pressure source which energize or turns the starter or which turns the turbine which turns a PMG which energize ignition and a fuel solenoid simutainously until proper rpm is reached which automatically disengages the pmg and ignition at self sustaining set speed.
I await everybody's slings and volleys



I would help you but it is not in the contract
User currently offlineG4Doc2004 From United States of America, joined Feb 2004, 123 posts, RR: 0
Reply 18, posted (8 years 11 months 3 weeks 5 days 1 hour ago) and read 4689 times:

Neither a sling or volley, but usually ignition, fuel, and ignition and starter cutout are controlled by a speed switch, not PMG. PMG is usually used to load or unload a generator being driven by the engine.


"Failure to prepare is preparing to fail"--Benjamin Franklin
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 19, posted (8 years 11 months 3 weeks 4 days 19 hours ago) and read 4645 times:

Quoting AGM114L (Reply 15):
That would be the venerable hajji hunter itself

The Name sounds Anti a particular community if thats what the literate meaning is.Whats the story on the Name.
regds
MEL



Think of the brighter side!
User currently offlineAGM114L From , joined Dec 1969, posts, RR:
Reply 20, posted (8 years 11 months 3 weeks 4 days 14 hours ago) and read 4620 times:

MEL,
Agreed that was an unprofessional statement on my part. The literal meaning of Hajji means one a Haj, or pilgramage. However, 'Hajji' is also slang for insurgents as a whole used in the community I work in, such 'Kraut' or 'Charlie' was used by GI's in the past as terms for the Nazis and Viet Kong. This forum is probably not the place to debat the merits of this, but I do apologize and will have no objections to the deletion of the posts.


User currently offline10MID From Singapore, joined Aug 2004, 198 posts, RR: 0
Reply 21, posted (8 years 11 months 3 weeks 4 days 9 hours ago) and read 4583 times:

Quoting Bri2k1 (Reply 10):
I guess you forgot to mention how a number of people will get a case of "diarrhea of the hands" and spew garbage into the thread.

Usually accompanied by constipation of the brain.


User currently offlineMasseyBrown From United States of America, joined Dec 2002, 5460 posts, RR: 7
Reply 22, posted (8 years 11 months 3 weeks 4 days 8 hours ago) and read 4576 times:

Is it still possible to start a modern turbine with a "grenade" as in the old days of 707s?


I love long German words like 'Freundschaftsbezeigungen'.
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 23, posted (8 years 11 months 3 weeks 3 days 19 hours ago) and read 4507 times:

Quoting AGM114L (Reply 20):
The literal meaning of Hajji means one a Haj, or pilgramage. However, 'Hajji' is also slang for insurgents as a whole used in the community I work in, such 'Kraut' or 'Charlie' was used by GI's in the past as terms for the Nazis and Viet Kong.

Did this term raise itself in Afghanistan.

Quoting MasseyBrown (Reply 22):
Is it still possible to start a modern turbine with a "grenade" as in the old days of 707s

You mean compressed Bottles.
Why not.The main requirement is the Required Pressure of Air for the Specific Duration.
regds
MEL



Think of the brighter side!
User currently offlineAvionicMech From United Kingdom, joined Jan 2005, 315 posts, RR: 3
Reply 24, posted (8 years 11 months 2 weeks 6 days 7 hours ago) and read 4414 times:

It has been mentioned a few times above that the APU is started by a starter/generator, the only APU that I have worked on that had a starter/generator was the one on the 737NG. The 757/767 APU has a separate starter and generator rather than the combined unit on the 737. But I have only worked on 737/757/767 though, so don't take it that the 737NG is the only aircraft with the starter/generator setup.

Avionic Mech


25 Post contains images Greasemonkey : The APU starter motor on most commercial aircraft is similar to that of an automobile starter . Current from a battery is directed to the starter moto
26 Post contains images HAWK21M : Found a few pics of the Lucas Aerospace CR201 GTS/APU regds MEL
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