redflyer
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Question Re Turbine Engine Start

Sat Oct 08, 2005 6:36 am

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
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Goldenshield
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 7:03 am

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?
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Ralgha
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 7:10 am

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.
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Goldenshield
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 7:14 am

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
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AGM114L
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 9:04 am

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

You forgot hydraulic accumulators.  Wink
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amtrosie
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 9:07 am

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.
 
AGM114L
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 9:31 am

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]
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bri2k1
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 9:34 am

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).
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727EMflyer
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 9:59 am

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.
 
MX757
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 12:01 pm

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
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bri2k1
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 12:14 pm

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.
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HAWK21M
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 2:22 pm

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
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FredT
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RE: Question Re Turbine Engine Start

Sat Oct 08, 2005 3:53 pm

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.
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G4Doc2004
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RE: Question Re Turbine Engine Start

Sun Oct 09, 2005 12:26 pm

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.
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HAWK21M
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RE: Question Re Turbine Engine Start

Sun Oct 09, 2005 4:11 pm

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
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AGM114L
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RE: Question Re Turbine Engine Start

Sun Oct 09, 2005 7:06 pm

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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HAWK21M
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RE: Question Re Turbine Engine Start

Mon Oct 10, 2005 3:26 pm

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

Whats the story behind the name.
regds
MEL
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Venus6971
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RE: Question Re Turbine Engine Start

Tue Oct 11, 2005 11:05 am

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.
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G4Doc2004
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RE: Question Re Turbine Engine Start

Tue Oct 11, 2005 11:42 am

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.
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HAWK21M
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RE: Question Re Turbine Engine Start

Tue Oct 11, 2005 5:41 pm

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
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AGM114L
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RE: Question Re Turbine Engine Start

Tue Oct 11, 2005 10:31 pm

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.
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10mid
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RE: Question Re Turbine Engine Start

Wed Oct 12, 2005 3:49 am

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.
 
masseybrown
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RE: Question Re Turbine Engine Start

Wed Oct 12, 2005 4:05 am

Is it still possible to start a modern turbine with a "grenade" as in the old days of 707s?
 
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HAWK21M
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RE: Question Re Turbine Engine Start

Wed Oct 12, 2005 4:56 pm

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
I may not win often, but I damn well never lose!!! ;)
 
AvionicMech
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RE: Question Re Turbine Engine Start

Mon Oct 17, 2005 4:59 am

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.

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Greasemonkey
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RE: Question Re Turbine Engine Start

Tue Oct 18, 2005 1:57 am

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 motor that is meshed to the APU gearbox, this turns the compressor, and when sufficient pressure is reached, fuel enters the combustion chamber, ignites and drives the turbine wheels. Once the APU has reached the point where the reaction is self-sustaining (~50%N2)the starter disengages and you are free to warm your hands under the hot exhaust when working the line on those frigid midwest winters.

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HAWK21M
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RE: Question Re Turbine Engine Start

Tue Oct 18, 2005 5:00 pm



Found a few pics of the Lucas Aerospace CR201 GTS/APU
regds
MEL
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