Another APU/jet systems question

n57flyguy

n57flyguy

Well-Known Member
Hey again guys. I had another APU question about APUs and how it relates to something else. If the APU is started by battery, how does the battery actually start the APU? Kind of basic yet seems to be complex.
My other question. How does the bleed air start the main engines on a turbine aircraft? I dont know if Im over complicating things again here, but I couldn't find it antwhere else, thanks again guys.
 
Well I dont fly turbine aircraft but ill give this a shot. The battery runs a starter just like in your car that spools up the APU to get sufficient airflow going then fuel is added and ignited. Bleed air is just compressed air taken from the compressor section of the APU. This air is used to spool up the engines to get enough airflow going to prevent a flame out and fuel is added and the engines starts. This is the simple answer, im sure people who know twice as much as I do can give you something more detailed.
 
n57flyguy said:
If the APU is started by battery, how does the battery actually start the APU?
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If the APU is electrically started, then it would have an electric starter, very similar to a car. The starter is nothing more than a small, strong electric motor, that spins the APU up until it can sustain itself with combustion.

How does the bleed air start the main engines on a turbine aircraft?
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In almost all modern jet engines with pneumatic starters, the bleed air is used to spin a starter motor (which does the same thing that an electric starter does). That bleed air is under alot of pressure, so it has no problem spinning the engine up!
 
n57flyguy said:
is it simaler to an alternator?
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The starter? Ummm . . . no. An alternator is turned by the engine, producing electricity. A starter TURNS the engine, using electricity to turn. Ever have an electric remote control car as a kid? Ever tear it apart and play with the tiny electric motor inside? Kinda like that!
 
mtsu_av8er said:
The starter? Ummm . . . no. An alternator is turned by the engine, producing electricity. A starter TURNS the engine, using electricity to turn. Ever have an electric remote control car as a kid? Ever tear it apart and play with the tiny electric motor inside? Kinda like that!
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Oh, thought I was off on that. Sorry this is kind of repeitive but, the power from the battery supplies electricty to the APU to turn the turbine shaft in the APU to start it, Bleed air from the APU is supplied to the main engines (this is where I get confused, Is it similar to the turbine fans that compress the air when an engine is running, except the compressed air is the product of the APU that enters at a smaller amount that gets the turbine shaft in the mains going?) then she makes the loudest most beatiful sound on earth,

Waht is a flameout?
 
n57flyguy said:
Oh, thought I was off on that. Sorry this is kind of repeitive but, the power from the battery supplies electricty to the APU to turn the turbine shaft in the APU to start it, Bleed air from the APU is supplied to the main engines (this is where I get confused, Is it similar to the turbine fans that compress the air when an engine is running, except the compressed air is the product of the APU that enters at a smaller amount that gets the turbine shaft in the mains going?) then she makes the loudest most beatiful sound on earth,

Waht is a flameout?
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Whoa - run on-sentence!!!:insane:

The battery supplies electricity to start the APU through a starter. The starter is a small, strong electric motor, that turns the APU until the APU can sustain itself (very simplified version).

The APU can then produce bleed air (it's taken from the compressor section of the APU), which can be used to spin a pneumatic starter. Thing of that starter as an air pump, working backwards. The very compressed air turns the starter, usually by rotating an impeller. this starter is connected, through a gearbox, to the shaft of the main engine. Once it's spinning, it will eventually be able to sustain itself.

Make more sense?

A flameout is when, for some reason, the combustion stops. Then, the engine stops!
 
n57flyguy said:
Waht is a flameout?
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Yeah! I can sort-of answer that question. Flameout is what it sounds like: Flame Out. The flame/fire which is contained in the combustion chamber of a jet engine is "suspended in mid-air," (dosn't touch the sides of the chamber), and basically creates the combustion of the fuel/air mixture by igniting it with it's super hot temperatures and creates high pressure and high velocity gases which essentially turn the turbines and propel the aircraft. However, the flame is not indestructible, it can be extinguished many ways: by a compressor stall, lack of fuel, lack of oxygen, ingestion of contaminates (water etc), mechanical malfunction etc. When this occurs, it is call a flameout.
 
One thing I think you may be confused about is this.

The APU is an entirely seperate engine, completely independant from the others hanging from the wings.

An APU is a "Turboshaft" engine which uses a jet engine to turn a driveshaft, thei driveshaft can be conected to anything. In adition to bleed air they can supply electrical power, hydraulic power, heat for the cabin, and anything else you want to hook it up to.

On most airliners they most comonly provide bleed air for starting, but that is not all they can do (depending on the model).
 
USMCmech said:
One thing I think you may be confused about is this.

The APU is an entirely seperate engine, completely independant from the others hanging from the wings.

An APU is a "Turboshaft" engine which uses a jet engine to turn a driveshaft, thei driveshaft can be conected to anything. In adition to bleed air they can supply electrical power, hydraulic power, heat for the cabin, and anything else you want to hook it up to.

On most airliners they most comonly provide bleed air for starting, but that is not all they can do (depending on the model).
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Yes I knew that it was seperate from the other jet engines, thanks for looking out though. I never knew that The APU actually had a drive shaft that actually powered some of the aircraft systems. Thanks again. TWO questions, i know this varies on certain model aircraft, but generally, where is the APU in an aircraft? I figured it would be towards the back. What do you mean by "its not all they can do"?
 
n57flyguy said:
TWO questions, i know this varies on certain model aircraft, but generally, where is the APU in an aircraft? I figured it would be towards the back. What do you mean by "its not all they can do"?
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My logical guess would be that it is located in/near the tail of the aircraft to assist in the expelling of the gases which the APU produces. (You can see it on a lot of aircraft real easy like the A318.

I also know that several (if not all) APU units also provide services while on the ground. They can harness the electricity needed to light the cabin etc, and they also handle the air and powering of the air conditioning unit (if equipped).
 
Jeppesen has a pretty good book that covers transport category aircraft systems, you might want to look for it. A more basic book that is also ok is The Turbine Pilot's Flight Manual.
 
ananoman said:
Jeppesen has a pretty good book that covers transport category aircraft systems, you might want to look for it. A more basic book that is also ok is The Turbine Pilot's Flight Manual.
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Cool, Ill check it out

BTW sorry for my crappy spelling before.
 
n57flyguy said:
Hey again guys. I had another APU question about APUs and how it relates to something else. If the APU is started by battery, how does the battery actually start the APU? Kind of basic yet seems to be complex.
My other question. How does the bleed air start the main engines on a turbine aircraft? I dont know if Im over complicating things again here, but I couldn't find it antwhere else, thanks again guys.
Click to expand...

The battery will start the APU just like the battery in a Warrior starts the engine, simply supplies power to a starter to spin the APU. Remember an APU is nothing more than a "small" jet engine and the starting cycle is fairly automated, just verify you have adequet battery voltage (22 volts usually) and press the start button.

Bleed air, either supplied by the APU or by a ground cart (huffer), turns a starter that is mechanically connected to the N2 section (core section) of the engine. As the N2 section is spun up, air is drawn through the front fan (N1) and that airflow starts spinning the N1 turbine at the rear. Once you reach a specific N2 (20- 25% on the DC8) and a specific time (30 seconds usually) you introduce fuel. You will get a lightoff in about 5 seconds, depending on altitude, but you keep the starter engaged until about 50% N2. When you release the starter (closing a valve that cutsoff the airflow to the starter) the engine will continue to acclerate to about 65% N2 where it is considered "stabilized". The reason for keeping the starter engaged is to keep the rotational speed of the engine up while fuel is being introduced. The most "dangerous" situation for a turbine engine is on startup, low airflow, and lots of fuel. A turbine remains cool be airflow through the engine, if fuel was allowed to just burn in side the combustion chamber with no rotation of the engine, you would cook that engine very quickly.

Clear as mud yet?
 
wow thanks! Love your sig! I had quoted Dark helmet before on my sig but I deleted it.

Yeah that makes alot more sense now. All that stuff I read in a G-IV Pilot training manual is making more sense now, connecting every thing. really in depth, thaks alot!
 
n57flyguy said:
Yeah that makes alot more sense now. All that stuff I read in a G-IV Pilot training manual is making more sense now, connecting every thing. really in depth, thaks alot!
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Please go over with me again how old you are and what ratings you have?
 
AlexF said:
Please go over with me again how old you are and what ratings you have?
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Ha ha! My freind who works at Flight Safety International, gives type ratings for the GV/IV, and flies GV/IVs and Hawkers gave it to me. I would be worried If I was type rated and asking these questions. I ask them because some things I don't understand in it. BTW, Im 15.
 
n57flyguy said:
Ha ha! My freind who works at Flight Safety International, gives type ratings for the GV/IV, and flies GV/IVs and Hawkers gave it to me. I would be worried If I was type rated and asking these questions. I ask them because some things I don't understand in it. BTW, Im 15.
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:) I thought it was something like that. Just remember, you'll be flying single-engine piston aircrafts, so make sure you know all you can about that first, cause when your engine fails, knowing why the compressor suddenly gave up, or how to "re-ignite" (for lack of a better word) a jet engine won't help one bit! It'll make you sound pretty smart though!
 
AlexF said:
:) I thought it was something like that. Just remember, you'll be flying single-engine piston aircrafts, so make sure you know all you can about that first, cause when your engine fails, knowing why the compressor suddenly gave up, or how to "re-ignite" (for lack of a better word) a jet engine won't help one bit! It'll make you sound pretty smart though!
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Yeah I know, I am just really interested in this stuff, its fun to learn and it does make me look smart. I know I should concentrate on piston arcraft right now and I have actually done more reading on it than Jet Systems, I ask all the Jet stuff here.:)

Thanks for all your help guys
 
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