787 APU?

clayfenderstrat

clayfenderstrat

Well-Known Member
Anybody know anything about the APU system on the 787? I know that there is no bleed air, so is the APU electric generators hanging off the engines, or is there a conventional APU? I have been looking around, but can't find a definitive answer.......
 
I googled "787 APU" and the first entry is from the Boeing AERO magazine.

There is a traditional APU present, but does not involve any bleed air.

On the engines, there are dual starter/generators... the starters are essentially reversing the flow of current, momentarily overdriving the generators as electric motors. The generators also function as controllers for the cabin pressurization compressors.
 
Minuteman said:
I googled "787 APU" and the first entry is from the Boeing AERO magazine.

There is a traditional APU present, but does not involve any bleed air.

On the engines, there are dual starter/generators... the starters are essentially reversing the flow of current, momentarily overdriving the generators as electric motors. The generators also function as controllers for the cabin pressurization compressors.
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Guess things changed then. We didn't get the contract last I heard Sunstrand did. Our engineeers at work were starting that the APU would be akin to a "starter" in a car and not a traditional APU!
 
Maximillian_Jenius said:
Guess things changed then. We didn't get the contract last I heard Sunstrand did. Our engineeers at work were starting that the APU would be akin to a "starter" in a car and not a traditional APU!
Click to expand...

That is actually a traditional APU. All that I have used contained starter/generators.
 
Minuteman said:
I googled "787 APU" and the first entry is from the Boeing AERO magazine.

There is a traditional APU present, but does not involve any bleed air.

On the engines, there are dual starter/generators... the starters are essentially reversing the flow of current, momentarily overdriving the generators as electric motors. The generators also function as controllers for the cabin pressurization compressors.
Click to expand...

Yeah, I read that article, and I don't understand what the traditional APU is for. From what I understand, the generator/starters are hanging from the engine. That is what has me confused....
 
I would think the conventional APU would function..well...conventionally, less bleed air function. To provide electrical power for the aircraft when the engines are not running.
 
clayfenderstrat said:
Yeah, I read that article, and I don't understand what the traditional APU is for. From what I understand, the generator/starters are hanging from the engine. That is what has me confused....
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The electricity for the starters has to come from somewhere. It obviously can't be the engines because they are not turning yet. That leaves either an APU or a GPU. Can't be a battery because they only last for minutes whereas APUs last for hours and GPUs are external.
 
JoelT said:
The electricity for the starters has to come from somewhere. It obviously can't be the engines because they are not turning yet. That leaves either an APU or a GPU. Can't be a battery because they only last for minutes whereas APUs last for hours and GPUs are external.
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If you are still carrying an APU onboard, what is the advantage of not using the bleed air that it is producing? I understand how an APU works, but what's the point of putting such a big piece of equipment onboard just to start the engines, then not use it for anything else? The 727 can only use it on the ground (not in-flight), and I thought Boeing didn't want to do that anymore.....
 
clayfenderstrat said:
If you are still carrying an APU onboard, what is the advantage of not using the bleed air that it is producing? I understand how an APU works, but what's the point of putting such a big piece of equipment onboard just to start the engines, then not use it for anything else? The 727 can only use it on the ground (not in-flight), and I thought Boeing didn't want to do that anymore.....
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We only can use our APU on the ground.

IIRC, all the 787 systems are electric. If so, the APU will power the AC at the gate and the lights and the galley. Even the little turd grinder in the lav will be powered at the gate.

Such a nice little gem that APU.
 
Polar742 said:
We only can use our APU on the ground.

IIRC, all the 787 systems are electric. If so, the APU will power the AC at the gate and the lights and the galley. Even the little turd grinder in the lav will be powered at the gate.

Such a nice little gem that APU.
Click to expand...
Why wouldn't they just use ground power (external) and save the trouble and weight of running an APU?
 
clayfenderstrat said:
Why wouldn't they just use ground power (external) and save the trouble and weight of running an APU?
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Some places may not have ground power, or it may be broken.

I have no idea about the 787, but the E170 (also a very electric jet) required very high quality ground power or it would get angry. The APU can provide that if the GPU can't.

Also, there may be places where you can't start engines at the gate either. In that case, starting the engines is a nice feature. Almost as nice as not sweating in the cockpit. ;)
 
clayfenderstrat said:
Why wouldn't they just use ground power (external) and save the trouble and weight of running an APU?
Click to expand...
We have to be able to start engines anywhere. We shut down all engines on taxiways all the time during ground stops to save fuel. The APU is what is powering the airplane then and sipping MUCH less gas. Basically, airliners have to be completely independent and self-contained.

Also, to cross oceans you have to have backup power supplies.
 
I understand all of the reasons of why you guys use an APU, but since they are putting one on the aircraft, why not use the bleed air that comes off of it? Isn't it inefficient to use it just for start or to supplement power while sitting on the ramp? If you have the resources, why not use them? I figured the electric conversion was supposed to save weight and make the systems less complex. If you are putting an APU on the aircraft anyway, why not completely utilize its potential?
 
clayfenderstrat said:
I understand all of the reasons of why you guys use an APU, but since they are putting one on the aircraft, why not use the bleed air that comes off of it? Isn't it inefficient to use it just for start or to supplement power while sitting on the ramp? If you have the resources, why not use them? I figured the electric conversion was supposed to save weight and make the systems less complex. If you are putting an APU on the aircraft anyway, why not completely utilize its potential?
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If no systems run off of bleed air, there is no reason to tap it off the compressor.
 
clayfenderstrat said:
I understand all of the reasons of why you guys use an APU, but since they are putting one on the aircraft, why not use the bleed air that comes off of it? Isn't it inefficient to use it just for start or to supplement power while sitting on the ramp? If you have the resources, why not use them? I figured the electric conversion was supposed to save weight and make the systems less complex. If you are putting an APU on the aircraft anyway, why not completely utilize its potential?
Click to expand...

Because, theoretically, the new brushless electric motors on the 787 are more efficient than using pneumatics. It all comes down to fuel usage.
 
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