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rjsampson
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A couple of questions about 787 Electrics

Mon Jun 19, 2017 6:01 pm

First: The engine starters, Each has two, and I have no doubt that each of them are some pretty weighty equipment. Would I be correct in assuming that it is for reduncancy? If both were inop conceivably (at which point I'm guessing you're not going anywhere with MEL): Can a GEnx be airstarted on the ground?

Second: The brakes. I'm trying to get my head around how electric brakes would work. It's very easy for mto visualize howerful hydraulic pressure pushing stator disks against rotors.

How do do electric brakes work? Are they mototized, spinning a jackscrew of some sort? If so, I would imagine those motors would need to spin up and down extremely quickly to have the same level of responsiveness and the same feel on the brake pedals as a hydralic system.
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
BravoOne
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 6:46 pm

They are starter generators so that is the reason you have two because the airplane needs al the electricity it can get. As for the brakes, Boeing does not really explain how they are built or designed, just some limits associated with their operations.

Sorry for the brief non technical answers but that's the way Boeing puts the FCOM together these days. If you can't do anything about it then you probably don't need to know anymore than the minimum knowledge required to operate the airplane safely.

Don't shoot the messenger.
 
ChrisKen
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 7:09 pm

rjsampson wrote:
Second: The brakes. I'm trying to get my head around how electric brakes would work. It's very easy for mto visualize howerful hydraulic pressure pushing stator disks against rotors.

How do do electric brakes work? Are they mototized, spinning a jackscrew of some sort? If so, I would imagine those motors would need to spin up and down extremely quickly to have the same level of responsiveness and the same feel on the brake pedals as a hydralic system.


You're right with the first bit of visualisation. Basically they work exactly the same way as 'normal', they're just commanded electrically rather than mechanically.
Electrics command pump/servo, pump drives hydraulics, hydraulics clamps discs/drum.




b787 units
http://utcaerospacesystems.com/cap/Docu ... 0Sheet.pdf
 
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Polot
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 7:16 pm

Electric brakes work via magnets. Electricity flowing though an electromagnet magnetizes it (magnet strength based on current strength), and through some clever engineering you can control and use the magnet's movement to force the brake shoes on the brake drums or the brake pad on the disc, depending on brake type, with braking strength dependent on magnet strength, and thus the amount of electricity you pump through it.

I doubt there is much response difference compared to hydraulic lines, electrons and magnetic forces move pretty fast ;) You just have to make sure you have a strong and consistent electrical current to the brakes, which has been the limiting factor for electric brakes in the past. As soon as the power is cut the electromagnet is no longer magnetized and you don't have breaking action.
 
RetiredWeasel
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 7:33 pm

ChrisKen wrote:
rjsampson wrote:
Second: The brakes. I'm trying to get my head around how electric brakes would work. It's very easy for mto visualize howerful hydraulic pressure pushing stator disks against rotors.

How do do electric brakes work? Are they mototized, spinning a jackscrew of some sort? If so, I would imagine those motors would need to spin up and down extremely quickly to have the same level of responsiveness and the same feel on the brake pedals as a hydralic system.


You're right with the first bit of visualisation. Basically they work exactly the same way as 'normal', they're just commanded electrically rather than mechanically.
Electrics command pump/servo, pump drives hydraulics, hydraulics clamps discs/drum.




b787 units
http://utcaerospacesystems.com/cap/Docu ... 0Sheet.pdf


I don't think there are any hydraulics involved in the 787's brakes. Watch this:

http://www.youtube.com/watch?v=n0nXiQPwHE8
 
ChrisKen
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 7:39 pm

EBA is an Electronic Brake Actuator - Brake Fluid under pressure moves the drum/clamps Fluid = Hydraulic.

The system is essentially the same. Just an electronic link between the pedals and the brakes, and the smaller hydraulic system is self contained on the wheel assembly,
 
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Polot
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 7:53 pm

ChrisKen wrote:
EBA is an Electronic Brake Actuator - Brake Fluid under pressure moves the drum/clamps Fluid = Hydraulic.

An electric actuator does not need fluid to work.

Again you use the the electromagnetic properties of electricity to make things move, and through careful design you control the movement and what it does. Every electronic device that does something mechanical (i.e. an electric pencil sharpener, or a fan) does not have fluid in it.
Last edited by Polot on Mon Jun 19, 2017 8:01 pm, edited 1 time in total.
 
RetiredWeasel
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 7:58 pm

ChrisKen wrote:
EBA is an Electronic Brake Actuator - Brake Fluid under pressure moves the drum/clamps Fluid = Hydraulic.

The system is essentially the same. Just an electronic link between the pedals and the brakes, and the smaller hydraulic system is self contained on the wheel assembly,


That's not my understanding. These EBA are all electrical/mechanical. There is no fluid under pressure doing any work.
 
ChrisKen
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Re: A couple of questions about 787 Electrics

Mon Jun 19, 2017 8:07 pm

I stand corrected. I thought the initial units were hybrids, I see it's moved on.
 
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Balerit
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Re: A couple of questions about 787 Electrics

Tue Jun 20, 2017 12:32 pm

As the OP suggested, they are all electric rotary jackscrews, probably operate similar to stepper motors.

A control unit drives the electric braking system and is linked to motors that convert electric power to drive gear wheels, which in turn drive a ball screw and nut. The screw is linked to a piston that presses the carbon disks together. The piston is no longer hydraulic.


http://www.ainonline.com/aviation-news/air-transport/2006-11-14/all-electric-brakes-set-boeing-787-year-end
Licensed Aircraft Maintenance Engineer (retired).
 
BravoOne
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Re: A couple of questions about 787 Electrics

Tue Jun 20, 2017 3:22 pm

Well that pretty much sums it up as I know a couple of these poster are qualified and as you can see Boeing does not give the information required to tell it like it is. Kind of pathetic IMO.
 
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rjsampson
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Re: A couple of questions about 787 Electrics

Wed Jun 21, 2017 5:43 am

Balerit wrote:
As the OP suggested, they are all electric rotary jackscrews, probably operate similar to stepper motors.

A control unit drives the electric braking system and is linked to motors that convert electric power to drive gear wheels, which in turn drive a ball screw and nut. The screw is linked to a piston that presses the carbon disks together. The piston is no longer hydraulic.




Huh. I didn't really expect to be correct on a hypothetical thought, for once (someone please tell my wife about this!) I was surprised to learn that, this was in fact how those brakes work. That Jackscrew must have some crazy torque tolerances to go back and forth, with the slightest/strongest pressure being applied and released by the pilot on those pedals. It boggles my mind that a screw can spin with the same responsiveness that a hydraulic application of pressure would apply, with hydraulics being a much simpler system.

I'll wait for YouTube videos in a few years showing this Jackscrew in action. I can only wonder what kind of beefy, agile motor could accomplish this.
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
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Balerit
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Re: A couple of questions about 787 Electrics

Wed Jun 21, 2017 6:38 am

He he he, just remember that the travel is very small and being geared it would apply quite a force, especially as there are a number of them. Will be interesting to see a video of how they work.
Licensed Aircraft Maintenance Engineer (retired).
 
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rjsampson
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Re: A couple of questions about 787 Electrics

Fri Jun 23, 2017 9:30 pm

Balerit wrote:
He he he, just remember that the travel is very small and being geared it would apply quite a force, especially as there are a number of them. Will be interesting to see a video of how they work.


Got it. (Being an IT guy, and using the only metaphor I can understand): I'm visualizing an old-school, spindle-style Hard Disk Drive and how incredibly responsive a hard disk's motor is, on constantly changing input. In my feeble mind, I suspect the motors on electrical brakes are very similar (and also geared), just with a massively higher current applied.

Thanks for the explanation! So anyone, feel free to correct me if my visualization is wrong: The motors braking a 787 are essentially like a spindle hard-disk drive with several orders of magnitude of power applied to the spline and subsequent gears. Tiny gears in an HDD, are analogous to much larger gears in electrical brakes, but they ultimately function to connect rotary plates with stator plates in a brake assembly, while retaining incredible sensitivity to the input applied. Am I on the right track?
"..your eyes will be forever turned skyward, for there.." yeah we know the DaVinci quote. But GA is so dang expensive these days! :(
 
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Balerit
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Re: A couple of questions about 787 Electrics

Sat Jun 24, 2017 9:56 am

The inner workings might be propriety information at the moment and the closest I can find is this view. Looking at the silver part it seems to have two gear sets and probably a linear actuator such as you're visualizing:

Image
Licensed Aircraft Maintenance Engineer (retired).

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