It's an interesting development. The publicity factor with a safety related issue always bears the potential to outweight the utility, and I know that with lithium-ion's reputation their will be no shortage of executives taking the opportunity to staid the concern. Mr McNerney himself will be keen to see the Dreamliner's presence in news ahead of the A350, given the latter has marginally better specifications. I've had the pleasure of working on the topical laminates for both jets during testing and I have to say, the budgetary restraints continued to lapse on either side as the project continued. Everything is related to appearances and neither side wants to be outdone in even the most remote aspect of test and dev, given the possible reprecussions the media has motivation to slant a certain way. I was at http://www.ventec-usa.com while the match-up was unfolding and there were optimisation concerns from both parties which at one point I thought would never be settled
Now it's almost 18 months since Dream's commercial introduction, and with the A350 hurtling towards us from beyond the horizon, it's about time there was a united front for all disciplines in all departments at Boeing! Although I'm certain the engineers have done a fantastic job despite the on-going pressure which to be frank, has been there since conception.
"The entire argument for the transition from hydraulic to electrical, especially combined with "battery storage," has been to reduce the requirement for the power being tapped off the engines on a continual basis. The idea is, you take a little bit of power, which is used to charge the batteries... and you discharge the batteries (in "lump amounts") as needed."
It doesn't matter if the power is being "tapped off the engines on a continual basis", the load on the engines and its effect on fuel consumption is only a function of the current being drawn, assuming the voltage being generated must be at fixed constant value.
The sizing of the generators depends on the peak current load that must be handled, but only the actual electrical load being delivered by the engine at any given moment, whether the load is constant, variable or intermittent, will affect fuel usage. No load, essentially no additional fuel usage except to cover minimal constant losses within the generator due to windage, bearing friction, etc.
They should have backpedalled to a less finicky, more extinguishable battery technology until they get their act together. I can't imagine that they could easily overcome Lion's overtemp/undertemp restrictions in the environs of a commercial aircraft.
Totally agree the swich from bleed air and hydraulics over to electrics is fuel burn. BTW, during revenue service before the grounding, airlines reported fuel burn bettered spec by 1-2%, which as you know is huge in the airline biz.
Other reasons for moving away from hydraulics are one less system to maintain, less weight, easier system integration into the airframe, better "coupling" to software control (and thus other systems), easier monitoring and maintenance, more environmentally-friendly, etc. Of course a leaky hydraulic fitting seems way easier to detect and fix than an error code on the maintenance console and the difference between hydraulics and electrics ought to add more reliability through "diversity".
I'm open to being wrong as I know you have a lot of knowledge too. I could very well be misinterpreting the Avweek link. Also as a "sparky", the battery capacity seems a drop in the bucket compared to total electrical load. There may be other battery banks that do what you describe....of course the LAST thing Boeing wants to do at this point is say..."oh, you know there's much larger batteries on the 787" ! :)
I hate to keep arguing with you... but this is the only reason that the aviation business has moved away from hydraulic systems to electrical system, for, say, actuating the gear, or the flaps, or the control surfaces in some cases.
They've moved away from direct hydraulic to electrically-powered devices for the EXACT reason I described. You can store up electrical energy, over time, and discharge it "on demand," reducing parasitic loses and thus improving overall fuel efficiency.
There is ZERO DOUBT about what I'm describing. This is hard fact.
There's quite literally no other reason to move from a hydraulic system to an electrical system, is there?
I'll concede that your point about "remote self-starting" COULD be accurate... though every airport I've ever deal with would just roll out a "start cart" to the aircraft, and I've never seen a commercial airliner have to start itself. The "in-flight restart" scenario does seem quite a bit more likely... and I know this is a real requirement, in fact. But normally, to do this, they'd switch of other in-flilght systems (lighting, entertainment, heating, etc). Today's aircraft use a lot more batteries than older ones, because they're supporting a far, far higher electrical load... including the various "replaced hydraulics" I mentioned previously.
I'm worried that maybe you think I'm stating a direct paralle between the passenger bus (which uses stored electrical power for MOTIVE FORCE) and the aircraft (which uses stored electrical power for momentary high-demand actions like, for example, deploying flaps or landing gear).
But the POINT remains the same, and I'm absolutely clear on this... the reason for switching from hydraulic to electrical power is to permit you to burn less fuel throughout the flight, by slowly charging the batteries and rapidly discharging them in burst to do brief "high demand" operations, rather than having a perpetual load on the engines capable of supplying that full load at any given instant, and serving only as losses the rest of the time.
That's the argument which was at the core of the 787's overwhelmingly electrically-driven design... all in the name of improving fuel efficiency.
This isn't a "debatable" point... so... you can take my word for it, or reject what I'm telling you... but I'm not going to bother to restate this another time.
Your comments are accurate, but not in the context of the 787. Here's a good link that explains my earlier comments: http://www.aviationweek.com/Blogs.aspx?plckBlogId=Blog:7a78f54e-b3dd-4fa6-ae6e-dff2ffd7bdbb&plckPostId=Blog:7a78f54e-b3dd-4fa6-ae6e-dff2ffd7bdbbPost:5e8acfbf-0e5f-4354-a6f3-f19ec0ac6556
You're right about ground power from a cable or "start cart". But that's not always available at the far corner of the maintenance or overnight parking ramp. The more important thing is twin-engine aircraft have a requirement called ETOPs to fly over water or isolated areas. The higher levels of ETOPS certification require a cold-start APU (cold in both the literal and figurative sense). In case of a double failure of both engines the APU needs to be started on a stand-alone basis after being cold-soaked to -70C in flight...not an easy requirement!
The batteries are definitely not used as a "reservoir" floating on the main bus or to handle parasitic losses as you describe with ground vehicles.
My knowledge is second-hand too...just from working with avionics, as an "aerogeek", and a private pilot. So, I could stand corrected but everything I've seen from Boeing, NTSB, FAA, etc. support the Avweek link.
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