Boeing wins $10 bln Qantas jet order (Its Boeing, baby! Another $10B that Airbus DOESNT get)

"The 787 is doing away with almost all that, and is switching to electrical powered devices."

That's news to me.

I haven't run the numbers, but it would seem that using bleed air from the efficient compressor stage of an engine would be much lighter in weight, simpler and much more reliable than adding a several hundred horsepower electric air compressor that the ship has to lug around all its life. Mechanical work still has to be done, so it gets robbed from the engine in some way, no matter what particular system gets used.

The engine is already a very efficient air compressor, so why not use it, rather than increase the load on the spindle shaft and require more shaft energy from the engine to run a larger, heavier generator, to run a several hundred horsepower electric motor, (more added weight) to run another air compressor (still more added weight) for cabin pressurization?

Something tells me that good ol' Boeing may have bitten off more than it can chew with this new 787.

Here's a good read on the reason for the switch to electrical. It's about a third of the way down:

http://www.cocardes.com/articles.php?lng=en&pg=366

Basically, they say pneumatic and (infant) electrical systems are currently equal in efficiency, but pneumatic technologies peaked around a decade ago while electronic stuff has the sky for the limit. Add to that only a few companies are in pneumatics (in a very limited and mature market) while everyone and his brother are working on improving power electronics and electrical devices in the quest for efficiency.

Also, forgot to mention that the bleed air system in an aircraft frequently requires maintenance and the extensive ducting takes up valuable space that could otherwise be used for revenue service. I also wonder about the compatibility of 350 degree bleed air ducts in a composite aircraft.

Dang, why can't you edit?

For you non-link readers, here's selected quotes from the the article.

Snip

Why did Boeing make the jump to electric pressurization? Some of the answers seem to be already fading in the mists of time and computer programs as the aircraft rapidly becomes more solid, and not everyone gives the same answers.

A top reason is that Boeing feels that power electronics, which are key to the all-electric airplane, are on a steep curve of performance and cost improvement, while pneumatic systems growth "tapped out" around 1995, says Michael K. Sinnett, Boeing chief engineer for 787 systems. At the moment, the performance of pneumatics and electrics is roughly similar, but electrics are poised for growth and pneumatics are not.

"This was a Boeing gut decision, and I think they made a good decision," says Clifton D. Jacobs, Hamilton Sundstrand Electrical Systems vice president and general manager. "If we get a high-temperature power transistor that needs less cooling, the system will be better later. I don't see much further improvement in bleed."

Along the same line, Boeing believes it should get its technology more from the broader industrial market, which has a large investment in better designs, rather than from what it calls the "boutique" aircraft business--and electrics have broader use than pneumatics. The billions of dollars being thrown at hybrid cars affects the motors and controllers that Boeing will use.

But do electrics burn less fuel? "When we decided on electric pressurization, it lowered aircraft empty weight 1,000-2,000 lb. and fuel burn was down several percent," Sinnett says. "But the numbers got muddied as the 787 got integrated. It's hard to say where the weight has gone."

He says the main reason the electric cabin is more efficient is that modern engines compress and heat the air too much--that energy is thrown away in the precooler and excessive expansion. At current 30-psia. bleed pressures, the amount of wasted energy is about 30%. The electric compressor creates less pressure and less waste.

"A top reason is that Boeing feels that power electronics, which are key to the all-electric airplane, are on a steep curve of performance and cost improvement, while pneumatic systems growth "tapped out" around 1995, says Michael K. Sinnett, Boeing chief engineer for 787 systems."

But he doesn't cite any specifics. His reasoning reeks of "Gee, we need to use the latest technology or else we will be laughed at," not that it produces a better airplane.

So far, there are good arguments against his reasoning for an all-electric airplane, but I wish him the best.

He says the main reason the electric cabin is more efficient is that modern engines compress and heat the air too much--that energy is thrown away in the precooler and excessive expansion. At current 30-psia. bleed pressures, the amount of wasted energy is about 30%. The electric compressor creates less pressure and less waste.

Yes, why spend the extra fuel to produce heat and excess compression that only goes to waste?

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