787 APU?

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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Some aircraft can only use the APU on the ground, as Polar mentioned. Others, like the one's I flew, have altitude restrictions to using the APU airborne, which is only done during an emergency anyway for either backup electrical or hyd.

As already mentioned, APUs are used for very specific reasons, and have limitations. They aren't always running.
 
clayfenderstrat said:
If you have the resources, why not use them?
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Drill Here, Drill NowARARGRGHAHRG!!!! :cwm27:

Having pneumatic systems that run off bleed air makes many more design operating points for an engine, which increases complexity... and weight, cost, and possibly, failure modes.

"But these systems have been used successfully in airmobiles for half a decade," you might wonder. True. I imagine the thinking was along the lines of "Hrmm... we seem to be the only industry to transmit large amounts of power using air as a working fluid. Everyone else is using hydraulics and electrics. This technology must be a dead end, let's do efficiency and performance studies for a bleedless jet."

And then they're all like "Wow, we can have common high-capacity electrical buses and throw out all the weight and cost of pneumatic plumbing. Let's go invent a near-supersonic transport just to mess with Rémy and Olaf."
 
Let's just see if if Boeing can get the plane into service. Yeah, their stock price is up big time, as are the delays with the 787. I expected better from them. As far as I am concerned they can start the thing with Squirrels on a conveyor belt.
 
Coming from an airliner that has no APU and relies heavily on a GPU, it is quite embarrassing when you think you can start on batteries, and the aircraft just cant do it and the passengers see that prop starting to go, then it stops. Alot of pax are already worried about flying on a "prop" aircraft.

Its annoying in a place like LGA when you have to push back, then you have to have a cart quickly drive out with a portable GPU, hook up, start up, then disconnect.

The Saab would be a much better aircraft with some bigger engines and an APU.

This has nothing to do with the 787, but for those questioning the point of an APU, welllllll its gotta be a nice thing to have.
 
Early flying machines used mostly electrics for systems. Then we got APUs and bleeds to provide pneumatics and other systems went to hydraulics. The 787, as I read, will still use some hyd but not the old stuff. 5000psi (like on Concorde). No bleed for engines.

The usual Boeing has a single generator on each engine and rated at 40KVa. The 787 will have DUAL 250KVa generators on each engine as well as dual generators on the APU. It will have almost 1.4GIGs of electrical power available to raise/lower the gear/flaps, nosewheel steering, power the air conditioning and pressurization systems, etc. No long pipes to contain the hot bleed air. No long hyd lines to leak.

V1valarob said
The Saab would be a much better aircraft with some bigger engines and an APU.
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I think that is called a Saab 2000. (just got caught at the tailend of the regional turboprop much like the Lockheed Electra with the early jets. Shame because the 2000 was/is probably more advanced than most of the RJ stuff that came after it. Unfortunately it just had those wingy-things whirling around outside the engines and most pax wouldn't know doodly about the advanced technology that went into the 2000 versus the RJ spam-can)
 
Orange Anchor said:
I think that is called a Saab 2000. (just got caught at the tailend of the regional turboprop much like the Lockheed Electra with the early jets. Shame because the 2000 was/is probably more advanced than most of the RJ stuff that came after it. Unfortunately it just had those wingy-things whirling around outside the engines and most pax wouldn't know doodly about the advanced technology that went into the 2000 versus the RJ spam-can)
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I sat in a Saab 2000 sim, and yes, it is completely different on the inside when compared to the Saab 340. I would put the Saab 2000 and Q400 in the same group. At least that's what it looked like from the drivers seat.

Unfortunately in the markets that I fly the Saab 340 in, a Saab 2000 is just too big
 
Orange Anchor said:
V1valarob said I think that is called a Saab 2000. (just got caught at the tailend of the regional turboprop much like the Lockheed Electra with the early jets. Shame because the 2000 was/is probably more advanced than most of the RJ stuff that came after it. Unfortunately it just had those wingy-things whirling around outside the engines and most pax wouldn't know doodly about the advanced technology that went into the 2000 versus the RJ spam-can)
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To continue in that vein, XJT came real close to buying the Saab 2000 instead of the ERJ in the mid-90's. At the time the longest route that was planned was still just over 500nm. The Saab could theoretically do those legs faster and more efficient than the E145.
 
v1valarob said:
I sat in a Saab 2000 sim, and yes, it is completely different on the inside when compared to the Saab 340. I would put the Saab 2000 and Q400 in the same group. At least that's what it looked like from the drivers seat.

Unfortunately in the markets that I fly the Saab 340 in, a Saab 2000 is just too big
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When one gets into all the tech the Swedes put into that airplane, it is truly sad that it was so long delayed due to the unacceptable changes in elevator feel when configuring. They used cray computers to design the props and also where to place the engines. It was part of a study of how noise enters the cabin including structure and aero. And as I remember, cruise RPM on those props is only 970.

Big picture (sorry) of the Hendricks race team's 2000. I believe Joe Gibbs also has one. Nice view of those props.

Haiti_Day8-112.jpg
 
Orange Anchor said:
They used cray computers to design the props and also where to place the engines. It was part of a study of how noise enters the cabin including structure and aero. And as I remember, cruise RPM on those props is only 970.
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I've seen those Hendricks airplanes in HPN a few years ago. Good looking airplane. I saw either 2 or 3 of them there, can't remember.

On the Q400, the cruise RPM is 850.
 
Getting back to APU bleed air....The "quality" of the air is such that it isn't always able to do the things that engine bleed air can do.

-For one we can't be running APU bleed air as Anti-Ice in flight. It isn't hot enough and the logic doesn't allow for it to be used in conjunction with the engines to pressurize and anti-ice simultaneously.

-Can't pressurize the cabin above FL250 (somewhere around there) just using the APU bleed. Not enough output.

I thought Hendriks ditched the Saab 2000 in exchange for a jet.
 
If you don't need to tap bleed air out of the APU then you can construct a smaller lighter more efficient APU to just drive the GEN. Anytime you tap bleed out of a compressor you are reducing its efficiency.
 
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