Boeing's possible answer to A380: B747A

On Mon, 31 May 2004 17:52:55 +0800, Nik wrote:


"devil" wrote in message
news
On Sun, 30 May 2004 16:55:50 +0000, nobody wrote:

Difference is, in your car you use waste heat. In your plane you use
mechanically compressed air.


Well - in order to generate electricity in the amounts that would be needed
to heat the wings (you might go from -60C down to a rainstorm at 1C), move
the control surfaces flaps etc, keep the pressure in the cabin, have the
gear to get up and down and all at the same time - you would either have to
have some HUGE generators attached to the engines in which case you are also
using mechanically compressed air to generate electricity (as mechanically
compressed air that is being heated is what drives a turbofan). Now these
generators have to be so big that each of them can individually produce
enough power to cover the entire requirement at maximum system demand for
extended periods of time. The alternative is that you will need to have two
HUGE APU's to do the same job in which case you will not only need the
generators but also some device to move the generator (unless you are into
fuel cells - but I sincerely doubt that you at this stage would consider
this technology mature enough to be sued in an aeroplane).

None of which is related to the point above.

Of course the generators will have to be large enough, but you try to
make it sound like it's a major issue. They'll have to replace that air
thing. So, roughly same thing.

Furthermore, you can establish a hierarchy of priorities/level of
redundancy. Surely maintaining cabin comfort does not rank as high as
powering control surfaces. Having a single system rather than an electric
plus hydraulic plus pneumatic does probably provide for some
simplification and avoidance of duplications.

All this this missing the key point though. The issue is not about making
the auziliary stuff more or less heavy and/or efficient, but its' about
running the engines efficiently. And it does appear that there is a
substantial price to pay in that respect by having to design engines that
have to be able to handle variable/unpredictable amonts of bleed.

A compressor (mostly, in this case, axial) is a delicate animal, and it
clearly does not like having to be designed for a broad range of variable
conditions.

Anyway, it appears the whole issue is driven by engine manufacturers, who
seem to be otherwise running out of ideas on how to make their beasts more
efficient.

Now add to this that electrically driven devices - such as will be needed to
move the control surfaces bring the cabin pressure up etc etc - are
significantly heavier than pneumatic driven ones and the savings seems to be
going out of the window. As far as weight is concerned, pneumatic devices
are hugely efficient.

I am skeptical on that. As far as *efficiency itself,* they are surely
poor compared with electric drive.

Furthermore. A modern turbo fan compresses air in several stages. Most
of the air that it sucks in only passes on the outside of the fan's
core. Only a small proportion is going through the second and third
stage of compression before getting to the camber where it is heated.

Camber? I think you mean the combustion chamber. I don't think the bleed
air comes from the fan stage, where pressure is way too low to be useful
for most stuff. So it would have to come from some intermediate compresor
stage. From what we have heard, it sounds like in order to keep the
effect on the overall engine performance, they bleed air in a roughly
uncontrolled manner, so as to maintain the effect on the compressor
reasonably manageable.

I suppose you could make the thing even more complicated, thereby
resulting in an even bigger engine efficincy penalty?

Now, there are therefore (theoretically) several possibilities for
taking out air from a turbofan. You could take out the passing through
air, air after the second stage compression in the core or air after the
third stage (but before the heating). Non of these air will be hotter
than what the compression itself causes. really hot air you'll only get
after the air has been through the central burning camber.

It would be interesting if someone with more exact knowledge could tell
where in the process the bleed air is actually taken. Perhaps (as some
illustrations indicate) more than at one stage. Who knows?

To me the entire thing seems more and more to be a Boeing prank -
something like the Sonic Elephant! The Electrical Airplane seems to me
to be an idea developed in the PR rather than in the R&D department.


Sounds like this is what you want to believe?

But (1) the thing is driven mostly by engine folks, not Boeing, and (2)
having been bitten badly in recent years, I suspect they are serious with
this one. There is a need for a 767 replacement in their line, and they
do need ways to make it efficient.

"devil" wrote in message
news
On Mon, 31 May 2004 17:52:55 +0800, Nik wrote:


"devil" wrote in message
news
On Sun, 30 May 2004 16:55:50 +0000, nobody wrote:

Difference is, in your car you use waste heat. In your plane you use
mechanically compressed air.


Well - in order to generate electricity in the amounts that would be
needed
to heat the wings (you might go from -60C down to a rainstorm at 1C),
move
the control surfaces flaps etc, keep the pressure in the cabin, have the
gear to get up and down and all at the same time - you would either have
to
have some HUGE generators attached to the engines in which case you are
also
using mechanically compressed air to generate electricity (as
mechanically
compressed air that is being heated is what drives a turbofan). Now
these
generators have to be so big that each of them can individually produce
enough power to cover the entire requirement at maximum system demand
for
extended periods of time. The alternative is that you will need to have
two
HUGE APU's to do the same job in which case you will not only need the
generators but also some device to move the generator (unless you are
into
fuel cells - but I sincerely doubt that you at this stage would consider
this technology mature enough to be sued in an aeroplane).

None of which is related to the point above.

Of course the generators will have to be large enough, but you try to
make it sound like it's a major issue. They'll have to replace that air
thing. So, roughly same thing.

Generators are heavy by nature.


Furthermore, you can establish a hierarchy of priorities/level of
redundancy. Surely maintaining cabin comfort does not rank as high as
powering control surfaces. Having a single system rather than an electric
plus hydraulic plus pneumatic does probably provide for some
simplification and avoidance of duplications.

This might help a little. But the only thing that might be turned off is the
cabin preasure. Or do you want to land without gear down? Without flaps?
With iceed wings?

All this this missing the key point though. The issue is not about making
the auziliary stuff more or less heavy and/or efficient, but its' about
running the engines efficiently. And it does appear that there is a
substantial price to pay in that respect by having to design engines that
have to be able to handle variable/unpredictable amonts of bleed.

Neither - it is about having the most efficient system overall.


A compressor (mostly, in this case, axial) is a delicate animal, and it
clearly does not like having to be designed for a broad range of variable
conditions.

Anyway, it appears the whole issue is driven by engine manufacturers, who
seem to be otherwise running out of ideas on how to make their beasts more
efficient.

Perhaps.


Now add to this that electrically driven devices - such as will be
needed to
move the control surfaces bring the cabin pressure up etc etc - are
significantly heavier than pneumatic driven ones and the savings seems
to be
going out of the window. As far as weight is concerned, pneumatic
devices
are hugely efficient.

I am skeptical on that. As far as *efficiency itself,* they are surely
poor compared with electric drive.

One major problem here is that I - albeit not an engineer - cannot think of
any other technical system that performs individual tasks similar to
individual tasks performed in an airplane where pneumatic is not the
preferred option as technical solution. Be it when you need brute force or
high precision. You might well help my memory here.


Furthermore. A modern turbo fan compresses air in several stages. Most
of the air that it sucks in only passes on the outside of the fan's
core. Only a small proportion is going through the second and third
stage of compression before getting to the camber where it is heated.

Camber? I think you mean the combustion chamber.

I am no native...

I don't think the bleed
air comes from the fan stage, where pressure is way too low to be useful
for most stuff.

I don't know. Some drawings seems to have more than one bleed air outtake
where one of them as far as I can see seems to take bleed from the fan
stage - perhaps this air is compressed enough to keep the pressure up in the
cabin? We'll need an expert to explain this to us.

So it would have to come from some intermediate compresor
stage. From what we have heard, it sounds like in order to keep the
effect on the overall engine performance, they bleed air in a roughly
uncontrolled manner, so as to maintain the effect on the compressor
reasonably manageable.


If you mean the article mentioned in this NG I will not put too much trust
in the details of what it is saying (I just read it again yesterday). To me
this article seems to be too much into propaganda. The affirmations it makes
seems to me to be more motivated in their PR value rather than in their
technical accuracy. But it doesn't prove, of cause, that Boeing is not
serious.

I suppose you could make the thing even more complicated, thereby
resulting in an even bigger engine efficincy penalty?

If you are to put an even bigger generator on the engine that will need to
produce different amounts of energy during flight will also represent a
problem.


Now, there are therefore (theoretically) several possibilities for
taking out air from a turbofan. You could take out the passing through
air, air after the second stage compression in the core or air after the
third stage (but before the heating). Non of these air will be hotter
than what the compression itself causes. really hot air you'll only get
after the air has been through the central burning camber.

It would be interesting if someone with more exact knowledge could tell
where in the process the bleed air is actually taken. Perhaps (as some
illustrations indicate) more than at one stage. Who knows?

To me the entire thing seems more and more to be a Boeing prank -
something like the Sonic Elephant! The Electrical Airplane seems to me
to be an idea developed in the PR rather than in the R&D department.


Sounds like this is what you want to believe?

No - that is not what I want to believe. It is what I fear the most. For any
industry to be dynamic and inovative we - the consumers - need to have as
much competition as possible on all levels. What I fear the most is that
Boeing has given up on that.


But (1) the thing is driven mostly by engine folks, not Boeing, and (2)
having been bitten badly in recent years, I suspect they are serious with
this one. There is a need for a 767 replacement in their line, and they
do need ways to make it efficient.



Ad 1) doesn't sound good. No one needs just to have a more efficient engine
per se but an overall more efficient airplane.

Ad 2) I certainly hope that you are right - but I fear that you are wrong.
Depends I believe on the time frame that the leadership of Boeing is capable
of working with. If it is only a few years - a Wall Street perspective that
might be supported by stock option-incentives to the key decision making
people in the organisation - I do sincerely believe that Boeing is in huge
troubles as far as being an actor in the commercial airplane marked is
concerned.

Nik

On Tue, 01 Jun 2004 07:57:58 +0800, Nik wrote:


"devil" wrote in message
news
On Mon, 31 May 2004 17:52:55 +0800, Nik wrote:



Of course the generators will have to be large enough, but you try to
make it sound like it's a major issue. They'll have to replace that air
thing. So, roughly same thing.

Generators are heavy by nature.

Everything is heavy by nature. At least whgen in a gravitational field...

Furthermore, you can establish a hierarchy of priorities/level of
redundancy. Surely maintaining cabin comfort does not rank as high as
powering control surfaces. Having a single system rather than an
electric plus hydraulic plus pneumatic does probably provide for some
simplification and avoidance of duplications.

This might help a little. But the only thing that might be turned off is
the cabin preasure. Or do you want to land without gear down? Without
flaps? With iceed wings?

All this this missing the key point though. The issue is not about
making the auziliary stuff more or less heavy and/or efficient, but
its' about running the engines efficiently. And it does appear that
there is a substantial price to pay in that respect by having to design
engines that have to be able to handle variable/unpredictable amonts of
bleed.

Neither - it is about having the most efficient system overall.

Repeating the obvious. Which is why engine efficiency overwhelms
auxiliary systems.


A compressor (mostly, in this case, axial) is a delicate animal, and it
clearly does not like having to be designed for a broad range of
variable conditions.

Anyway, it appears the whole issue is driven by engine manufacturers,
who seem to be otherwise running out of ideas on how to make their
beasts more efficient.

Perhaps.


Now add to this that electrically driven devices - such as will be
needed to
move the control surfaces bring the cabin pressure up etc etc - are
significantly heavier than pneumatic driven ones and the savings
seems
to be
going out of the window. As far as weight is concerned, pneumatic
devices
are hugely efficient.

I am skeptical on that. As far as *efficiency itself,* they are surely
poor compared with electric drive.

One major problem here is that I - albeit not an engineer - cannot think
of any other technical system that performs individual tasks similar to
individual tasks performed in an airplane where pneumatic is not the
preferred option as technical solution. Be it when you need brute force
or high precision. You might well help my memory here.

I am an engineer. With a background in aerospace and in propulsion
incidentally, although not specifically about engine details.

And you are quite wrong in your last statement. Pneumatic is often not
the option of choice, and it's very doubtful that it leads to lighter
solutions.

It used to be in aerospace before the advent of computers and electronics,
but it has been losingh ground ever since. Wires are usually lighter and
easier to deal with than pipes. And air power tends to require relatively
large airflows. Finally air turbines tend not to be terribly efficient.
But if you go to WWII era planes, you'll see lots of air driven things,
because electric system were still hard to handle and relatively
inflexible. These things have changed a lot since, though.


Furthermore. A modern turbo fan compresses air in several stages.
Most of the air that it sucks in only passes on the outside of the
fan's core. Only a small proportion is going through the second and
third stage of compression before getting to the camber where it is
heated.

Camber? I think you mean the combustion chamber.

I am no native...

Neither am I.

But camber is a totally difference thing. One of these angles that need
to be adjusted when you get your car wheels to be aligned.

I don't think the bleed
air comes from the fan stage, where pressure is way too low to be
useful for most stuff.

I don't know. Some drawings seems to have more than one bleed air
outtake where one of them as far as I can see seems to take bleed from
the fan stage - perhaps this air is compressed enough to keep the
pressure up in the cabin? We'll need an expert to explain this to us.

So it would have to come from some intermediate compresor
stage. From what we have heard, it sounds like in order to keep the
effect on the overall engine performance, they bleed air in a roughly
uncontrolled manner, so as to maintain the effect on the compressor
reasonably manageable.


If you mean the article mentioned in this NG I will not put too much
trust in the details of what it is saying (I just read it again
yesterday). To me this article seems to be too much into propaganda. The
affirmations it makes seems to me to be more motivated in their PR value
rather than in their technical accuracy. But it doesn't prove, of
cause, that Boeing is not serious.

Was that the article in Aerospace America? If it is, I should have a
copy at the office. The cursory glance I gave it left me with the
impression that it was reaonably balanced, by no means Boeing spin. Last
time I checked, Aerospace America was a publication of the AIAA, not a
Boeing advertisement.

This said, of course, these days anyone does its best to play tge spin
game (don't you in your everyday life, like the rest of us?), so
obviously, both Boeing and airbus would try to get the most of any
interview/article.


I suppose you could make the thing even more complicated, thereby
resulting in an even bigger engine efficincy penalty?

If you are to put an even bigger generator on the engine that will need
to produce different amounts of energy during flight will also represent
a problem.

Your generator is never going to be of a significant size compared to the
power that they turbine needs to produce to drive the compressor. And
since your electrical system is going to be perhaps twice as efficient,
the power that will be needed will be half the power that the turbine
needs to produce for bleed air.



But (1) the thing is driven mostly by engine folks, not Boeing, and (2)
having been bitten badly in recent years, I suspect they are serious
with this one. There is a need for a 767 replacement in their line,
and they do need ways to make it efficient.



Ad 1) doesn't sound good. No one needs just to have a more efficient
engine per se but an overall more efficient airplane.

Hence more efficient engines. They are the lion's share of the power
needed.

"devil" wrote in message
news
On Tue, 01 Jun 2004 07:57:58 +0800, Nik wrote:


"devil" wrote in message
news
On Mon, 31 May 2004 17:52:55 +0800, Nik wrote:



Of course the generators will have to be large enough, but you try to
make it sound like it's a major issue. They'll have to replace that
air
thing. So, roughly same thing.

Generators are heavy by nature.

Everything is heavy by nature. At least whgen in a gravitational field...

Yeps - something is heavier than other things though. An electric drill is
much heavier than a pneumatic one.

- sip -


Was that the article in Aerospace America? If it is, I should have a
copy at the office. The cursory glance I gave it left me with the
impression that it was reaonably balanced, by no means Boeing spin. Last
time I checked, Aerospace America was a publication of the AIAA, not a
Boeing advertisement.

This said, of course, these days anyone does its best to play tge spin
game (don't you in your everyday life, like the rest of us?), so
obviously, both Boeing and airbus would try to get the most of any
interview/article.



We all try to give things the best possible spin - sometimes, however, it
seems more obvious than at others times. To me the article smelled too much
of rat for comfort. This, of cause, is very much a subjective judgement.

I suppose you could make the thing even more complicated, thereby
resulting in an even bigger engine efficincy penalty?

If you are to put an even bigger generator on the engine that will need
to produce different amounts of energy during flight will also represent
a problem.

Your generator is never going to be of a significant size compared to the
power that they turbine needs to produce to drive the compressor. And
since your electrical system is going to be perhaps twice as efficient,
the power that will be needed will be half the power that the turbine
needs to produce for bleed air.



And your are able to calculate this right on hand? You must be a very
cleaver engineer indeed!

Nik.

bleed air

I did a bit of reading.

Bleed air is taken generally from the last compressor stage prior to the
combustion chamber, from the actual jet engine (core, not the bypass). They do
this to extract the highest pressure air at the hottest temperature (about
500C). However, that air is cooled down to about 200C in the engine pylon.

There are also other bleed air adjustable vales that operate to maximise
engine efficiency at lower regimes (iddle for instance). Those are generally
"autonomic" controls part of the engine and not controlled by the aircraft/cockpit.

The "real" bleed air is also adjustable. For instance, aircraft generally
switch from "high flow" to "normal flow" once reaching cruise altitude in
order to save fuel.

However, with high bypass engines, most of the thrust is generated by the
bypass section which is highly impacted by a small reduction of efficiency in
the relatively small core. As a result, taking bleed air from the core does
have significant impacts on engine performance.

(since the core processes relatively little air, any buit you take has greater
impact than if you were to take that same amount from the bypass section).

What I do not understand is why they have continued to take air from the most
compressed/hottest section of the compressor instead of taking bleed air from
the bypass section.

Taking air from the bypass section would also have a big advantage of not
requiring a duct to pass through the bypass to reach the core. (hampering air
flow in the bypass section).


Jet engines already have the gearing to provide torque to drive alternators,
so it is a question of putting in beefier alternators (and gearing).

The winning solutio is probably a hybrid one where air from bypass is taken
for cabin air and additional heat added electrically for wing de-ice.

Taking air from the core is inefficient.

Nik wrote:
Generators are heavy by nature.

Yep. So are electric compressors needed to move/compress sufficnet air for cabin.

Question is whether increased engine efficiency will make up for heavier machinery.

As far as dropping gears, moving surfaces etc, currently, those are not driven
by bleed air, but by gears from engines which drive hydraulic pumps. (possible
the same gearing which also drives alternators).

Hydraulic pumps are very efficient from a weight point of view because you
only need one send of redundant pumps running on the redundant hydarulic
lines. Then each "motor" is light and is just activated by a valve that allows
hydraulic fluid to flow of not.

If you dump all hydraulics and replace with motors, then you have much heavier
equipment (electric motor) at each location that needs something moved, and if
you 're paranoid, you'll need not only dupliocate electrical sustems, but also
duplicate motors.

So perhaps Boeing will opt for electrically driven hydraulic systems. But
then, since they already have the gearing from engines to drive alternators,
why not also drive hydraulic pumps that way too ?

On Thu, 03 Jun 2004 03:29:04 +0000, nobody wrote:

bleed air

I did a bit of reading.

That's good.

Bleed air is taken generally from the last compressor stage prior to the
combustion chamber, from the actual jet engine (core, not the bypass). They do
this to extract the highest pressure air at the hottest temperature (about
500C). However, that air is cooled down to about 200C in the engine pylon.

That's more or less what I expected. To use as a source of mechanical
power, you want as high a pressure as you can get. Note however that the
cooling, even if by exchange with cold outside air, is a net energy loss.

Also, that pressure will be way too high for cabin air.


There are also other bleed air adjustable vales that operate to maximise
engine efficiency at lower regimes (iddle for instance). Those are
generally "autonomic" controls part of the engine and not controlled by
the aircraft/cockpit.

Compressor surge control and the like, sure.

The "real" bleed air is also adjustable. For instance, aircraft
generally switch from "high flow" to "normal flow" once reaching cruise
altitude in order to save fuel.

What I do not understand is why they have continued to take air from the
most compressed/hottest section of the compressor instead of taking
bleed air from the bypass section.

Because they need high pressure for air power. The pressure gain in the
fan is minimal, probably not even enough for cabin air.

Think of these high bypass engines as in effect ducted turboprop.

Taking air from the bypass section would also have a big advantage of
not requiring a duct to pass through the bypass to reach the core.
(hampering air flow in the bypass section).

Look at the fan. How can you take air from it? The fan mostly pushes air
down, providing thrust, especially at takeoff.

Jet engines already have the gearing to provide torque to drive
alternators, so it is a question of putting in beefier alternators (and
gearing).

Obviously. And making sure the core is large enough to provide the power
needed.

I suspect the issue is balance between compressor and turbine. Having a
flow rate in the compressor that varies independently of that in the
tubine.

The winning solutio is probably a hybrid one where air from bypass is
taken for cabin air and additional heat added electrically for wing
de-ice.

For cabin AC, it might be possible not to take any air from the engines at
all. Although that may not be very efficient either.

devil wrote:
That's more or less what I expected. To use as a source of mechanical
power, you want as high a pressure as you can get. Note however that the
cooling, even if by exchange with cold outside air, is a net energy loss.

Bleed air is not use for mechanical power. It is used for cabin, and wing
de-icing, and to help start the other engines (first one started with bleed
air from APU).

Also, that pressure will be way too high for cabin air.

Pressure isn't the issue. It is the volume of air. The higher the pressure,
the smaller pipe you need to transport the same volume. It is the volume of
air that you input into the cabin that counts. The pressure will drop to cabin
pressure as soon as the high pressure air escaped from pipe and is dumped into
the cabin.

Because they need high pressure for air power. The pressure gain in the
fan is minimal, probably not even enough for cabin air.

There is still significant pressure gain from the fan section on most engines.
Recall that the outlet is much narrower on most engines than the inlet. Also,
due to the fact that bypass air helps keep the core cool by flowing around it,
bypass air is also warmed. When you warm air in a confined space, it
increases the pressure.

Look at the fan. How can you take air from it? The fan mostly pushes air
down, providing thrust, especially at takeoff.

Since it is ducted and since on most engines, the duct narrows, it means that
there is higher pressure inside than outside. This means that an orifice in
the duct would let air out. Capture it into a pipe and you've got bleed air.

However, it may not be warm enough to de-ice wings.

Consider a plane going at 200 knots during descent through clouds, with
engines at almost idle. That phase of flight still requires lots of energy to
keep the wings warm and toasty to prevent ice formation, and with 250 knot
wind constantly cooling the wing, you need to pump a lot of heat into the wing.

I suspect the issue is balance between compressor and turbine. Having a
flow rate in the compressor that varies independently of that in the
tubine.

Which is why newer engines have gearing between the turbine and the bypass
fan, meaning that the later doesn't spin at the same rate as the turbine so
that both can operate at their most efficient speed.

For cabin AC, it might be possible not to take any air from the engines at
all. Although that may not be very efficient either.

What boeing wants to do is to add an electric compressor to take outside air
and compress it to mimic bleed air. Remember that you not only need compressed
air, but also large volume of air to keep passengers alive.

"nobody" wrote in message
s.com...
bleed air

I did a bit of reading.


Well done - interesting and enlighting post!

Bleed air is taken generally from the last compressor stage prior to the
combustion chamber, from the actual jet engine (core, not the bypass).
They do
this to extract the highest pressure air at the hottest temperature (about
500C). However, that air is cooled down to about 200C in the engine pylon.

There are also other bleed air adjustable vales that operate to maximise
engine efficiency at lower regimes (iddle for instance). Those are
generally
"autonomic" controls part of the engine and not controlled by the
aircraft/cockpit.

The "real" bleed air is also adjustable. For instance, aircraft generally
switch from "high flow" to "normal flow" once reaching cruise altitude in
order to save fuel.

However, with high bypass engines, most of the thrust is generated by the
bypass section which is highly impacted by a small reduction of efficiency
in
the relatively small core. As a result, taking bleed air from the core
does
have significant impacts on engine performance.

(since the core processes relatively little air, any buit you take has
greater
impact than if you were to take that same amount from the bypass section).

What I do not understand is why they have continued to take air from the
most
compressed/hottest section of the compressor instead of taking bleed air
from
the bypass section.

Taking air from the bypass section would also have a big advantage of not
requiring a duct to pass through the bypass to reach the core. (hampering
air
flow in the bypass section).


Jet engines already have the gearing to provide torque to drive
alternators,
so it is a question of putting in beefier alternators (and gearing).

The winning solutio is probably a hybrid one where air from bypass is
taken
for cabin air and additional heat added electrically for wing de-ice.

Taking air from the core is inefficient.

Now - let us see where we stand:

In the article about the 7E7 the plane was supposed to be an all electric
aircraft. The article said (as far as I remember) that they were going to
use electricity for moving up and down of the gear as well as moving the
control surfaces about. The reason for that being that bleed air - that we
now knoe is taken from the core - involves quite some loss of efficiency.
Now it seems as if the moving up and down of the landing gear and the moving
about of the control surfaces is not done by bleed air after all but by an
ordinary hydraulic system that is powered by a pump attached to the core of
the engine. That means that if it is to be more efficient to change these
systems from hydraulic to electric systems and you want to save something,
then the saving should be because either:

1 - it allows for a lighter construction. - Doubtful due to the weight of
electric motors and bigger generators.

2 - it is cheaper to construct - hence a cheaper aircraft - hence lower
capital costs. -Perhaps?

3 - lower maintenance costs - perhaps?

4 - energy efficient. Possibly only very marginally. You only move the gear
up and down once during flight and reconfigure the wing twice during flight
(take off and landing) so no doubt: the weight issue, capital cost issue and
maintenance issue will be more important in the bigger picture.

There are also indications in the same article that they do not intend to
stop using hydraulic systems as they - as in the 380 - want to increase the
hydraulic pressure. Wonder what they want to use this high pressure power
for?

This means that there are actually only two areas where bleed air is used.
That is in the heating of the wings and in keeping the cabin pressurized.

One interesting question here is when is there a need to heat the wings? In
flight? Is there enough moisture in 33000 feet to allow for ice to form on
the wings? Or do you only need to heat the wings at lower altitudes - that
is during take off (in cold and rainy wether) and landing?

Keeping the cabin pressure up seems to be an interesting question. However,
if another system than bleed air would be would be developed here it seems
to me as if it would not be a major problems to redevelop existing systems
to include it.

Nik.

On Fri, 04 Jun 2004 07:47:12 +0800, Nik wrote:


In the article about the 7E7 the plane was supposed to be an all electric
aircraft. The article said (as far as I remember) that they were going to
use electricity for moving up and down of the gear as well as moving the
control surfaces about. The reason for that being that bleed air - that we
now knoe is taken from the core - involves quite some loss of efficiency.
Now it seems as if the moving up and down of the landing gear and the moving
about of the control surfaces is not done by bleed air after all but by an
ordinary hydraulic system that is powered by a pump attached to the core of
the engine. That means that if it is to be more efficient to change these
systems from hydraulic to electric systems and you want to save something,
then the saving should be because either:

1 - it allows for a lighter construction. - Doubtful due to the weight of
electric motors and bigger generators.

These issues don't strike me as being the crucial ones. (And I confess
I forgot about going back and digging for the Aerospace America article.)
This said, I would not be surprised if an electric system would be lighter
than a hydraulic one. I wonder why you seem to think the opposite.



4 - energy efficient. Possibly only very marginally. You only move the
gear up and down once during flight and reconfigure the wing twice
during flight (take off and landing) so no doubt: the weight issue,
capital cost issue and maintenance issue will be more important in the
bigger picture.

An electric system would probably have a higher effciency. Not sure this
matters all that much.

Getting back to the core issue: it appears that current bleed air systems
have a negative impact of some significance on engine performance. And
thats' what being addressed.