A350 vs. 787

If I recall, the complex and heavy system passes 3 or 400 degree
pressurized air to be used for heating, cooling via an air-cycle
machine, de-icing, and engine starting. Electric power seems more
efficient just as an electric drill is more efficient than a pneumatic
drill for the cost of energy to power the aircompressor vs. powering
the drill directly. The airflow has losses as it passes through the
ducts and around corners which lowers its efficiency. Airlines have
been criticized for cutting back on pax airflow to save fuel burn due
to a little extra bleed air.

The hot ducts made of stainless must be insulated and kept clear of the
structure so as not to overheat the aluminum it comes close to. This
would be even more of a concern as the composite can not stand as high
a temperature as aluminum. Also the duct has to pass through holes in
the structure, especially the pylon. The 787 pylon will be much thinner
as it does not have to carry the engine loads and have holes cut out
for the ducts. The 787 pylon will be thinner than we are used to seeing
on transports. The thinner pylon will be less of a disruption to the
complex airflow around and between the engine cowling and the wing.
This will improve the performance of the wing. Investing in a thinner
pylon means this is a one-way decision for Boeing. They cannot easily
switch back to bleed air system. The ribs, fuselage, bulkheads, and
frames will also not have to have holes for ducts. Holes usually
require reinforcement so they end up heavier and less efficient than no
holes. Only when structure is loaded below minimum gauge are lightning
holes able to save weight.

As usual, all systems on an aircraft are interrelated. You cannot
change one without affecting *all* other systems. So changing from
bleed to non-bleed has effects that spread throughout the aircraft.
Airbus will make comments to marginalize the bleedless differences, but
the final 787 will proove the combined advantages now and in the future
as electric power technology inproves.

I heard that the 787 cowling de-icing will still be done by bleed air.
This makes sense as it can be done without going through the pylon and
will not require much air. The engine cowling is a specialized
environment so I am not surprized to hear that bleed air is still be
best solution to de-icing the cowl.

The engine cowel environment is more demanding than the general
airframe structure. It has to deal with heat, fuel, vibration,
acoustics, and high loads all in a confined, inaccessible space.
Attachments must be double locked and nothing can be allowed to come
loose and get sucked in the engine. For example (correct me if I do not
remember this correctly), the CFM-56 on the DC-8 has mechanisms to open
vents that need to be actuated. Instead of routing a power line to run
a motor, a cable to pull a lever, or a hydraulic line, the designers
used another pressurized fluid which was already available in the
cowel, the fuel itself is used to power the actuator. It makes sense
and saves adding another power system.

James

Much of the fuel savings on the B787 is do to the fact that bleed air
from the engines will be used only for thrust.

Not entirely accurate, as the GEnx engines on 787 do not have bleed
air. All engine power goes to thrust. (I know...semantics) The GEnx
will have some pretty stout generators though, to power all the things
normally accomplished by bleed air from the compressor.

The GEnx engines will also be on the A350 but plumbed for bleed air.
Ditto for the 747 Advanced (if it ever gets built).

Airbus says there's only 10% commanality in parts numbers between A330
and A350.

I sat through a Boeing presentation on the 787 compared to the
A380, and the 787's max fuel burn per seat-mile was lower that the
lowest that the Airbus folks were predicting. This is, of course, a
Boeing perspective, but even if there's some exaggeration it still
looks pretty good.

Dan

Dan,

I sat through a Boeing presentation on the 787 compared to the
A380, and the 787's max fuel burn per seat-mile was lower that the
lowest that the Airbus folks were predicting. This is, of course, a
Boeing perspective, but even if there's some exaggeration it still
looks pretty good.

I'm still trying to figure out how the 787 can save 15-20% (I forget what
Boeing claims) when everyone jumps for joy when something will save 1%. Any
ideas or literature out there?

Thanks,

Hilton

I'm still trying to figure out how the 787 can save 15-20% (I forget what
Boeing claims) when everyone jumps for joy when something will save 1%. Any
ideas or literature out there?

Widespread use of composites might save 8-10% in structural weight
(that's just a WAG though), which means you need less lift, which means
a little less drag. Also, the engines are of higher bypass ratio than
anything else used so far. Combine that with more aerodynamic
improvements and use of all-electric systems, I can see where you might
get up to 15-20% fuel savings.

wrote:
I sat through a Boeing presentation on the 787 compared to the
A380, and the 787's max fuel burn per seat-mile was lower that the
lowest that the Airbus folks were predicting. This is, of course, a
Boeing perspective, but even if there's some exaggeration it still
looks pretty good.

Dan

Well here's another metric to think about: fuel burn per empty seat.
Boeing's contention has for a long time been that there's more to be
gained by having more direct routes served by smaller planes.

In this view, the Sonic Cruiser was a strategic feint which forced
Airbus to put all its resources behind the A380, while Boeing's plan
all along was for something more like the 787. The A380 will turn a
profit but the 787 may be a much bigger financial success, and now
Airbus has to play catch-up me-too with the A350.

Needless to say, if he were alive today I suspect Howard Hughes would
find both companies equally gutless and addicted to the governmental
teat. It may be that the cost of developing a new transport is such
that there is simply no other way to do it anymore, but I see no heroic
entrepreneurialism in either one.

-cwk.

Boeing's contention has for a long time been that there's more to be
gained by having more direct routes served by smaller planes.

Apparently Airbus shares that view, hence the A350 : )

Although both companies are still pretty far apart on their forecast
numbers for large transports. If Boeing gets off its arse and makes a
decision on the 747 Advanced that'll be a much less risky option than
the A380 for many airlines.

cwk wrote:
Well here's another metric to think about: fuel burn per empty seat.
Boeing's contention has for a long time been that there's more to be
gained by having more direct routes served by smaller planes.

In this view, the Sonic Cruiser was a strategic feint which forced
Airbus to put all its resources behind the A380, while Boeing's plan
all along was for something more like the 787.

The day the Sonic Cruiser was announced, I told my friends in Seattle that
there was absolutely no way Boeing could deliver on their promises and that
the plane would never be built. Given that the guys at Airbus are obviously
a LOT more knowledgable than me about aerodynamics, I would bet they didn't
lose a second of sleep over it, let alone change their entire company
strategy.


The A380 will turn a
profit but the 787 may be a much bigger financial success, and now
Airbus has to play catch-up me-too with the A350.

While you might be correct, this statement is just too biased to consider.
If you would like to just stick to the facts, i.e. numbers, the company
playing catch-up right now is Boeing since they are being outsold by Airbus.
FYI: I'm not a Boeing-basher or an Airbus-lover, I'm just telling the facts
as they are. I think they both make excellent aircraft. The 747-400 on
short final is a thing of beauty.

Hilton

I sat through a Boeing presentation on the 787 compared to the
A380,


Except the thread is about comparing the 350, not the 380...

--
Thomas Borchert (EDDH)

Except the thread is about comparing the 350, not the 380...

Ah. Missed that! Need more coffee...

Dan