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