Boeing is building the 787 with engines that will not output bypass air,
stating that this will offer great savings.

As a sanity check, will a bleedless aircraft really save that much money
overall ?

Once you add the extra generator torque needed, and all the weight of
the new systems that will use electricity instead of bleed air, will it
still represent a significant savings ?

Are the electrical stand-alone systems (such as cabin air compressors)
already available and known quantities (such as weight, and electrical
requirements), or are those still being developped ?

I assume that the starter motor will act as a generator once the engine
has started ? Or will the two be separate entities ?

Also, has there ever been consideration of having variable bleed air by
having a valve at/near the core so that when the aircraft is at cruise,
less bleed air would be "wasted" than when , of instance, aircraft is
descending through clouds and de-icing is needed ?

(P.S I tried posting this before, but post never showed up, so I am

moderators.isc.org as email ).

I would expect the latter. Apparently they are two different beasts.
A friend who worked on these things at Ford explained that a starter,
at least for a piston engine, is a low-speed, high-torque device, while
the generator is a high-speed, low-torque device. They require different
design tradeoffs. Some folks at Ford built a combination unit, but it
required a trunk full of high-power electronics to make it work.

<snip>

--
-Stephen H. Westin
Any information or opinions in this message are mine: they do not
represent the position of Cornell University or any of its sponsors.

IIRC, on the 787 the generators will act as starters.  The power-management
complications of combining starter/generator functions are not that
difficult anymore.

We're all familiar with how the performance of computer chips has increased
in the last several years, while the costs have been dropping.  The same
advances in semiconductor manufacturing have been applied to power
transistors, which are continually getting cheaper and better.  Add in
lower-cost microprocessor control, and you can (theoretically) manage
complex variable phase/voltage power electronics with good efficiency.
Boeing is quoted as saying that an "electric airliner" wouldn't have been
practical just 10 years ago, and that's likely true.  For Boeing's sake I
hope that they and their subcontractors have enough clever electronics (as
opposed to electrical) engineers working for them.

Now if they would just replace the electro-hydraulic control surface
actuators with all-electric actuators, they truly would have an "all
electric" aircraft.

--
Frank

[snip]

The Douglas DC-8 has cabin pressurization from a dedicated pump (inlets
visible
on either side of the nose) rather than by engine bleed air, and it
seems to work
well enough. The British Vickers Valiant medium bomber, roughly B727
sized,
was all-electric.

On the automotive side, the motor generators of a hybrid car also serve
as the
starter. 2004 and later Toyota Prius has electric air-conditioning
compressor.
A single starter/generator will be standard on all cars as part of the
proposed
switch to 43 volt electrical systems, which will power most auxiliary
systems
electrically. One BMW model uses electric pumps for coolant
circulation,
apparantly a better match for coolant flow demands at various speeds.

                                                 Peter Wezeman
                                              anti-social Darwinist

Actually they are using starter/generators on the 787.  Aviation Week
has had some good articles about the technology being used in the 787.

-MZ
--
<URL:mailto:megazoneatmegazone.org> Gweep, Discordian, Author, Engineer, me.
"A little nonsense now and then, is relished by the wisest men" 508-755-4098
<URL:http://www.megazone.org/>  <URL:http://www.eyrie-productions.com/> Eris

On 13 May 2005 08:09:50 -0400,

Actually the 787 is the first large turbofan engine to use a starter
generator. They are in fact quite common on turboshaft engines in the
500-1500hp range. The APU on the 787 is rated 400kva, and each engine
will have a pair of alternators rated 250kva each...The 787 APU has no
pneumatic capability, so it is electric start, or no start...

[snip]

The starting speed-torque characteristics of a turbine engine differ
from that of a piston engine. In addition, aircraft generators are going
to variable speed AC generators with solid state constant frequency
power converters. With generators and conversion electronics sized to
handle the loads on a bleedless aircraft, the additional cost for
electric starting will be minimal.

--

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