Re: That 'efficiency' thing again

On 23 Jul, 13:16, Cathode Ray <RBCat...@xxxxxxx> wrote:
OK. Now for the next question :)

At what distance does a plane "break even" between flying all the way
with more drag at 15,000ft, compared to the extra fuel required to
attain 30,000ft?

Good question - more research needed.

I'm also a bit concerned about the apparent assumption that the time the
plane is flying at 15,000ft is part of the LTO phase and uses the same
amount of power as when climbing. Surely it doesn't take much longer (at
climbing power) to do the actual climbing in two sections, it's merely
that there's a gap on non-optimal cruising in between.

No, because the plane is going to have to spool up to 90% thrust or so
from a cruise and hence is going to have two periods of
"accelleration"



And isn't the "L" bit of LTO done at pretty low power - just the "TO"
phase that's soaking up a lot more joules? Or does your "25%" LTO
calculation factor that in, eg Very high power doing "TO" for maybe
12.5% of the time, medium power when cruising, and low power when "L"
for a second 12.5% of the time?

As Sam Wilson points out, the Landing phase is energy intensive as
well because of the extra drag of flaps, then flaps plus landing gear
all requiring much more than "idling" thrust to avoid stall speed,
plus the need for any course adjustments and manoeuvering during the
approach with the flaps deployed (the aircraft in this configuration
has maximum drag). Then reverse thrust during braking after touch
down. A pilot of my acquaintance states that a typical reverse thust
operation uses more fuel as accellerating the aircraft to its take off
velocity (rotate) during take off and climbing the first 500' feet.

I always remember the approach to Kai Tak airport in Hong Kong - glide
down gently then all hell breaking loose as the pilots applied a lot
of thrust, heavy banking left and right, viewing the rooftops, turning
right at the end of Kowloon High St and them slamming on the brakes
with maximum reverse thrust to stop the plane before it took a swim in
the harbour. Not very efficient, but a lot of fun.

If any one has hard and fast data on the reverse thrust fuel usage,
I'd be pleased to learn of it. Use of reverse thurst is encouraged
with those aircraft fitted with the system (The BAE146 and its
siblings for example don't have it), becuase its a lot cheaper than
brake pads and discs, so I'm told

Ray

My experience of recent flights - it would depend on the runway
length, but recently as a passenger on short haul flights (e.g. BHX-
AMS) suggest that whilst the thrust reversers are deployed, there
isn't the change in noise that suggests the engines are being
seriously spooled up. I think noise regs may mean that the braking is
done via the wheels. You could try asking on Airliners.net or PPRUNE,
both of which appear to have industry professionals (plus the usual
nutters :-) as regular contributors. Come to think of it, somebody
might be really nice and indicate what the typical fuel burn for a
trip might be in various 737/A320 series aircraft.

Regards
.

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