Question on fuel consumption

"Shemp McGurk" wrote in message
m...
[...]
And from the example you give above, it's about what I thought it
would be: a 747 with, say, 300 passengers is consuming 6 gallons per
mile and a Gulfstream with, say, 10 passengers is consuming 1 gallon
per mile, the per-passenger miles-per-gallon is WAY higher with the
private jet than with the commercial jet.

You have that backwards (maybe because the mileages were stated reverse from
what we're used to in the US).

300 passengers in an airplane that's getting 1/6th of a mile per gallon
winds up being 50 passenger-miles per gallon, while 10 passengers in an
airplane that's getting 1 mile per gallon winds up being 10 passenger-miles
per gallon. The 747 has better mileage by a factor of 5.

That's assuming the figures are actually correct, of course. I can't speak
on whether they are or not.

Pete

"Peter Duniho" wrote in message ...
"Shemp McGurk" wrote in message
m...
[...]
And from the example you give above, it's about what I thought it
would be: a 747 with, say, 300 passengers is consuming 6 gallons per
mile and a Gulfstream with, say, 10 passengers is consuming 1 gallon
per mile, the per-passenger miles-per-gallon is WAY higher with the
private jet than with the commercial jet.

You have that backwards (maybe because the mileages were stated reverse from
what we're used to in the US).


Yes, you're right...I have it backwards but I meant to say what YOU
say below. Instad of saying "WAY higher" I meant to say "WAY worse".


300 passengers in an airplane that's getting 1/6th of a mile per gallon
winds up being 50 passenger-miles per gallon, while 10 passengers in an
airplane that's getting 1 mile per gallon winds up being 10 passenger-miles
per gallon. The 747 has better mileage by a factor of 5.

That's assuming the figures are actually correct, of course. I can't speak
on whether they are or not.

Pete

Of course this is basically true for any two aircraft, the larger one will
get more passenger miles per gallon. A 777 gets a lot more than a 737 for
instance. The same is true for car vs bus.

Mike
MU-2

"Shemp McGurk" wrote in message
om...
"Peter Duniho" wrote in message
...
"Shemp McGurk" wrote in message
m...
[...]
And from the example you give above, it's about what I thought it
would be: a 747 with, say, 300 passengers is consuming 6 gallons per
mile and a Gulfstream with, say, 10 passengers is consuming 1 gallon
per mile, the per-passenger miles-per-gallon is WAY higher with the
private jet than with the commercial jet.

You have that backwards (maybe because the mileages were stated reverse
from
what we're used to in the US).


Yes, you're right...I have it backwards but I meant to say what YOU
say below. Instad of saying "WAY higher" I meant to say "WAY worse".


300 passengers in an airplane that's getting 1/6th of a mile per gallon
winds up being 50 passenger-miles per gallon, while 10 passengers in an
airplane that's getting 1 mile per gallon winds up being 10
passenger-miles
per gallon. The 747 has better mileage by a factor of 5.

That's assuming the figures are actually correct, of course. I can't
speak
on whether they are or not.

Pete

"Mike Rapoport" wrote in message
ink.net...
Of course this is basically true for any two aircraft, the larger one will
get more passenger miles per gallon. A 777 gets a lot more than a 737 for
instance. The same is true for car vs bus.

There are two numbers that tell the story: the weight of the vehicle per
passenger carried, and the speed at which the vehicle travels. All other
things being equal, it takes more power (and thus fuel) to carry more
weight, and more power to carry a given amount of weight faster. If I
throttled my 172 back to highway speeds I suspect the fuel consumption would
rival many cars', though the same could not be said for payload.

One interesting result of this is that many more modern passenger jets
consume less fuel per passenger-mile than high-speed trains, because the
trains carry a lot more dead weight per passenger. It is thus quite likely
that the BOS-LGA shuttle operates at a higher fuel efficiency than the
high-speed Amtrak train making the same trip.

Best,
-cwk.

On Fri, 22 Oct 2004 at 23:50:36 in message
. net, C Kingsbury
wrote:
One interesting result of this is that many more modern passenger jets
consume less fuel per passenger-mile than high-speed trains, because the
trains carry a lot more dead weight per passenger. It is thus quite likely
that the BOS-LGA shuttle operates at a higher fuel efficiency than the
high-speed Amtrak train making the same trip.

What matters is drag. On the level a train may be heavy but its friction
is very low.

Rolling friction is roughly 10lb per ton or a bit less. So a train on
level ground has a 'Lift' to drag ratio of 224. Air drag is a tiny part
of a train's drag but let's say assume it easily achieves 200 to 1. For
a 747-400 let's assume that it has a lift drag of 20 to 1. In cruise at
say 705,000 lb. the 747 drag is therefore around 35,250 lb.. A train
with about the same drag could therefore weigh around 3,250 tons. They
both need a lot of energy to climb and the weight of train may be
against it but they don't have to climb above 30,000 ft! :-)

Let me know the AUW weight of the high speed Amtrak train and I can do
it bit better.

Incidentally the drag of an aircraft flying at its maximum Lift/Drag
ratio is the same at all heights. But at altitude it flies much faster
for the same drag.

So some questions for you. Do modern jet airliners use less fuel per
mile by flying higher or do they just get there quicker? Or perhaps the
engine efficiency is much higher at altitude? Better specific thrust
perhaps?
--
David CL Francis

"Mike Rapoport" wrote in message
ink.net...
Of course this is basically true for any two aircraft, the larger one will
get more passenger miles per gallon. A 777 gets a lot more than a 737 for
instance. The same is true for car vs bus.

Not always the case. From the SAS website, the efficiencies of their various
aircraft:

Family: DH-Q400 (turboprop)
Q400, 58-72 seats, 0.039 liters per seat / km

Family: MD-80 series
MD87, 110-125 seats, 0.048 liters per seat / km (JT8D)
MD81/82, 141-145 seats, 0.043 liters per seat / km (JT8D)
MD90, 147 seats, 0.039 liters per seat / km (V2500)

Family: 737 (all CFM-56)
737-600, 123 seats, 0.044 liters/seat km
737-700, 131 seats, 0.042liters/seat km
737-800, 179 seats, 0.034 liters/seat km

Family: A320
A321-200, 184 seats, 0.031 liters / seat km (V2500)

Family: A330/A340
A330-300, 261 seats, 0.035 liter / seat km (Trent 700)
A340-300, 261 seats, 0.039 liters / seat km (CFM-56)

The bigger planes or those with more efficient engines in each family
do better but it's not necessarily the case that the bigger planes are
more efficient. However you can't compare these exactly as the
short haul are probably more densely packed with seats than long
haul.

Paul