Question on fuel consumption

"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.

"John Galban" wrote in message
om...
"C J Campbell" wrote in message
...
snip
Some small single piston engine planes can easily exceed thirty miles
per
gallon, while others are doing well to get a third of that.


I'm curious, CJ. Can you provide an example of a light single that
easily exceeds 30 mpg? Most of the more common ones (172, PA28,
etc..) tend to average around 15 to 18 mpg, depending on the power
setting.

I know a guy who claims to get that in his Long-EZ, and my brother says he
will probably get that or better in his Celerity Mirage if he ever finishes
it.

John Galban wrote:

I'm curious, CJ. Can you provide an example of a light single that
easily exceeds 30 mpg? Most of the more common ones (172, PA28,
etc..) tend to average around 15 to 18 mpg, depending on the power
setting.


The Van's Aircraft RVs get right around 25 mpg

--
Chris W

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"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

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