Naked Science Forum
Non Life Sciences => Technology => Topic started by: teragram on 22/08/2008 17:22:08
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Can anybody tell me how much fuel an airliner (say 747,380, or others)uses during a flight from London to New York. My interest is from the point of view of climate change.
I believe that any solid body not supported by the ground or other structure, even in level flight, is actually accelerating continously at the rate of at least 9.81 m/sec., requiring a huge amount of fuel.
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According to info on the web a Boeing 747 uses about 36,000 gallons in a 10 hour flight. This is about 5 gallons per mile. Given that it may carry more than 500 passengers this works out at about 100 miles per gallon per person.
The statement about the equivalence of using as much fuel as accelerating at 9.81 m/sec is actually not true. It is much less. The energy to gain altitude is close to the gain in potential energy, but the energy to maintain level flight is much less (thanks to the presence of an atmoshere). The power from the engines must just overcome the "drag" in order to maintain level flight. It may seem like something for nothing, but this is not the case. There is no change in the potential energy of the aircraft. Gliders use no power but can glide at a very shallow angle and do not plummet at an accelerating rate to the ground so it definitely works!
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Imagine that, rather than a plane, held up by air, you had a raised railway held up with concrete and steel.
You could travel along that rail with very little expenditure of energy. The 9.81m/s/s doesn't enter into the calculation (at least not directly)
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If you kept the plane in the air using a rocket engine, pointing down, you would use an awful lot of fuel. However, an aeroplane wing produces LIFT at a fraction of this power output. A balloon would use even less fuel to keep it up.
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Thanks all for the responses. I think though that the 100mpg per passenger is rather optimistic. The raised railway is not relevent because no energy is required to maintain height. Also, a ballon, if it was a helium ballon, would require no energy to maintain height.
Thanks again, further comments welcome
teragram
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Thanks all for the responses. I think though that the 100mpg per passenger is rather optimistic. The raised railway is not relevent because no energy is required to maintain height. Also, a ballon, if it was a helium ballon, would require no energy to maintain height.
Thanks again, further comments welcome
teragram
Actually 100mpg per passenger maybe rather optimistic for a Boeing 747 as it is rather old, although this is a number quoted. More modern large long haul aircraft maybe a little better than this. Figures from Airbus suggest the A380 is a slightly better.
I think the point made by others is that it does not take much energy to maintain height. This may be surprising but is true. The main energy needed, after acheiving optimum altitude, is to overcome air resistance.
The average actual fuel consumption per passenger varies with the number of passengers carried, the specific type of vehicle and the distance travelled. But on average actual results are:
Courtesy of Wikipedia...
The US Transportation Energy Data Book states the following figures for Passenger transportation in 2006: [39]
Transport mode Average passengers
per vehicle Efficiency
per passenger
Vanpool 6.1 1,322 BTU/mi 2.7 L/100 km (87 MPGeUS)
Motorcycles 1.2 1,855 BTU/mi 3.8 L/100 km (62 MPGeUS)
Rail (Commuter) 31.3 2,996 BTU/mi 6.1 L/100 km (38 MPGeUS)
Rail (Transit Light & Heavy) 22.5 2,784 BTU/mi 5.7 L/100 km (41 MPGeUS)
Rail (Intercity Amtrak) 20.5 2,650 BTU/mi 5.4 L/100 km (43 MPGeUS)
Cars 1.57 3,512 BTU/mi 7.2 L/100 km (33 MPGeUS)
Air 96.2 3,261 BTU/mi 6.7 L/100 km (35 MPGeUS)
Buses (Transit) 8.8 4,235 BTU/mi 8.7 L/100 km (27 MPGeUS)
Personal Trucks 1.72 3,944 BTU/mi 8.1 L/100 km (29 MPGeUS)