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The fuel use increases with weight, but not as much as you might think, for two reasons: 1. Even at max weight, the cargo is only half the mass of a 747; put a
by repiret 12y ago
The fuel use increases with weight, but not as much as you might think, for two reasons:
1. Even at max weight, the cargo is only half the mass of a 747; put another way, you only decrease the mass by 50% when going from full cargo to empty.
2. Reducung weight reduces induced drag (the drag that you get in exchange for lift) but leaves parasitic drag unchanged. I don't know for sure, but I'd be willing to bet the parasitic drag dominates.
- spectre256 12y agoPilot here. Induced drag dominates at low airspeeds (i.e. takeoff and landing). At higher speeds parasite drag does dominate, so for most of the flight its what matters the most.
- hydrogen18 12y agoI don't know the specifics of the 747 mass characteristics, but you are correct in saying that a good chunk of the mass is the plane itself. Not the cargo. But of the plane's mass, a huge amount of that is just fuel. Whenever they take on fuel, they measure it in thousands of pounds (in the US at least). So by reducing the cargo weight, you reduce the fuel weight as well because you can takeoff with less fuel. In other words, removing a pound of cargo can reduce the total weight of the plane by several pounds. For extremely short trips this would be irrelevant, because fuel costs are dominated by takeoff, landing, and waiting for clearance for those two things.
- repiret 11y ago> In other words, removing a pound of cargo can reduce the total weight of the plane by several pounds Thats only true if a significant amount of fuel is used to overcome induced drag when in cruise. If most of the fuel is used to overcome parasitic drag, then reducing your weight doesn't significantly affect how much fuel you need to carry.