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| ▲ | repiret an hour ago | parent | next [-] | | Don’t conflate airplanes with rockets. On an airplane, most of the energy in cruise is spent overcoming parasitic drag, not induced drag. It’s spent pushing the airmass out of the way as it moves forward, not creating lift to stay aloft. For that reason, a change in weight does not significantly change cruise fuel usage. Weight is still precious, but that’s because airplanes’ load are more often weight constrained than volume constrained, and capital and operating costs are such that you want to maximize the load. | |
| ▲ | arijun 2 hours ago | parent | prev | next [-] | | > you're carrying all that dead weight for the rest of the flight If you're recharging the batteries for extra go-arounds during landing, they are as dead weight as the fuel you would otherwise reserve for that purpose. And if you have 30% more efficient engines, meaning less fuel and smaller engines, it's possible you could come out ahead, weight-wise. > what if you need two go-arounds I assume that a go-around requires less sustained power output than a full climb from takeoff, so you will probably get more than one go-around anyway, and we don't know how much over-capacity they're designing for. In any case, any design will require tradeoffs in safety, and having more engine-out capabilities might improve safety enough to overcome the higher risk with go-arounds. Not saying this project is will work out or that you're even wrong necessarily (this could be the equivalent of a concept car for Pratt & Whitney). | | |
| ▲ | vablings an hour ago | parent | next [-] | | Due to various penalties, wind resistance. gear down and aircraft configuration. A go-around consumes a huge amount of fuel, not as much as climbing to cruise but its alot | |
| ▲ | dmitrygr an hour ago | parent | prev [-] | | > I assume that a go-around requires less sustained power output than a full climb from takeoff, No. Source 1: PE = mgh Source 2: am pilot | | |
| ▲ | fransje26 2 minutes ago | parent | next [-] | | So, for a Dash 8-100, at 13,000 kg, disregarding drag, engine efficiency, etc, to take-off and climb to 1000m and accelerate to 150 knots (77 m/s), you will need: - 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude - 0.5 * 13000 * (77)^2 = 38.5 MJ, to accelerate to your climbing speed. Total: 127 + 38.5 = 166 MJ, or about 46.11 kWh For a go around, re-accelerating from 1.3 * stall speed (85 knots / 44 m/s) to your climbing speed, and going to your missed approach altitude of 1000 m, you will need: - 13000 * 9.81 * 1000 = 127.5 MJ, to reach your altitude - 0.5 * 13000 * (77^2 - 44^2) = 26 MJ to accelerate back to your climbing speed. Total: 127 + 26 = 153 MJ, or about 42.5 kWh | |
| ▲ | arijun an hour ago | parent | prev | next [-] | | You don't do a full climb after a go-around, so the heights are not equal, and the mass is less since you've expended fuel. You also retain some kinetic energy but I assume that is closer to a negligible effect. Also, PE = mgh is probably an not a great formula for energy cost of takeoff/go-around, as there are probably large costs it ignores (gravity loss, less efficient engine use, maybe less efficient turbines?). For your source 2 I have no rebuttal so will have to defer to you, but would ask for an explanation. | | |
| ▲ | SoftTalker an hour ago | parent | next [-] | | Not a pilot, but on approach for landing you bleed off a lot of energy. For a go-around you need to reverse your descent and build up enough energy to fly away again. Take-off/Go-around tends to be the same throttle setting, AFAIK. Of course it also depends on how early you decide to throw away the approach and go around. Doing it at 1000 feet is different from bouncing it off the runway. | | |
| ▲ | serf an hour ago | parent [-] | | but the point they were making is that it inevitably takes less energy to get to a level flying state (in similar weather conditions) due to fuel consumption. so, unless the pilot is fighting weather it would make sense that equal throttle levels and equal pitch plans in equal weather conditions would require less and less fuel burn until the tanks are empty. |
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| ▲ | dmitrygr 36 minutes ago | parent | prev [-] | | > You don't do a full climb after a go-around, an IFR missed approach can have you climb quite high, especially in areas with serious terrain. Example: https://aeronav.faa.gov/d-tpp/2607/00346IZLZ17R.PDF airport is at 4400 feet over sea level, but missed approach says: climb to 13,000. Also, some go arounds will lead you to have to divert to an alternate airport, getting there may require climbing high to clear terrain or gaining required engine efficiency to fly the distance. > And the mass is less since you've expended fuel In our theoretical aircraft with batteries, mass is the same. > You also retain some kinetic energy but I assume that is closer to a negligible effect. Negligible indeed. | | |
| ▲ | arijun 26 minutes ago | parent [-] | | > you climb quite high, especially in areas with serious terrain. Interesting, thanks. > In our theoretical aircraft with batteries, mass is the same. The fuel that's expended during cruise reduces the mass. |
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| ▲ | card_zero an hour ago | parent | prev [-] | | Time to invent regenerative air brakes, like fold-out windmills. | | |
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| ▲ | 0cf8612b2e1e 2 hours ago | parent | prev | next [-] | | Energy density of liquid fuels cannot be beat by batteries, so this is not competitive if you are looking to maximize cargo. However, there are plenty of short haul flights: private jets, island hopping, regional routes where you need to move little mass. | |
| ▲ | xattt 2 hours ago | parent | prev | next [-] | | Exactly. A hybrid passenger car can tolerate unpredictable power output that may come with an auxiliary power setup that may or may not be available when stronger dynamics are called for. A plane doesn’t have this luxury and needs predictable output. The fossil fuel engine either needs a sacrificial “overboost” mode for emergencies (at the cost of wear/long-term longevity), or has to be sized for full power at the ultimate cost of efficiency. | |
| ▲ | an hour ago | parent | prev | next [-] | | [deleted] | |
| ▲ | Tade0 2 hours ago | parent | prev | next [-] | | On anything but very short flights most of the fuel is spent on cruising. | | |
| ▲ | dmoy 35 minutes ago | parent [-] | | I would classify 290 miles as a very short flight, that's like 1-2 hours or something? |
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| ▲ | hobonation 2 hours ago | parent | prev [-] | | Valid. Perhaps it's not all negative: the electric portion could give a pilot a bit more glide than the gas portion dies. |
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