| What do you mean? Modern LFP cells have quite high power density. LTO is even higher. An e-bike with a 100Wh battery and a 300W motor would be extremely useful if it were light enough: you could carry it up stairs, onto trains, etc easily, and it would give plenty of boost to navigate traffic for short distances and make it easier to go up hills. The idea would be that most of the energy would come from the rider. 100Wh of modern LFP cells doesn't weight very much, but you still need to carry around the motor and the structure to support the motor. In an airplane, you need a lot of power to take off, and weight is a big deal. |
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| ▲ | eightysixfour 3 days ago | parent | next [-] | | As the other poster mentioned, they're not even on the same planet as the current power density of the combined engine + fuel in an aircraft. There are two things you are missing in these examples: 1. The motor won't scale down to a 2lbs and a few hundred watts. That's just not how it works. 2. The weight of the battery pack is partially about energy density, but it is also about the ability to discharge, which takes more batteries to make up for it. Let's say you wanted one of the motors in the article giving your device a "boost" of 500hp (sure, we can scale it back, but roll with me), your battery needs to output 400kW instantaneously. If it was a 48v pack, which is 13 cells in series, they would need to deliver 8,333 amps. Most cells are rated for something like 20a, so you need to put more in series to get the voltage high enough to get that to a reasonable number. A 400v car architecture is 112 lithium cells in series for example. This is before packaging considerations, the increase in complexity of the base system, etc. When you look at the overall system, you're just not gaining that much. Cars are actually uniquely good for hybridization and electrification. | |
| ▲ | dghlsakjg 3 days ago | parent | prev | next [-] | | I'm super excited to fly an electric plane, but a lightweight motor isn't what's gonna make that possible. My current plane carries 561 kwh of energy in a 100 pound/ 17gal gas tank. Even if you say the engine is only 30% efficient, I am still carrying an absolute shitload of energy compared to what I could get out of 100 lbs of lithium batteries. The actual motor only weighs 84 lbs. So I have a weight budget of 184 lbs (call it 200 just to be safe), and current capability is a peak output of 50hp (37 kw) and a cruise endurance of 3 hours at 40 hp / 30 kw. So just to math it out, lets say I will need about 90kwh of battery to come close to my current gas capabilities. At an extremely generous 5 kg per kwh, I need a battery that weighs more than my entire airplane's max gross takeoff weight. The weight of the engine is completely irrelevant when the battery weighs 450kg. | | |
| ▲ | amluto 3 days ago | parent | next [-] | | Quoting myself: > Hybrid aircraft? What if there was a battery large enough for takeoff and landing, a small motor (or pair for redundancy) for cruising and to recharge the battery, and motors and fans or propellers wherever is best from an aerodynamic perspective. Would this be better than a conventional design? I don’t know, but availability of very high power-to-weight motors would help. | | |
| ▲ | dghlsakjg 3 days ago | parent [-] | | Possibly in some designs. It comes down to a weight issue. The entire weight of the hybrid system needs to be less than the weight saved by installing a smaller ICE engine. Take a modern small aero engine like the Rotax 912. It comes in an 80hp version that weighs 122 pounds and a 100 hp version that weighs 144 lbs. Hard to beat that kind of power/weight. |
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| ▲ | danjermaus 3 days ago | parent | prev [-] | | Very curious about this because there are electric planes in testing, so how do they do it? | | |
| ▲ | dghlsakjg 3 days ago | parent | next [-] | | They are designed for drastically shorter flight times. Of the airframes I am aware of that are actually flying right now (pipistrel and the beaver) both of them have a range of approximately 1/5 that of the same airframe with a gas engine. The Pipistrel is probably the best example since it is actually available for sale in both ICE and electric. The ICE version of the airframe has 5.5 hours of cruise fuel good for 650 NM. The electric version has a payload that is 60% less, a ceiling of 12k feet compared to 18k feet, and an endurance of 56 minutes. Basically, they are sacrificing range and payload. It isn't that electric planes can't possibly fly, its that ICE powered planes have a pretty hard to overcome advantage until battery power density increases by an order of magnitude. There are already mass market brushless motors that could replace most aircraft engines at a lower weight. The problem is that gasoline holds 30x or more energy than a battery by weight. It doesn't matter if I can get replace 100 lbs of ICE engine with 1 lb of electric engine, because the real issue is that I would need a literal ton of batteries to replace less than 100 lbs of gas. | |
| ▲ | asynchronous13 3 days ago | parent | prev [-] | | The existing electric planes are designed to be trainers. They are primarily for takeoff and landing and flying in the pattern at an airport. Less than 1-hour endurance, and useful load that can barely accommodate two people. |
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| ▲ | danjermaus 3 days ago | parent | prev [-] | | I have a trek ebike and it's fairly light if I take the battery off. Could easily carry it up stairs. I don't think the motor is the problem |
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