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Comment by swingbridge | Hacker News Reader

A lighter engine certainly doesn't hurt, but it's really the batteries that are an issue. The other item often overlooked on the battery front is that most batteries hate the cold. Take your iPhone out on a 20F day and see how long that 100% charge lasts! (Spoiler alert, likely a few minutes).

As you go up in the atmosphere things start to get really cold. Until the weight and temperature issues with batteries can be address electrical aviation will be mostly a concept on paper.

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Waste heat can help keep the batteries warm. For example, Teslas have a battery heater which really eats power when you start out in the cold, but after you're cruising it reaches a steady state. Range still suffers, but at that point it's cabin heating and air resistance. Assuming the batteries are in the wings, you'll probably have voids between the batteries and the skin (since batteries don't have very a aerodynamic shape) which could be insulated without too much of a weight penalty.

Battery weight is a huge problem, of course. A Tesla battery pack is 1200 pounds. That's more than many light airplanes weigh total! The motor and inverter are only about 350 pounds, by comparison.

It looked like the battery pack was just behind the motor, in the nose, on the test plane. Easy to let the heat just flow back from the motor.

Of course, it's a small battery pack.

FYI: I've ridden a few regional flights from Sacramento to the bay that are only about 15 minutes in the air, so the battery life might be as huge an issue as it's made out to be. Use electric for regional hops, and jets for longer "traditional" flights.

The motor won't produce much heat, since it's really efficient, but that will definitely help.

And yes, there are certainly use cases which come up as the technology improves. Actually, the airplane I want to buy when I win the lottery is electric: the Antares 20E. It's an electric self-launching glider, so it really just needs enough to get airborne and to a decent altitude, then you use the weather to stay up. Short hops like you describe would work too.

It's much like cars. The Nissan LEAF was useless for driving across the country but great if you just need to get around the city, and then better technology and range increase the number of scenarios where the cars are useful.

Some flights may only be 15 minutes in the air, but that doesn't reflect the fuel requirements for aircraft. I believe you're required to keep something like 30-45 minutes worth of fuel beyond what you need to get to your destination.

I've noticed that people who work on electric aircraft express some annoyance with this.

> I believe you're required to keep something like 30-45 minutes worth of fuel beyond what you need to get to your destination.

> I've noticed that people who work on electric aircraft express some annoyance with this.

As someone who was just on a plane who was recently on a plane that was diverted to another airport due to dangerously bad weather, I'm glad those requirements are there. There's all kind of things that can go wrong that could delay a landing for a half hour or more.

That's right. It's 30 minutes for VFR, 45 for IFR. Minimum. Nearly all commercial flights are IFR.

Thanks for pointing that out, you two. Still, it's probably doable to increase the battery capacity a bit to get cruising time (vs high powered climbing time).

Not there yet, but probably within spitting distance of a few of the use cases on the edges.

Unfortunately, no. For IFR you have to be able to fly an approach at the primary airport, then fly to the alternate airport, do an approach there, and then fly an additional 45 minutes. After each approach you have to be able to fly the missed approach procedure, which necessarily involves climbing. Just being able to cruise for 45 minutes is not enough.

https://www.law.cornell.edu/cfr/text/14/91.167

> Take your iPhone out on a 20F day and see how long that 100% charge lasts! (Spoiler alert, likely a few minutes).

It's true that Lithium Ion batteries lose their charge in cold temperatures, but not that quickly. An iPhone in 20F will still hold charge for several hours. (And of course, under normal operating conditions the iPhone will be close to a warm human regardless of ambient temperature, so they rarely get that cold.)

iPhones have a single Li-ion cell and the phone itself sips power compared to an aircraft motor.

For a slightly more apt comparison-- try flying your DJI phantom on a cold winter day. It won't last long.

Grandparent is right IME

HN chain from when it happened to me - https://news.ycombinator.com/item?id=10939215

Your account sounds different from what GP describes (ie. descending from full charge in a few minutes, without any activity).

I live an a region that regularly stays below 0F for months on end and have never seen or experienced such extremely accelerated loss of charge. However, I have certainly seen phones lose charge more quickly once they get below 50% or so--I never correlated that with the cold, but maybe there is a connection.

I wonder if we have different phones? Mine is a regular iPhone 6

Theses reports seem pretty anecdotal. Is there some hard evidence or an official spec? Seems like a bunch of people with glitchy phones or batteries blaming the weather.

It has only happened once, but I doubt the phone is faulty - it instantly went from a charge in the 40% range to a 'plug in now' screen, eventually returning to normal with the same amount of charge once I got out of the cold.

Could be that different phone models behave differently in low temperatures?

> As you go up in the atmosphere things start to get really cold.

Yes, but the air is also much much thinner. So it doesn't suck heat away nearly as quickly as it might at ground level. So the batteries should retain heat.

I don't see the energy-density math. Batteries in passenger planes would have to last far longer than the flight duration, at least 30minutes more to accommodate safety margins such as diversion to other landing sites. And a battery-powered plane must suffer from the fact that it hauls around dead batteries. A gas-powered plane gets lighter as it burns fuel. So, by my math, an electric passenger plane would require an energy density at least four or five times that of current battery tech. The motor isn't the issue.

You go higher so you can go faster. The next heat exchange with the thinner air remains about constant.

If the batteries were exposed. But I assume they will be behind some cover, in relatively stable air. If the worry is that the batteries will get too cold, keeping them out of the wind is easy. (also, more aerodynamic)

I still find the same issue with cars, they have to haul around dead batteries until recharged. They just have the advantage that they cannot fall off the earth. The weight has come down a lot for both applications, the idea we need to haul over 400kg of batteries for so little range is mind boggling. Of course with increased power density comes the issue of charging it quickly and safely.

Enough tech is here to make these types of cars a viable second or third car for many people but they aren't viable replacements in general. I am still surprised the government never pushed to move all school buses to electric, they have space and spare capacity to support them and ideal spots to recharge many times a day

I would assume a bunch of the battery usage would be from take off. You'd think they could design a system like a rocket launch with a "staged" plane launch, where the batteries used for take off would then be jettisoned to land safely back at the airport for re-use.

A while ago I read a proposal by airbus for an electric airliner. They speculated that batteries could be recharged on the way back down from altitude. If you want to recover energy, you;ll need those empty batteries.

I'm curious if a heater coil gives better battery life or worse battery life in cold environments. Also, better batteries has been an issue forever. Electric cars were invented alongside the combustion car, but they didn't have good batteries, and died because of it.

Heater coil gives way worse battery life, but can still be useful to preserve other electronics in the cold.

Assuming you start the flight from mains power it depends on volume. https://en.wikipedia.org/wiki/Gigantothermy

For a poorly insulated toy quadcopter it is a bad idea. Something, the size of a 747 might need active cooling. Between them there is a point where a little heating is probably a good idea.

If the batter is working hard delivering power, it will generate some heat on it's own.

Batteries don't like the heat either, which is why battery packs in electric vehicles have their own cooling systems, so I wonder if there is a way to design the layout and power curve of battery-motive systems to balance the two out and maintain ideal temperature.

I wonder if hydrogen and fuel cell would have the right P/W ratio to power this. Hydrogen fuel cell should work well at low temperatures.

However, first instinct is to not get into any aircraft with hydrogen on board ;)

Well, in terms of energy density, you are alright: https://en.wikipedia.org/wiki/Energy_density

... but not in terms of power/weight for the motor (or fuel cell in this case): https://en.wikipedia.org/wiki/Power-to-weight_ratio#Fuel_cel...

... You'd need about 1000lbs of weight for 200hp equivalent.

Also, I'd rather be surrounded by hydrogen than jet fuel :).

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