In a gas plane, checking fuel level during preflight is a matter of direct observation. It's at a certain level, it's the right color, and it doesn't have any gunk in it. In an electric plane, you can't directly observe a battery's state of charge; rather, you can measure the voltage using a device of some kind, and then extrapolate range/capacity from that. That's not hard to do correctly, but it does involve more moving parts than just looking at the gas.
Batteries don't change their weight based on state of charge, which is nice for flight planning that normally has to take into account a heavier plane at takeoff than landing. But it is nice to be able to trade off range for performance by fueling to different levels in a gas plane, and you can't do that in an electric one.
Batteries perform worse in the cold. Gas engines don't (though normally aspirated/carbureted engines do). The eFlyer is a pretty small plane, though, and it probably won't go high enough to reach altitudes that are consistently very cold.
If your gas engine catches fire, there are a bunch of things you can do to put it out without killing yourself. But I'm not sure what you do if your batteries catch fire, except hope you're very close to a surface you can land on.
It's not as applicable for this class of aircraft, but batteries do not get lighter as they are expended.
Liquid fuel provides the same amount of unit energy until it is completely gone; batteries lose power over time, leading to lower horsepower at the end (this is a simplified point of view).
There are many more systems required to deal with the many failure modes of internal combustion engines:
- Heating systems to address carburetor icing. Typically manually activated by pilot since they have a performance cost.
- Fuel pumps since gravity cannot always be depended upon especially in acrobatic airplanes.
- Fuel sump systems which must be checked before every flight to address water in the fuel tanks & lines.
- Lubrication systems which require pre-flight fluid level checks.
- Magneto system to ensure that sparks can be generated independently from the electrical system.
- Redundant magneto system since carbon tends to build up and prevent proper sparking.
- Possibility of an engine running rough/cylinder firing out of sync due to carbon or other issues.
- Fuel/air mixture control so that the optimal mix can be selected based on altitude. Requires pilot intervention.
- Extreme thermal stresses on the engine, requiring engine run-up checks before takeoff.
- Radiators, and airflow rediretion to address cooling issues resulting in loss of performance and weight.
- Exhaust system, and noise vs. weight tradeoffI imagine a big challenge would be keeping the batteries at a happy temperature.
A quick google search shows the Solar Impulse team ran into this problem: (https://www.theverge.com/2015/7/11/8933295/solar-impulse-sus...)
Edit: The above in regards to providing reliable electric power. Disregarding this, the main downside by far is energy density of currently available batteries.
From https://en.wikipedia.org/wiki/Energy_density, a kilogram of gasoline has about 53x as much energy as a comparable 1 kilogram lithium ion battery.
Gasoline 46.4 Lithium-ion battery 0.36–0.875 (MJ/Kg)
Also, it's possible to have multiple, small electric motors with no loss of efficiency. That isn't possible for a gas engine. Multiple motors could add some redundancy, and therefore reliability.
I think we're going to see a lot of interesting designs over the next few years.
That varies wildly based on what kind of engines and motors you're talking about; it's not consistent enough to be applied as a generalization, especially when it comes to aircraft (turbines are some of the most power-dense engines available). See here [1] for examples.
[1] https://en.wikipedia.org/wiki/Power-to-weight_ratio#Examples
Chip Yates flew a few experimental electric airplanes a few years back. The last I heard, he had devised some sort of battery hot-swap system so he could fly from NY to England in one go. I don't think that one ever made it off the drawing board.
The issue will always be that fuel is consumed and causes the aircraft to be lighter the longer you fly.
Also, cabin heat affect on range will need to be well understood and incorporated in performance and endurance charts for flight planning purposes.
For pressurized airplanes, the power cost of pressurizing and cooling. I'm not sure what that cost to endurance would be, but it would need to be accounted for.