Lessons learned and consolidation for the electric plane I’m designing.
This might not materialize as a thing I do, mostly due to lack of space to build the thing in.
Most of my calculations have used a Sling 2 as the “base” plane - using their published figures for MGTOW, and applying the 50% rule, I can comfortably fit 100 kilowatt-hours of battery and still have weight for a passenger + light amount of cargo.
I’m still deciding between the tailwheel variant or not, because removing the drag from the nosegear is really tempting.
I’ve also run the numbers for other, more readily available, experimental aircraft, but the Sling still works out to be my best bet.
When I started this project, I thought I might use salvaged Tesla batteries. As I did more research I realized that the Tesla battery packs are severely over engineered for my needs.1. I can build a battery system that’ll be not as good as a Tesla system, but it’ll be good enough, and much lighter than a Tesla system.
I’m still working through this.
Current thought is to use LG MJ1 cells, which, as of early 2019, have the highest energy density (just under 260 watt-hours per kilogram) of any battery cell available. This might change by the time I get around to being ready to manufacture the battery packs.
Current thought is to build a small (1 to 4) number of 108s battery packs, with as few parallel strings as I can get away with. This is doable with off-the-shelf BMSs, and reduces the engineering challenges.
Initially, I thought I’d mount the batteries where the gas tanks would go - there’s plenty of space, the wing spar will handle the load, etc. But, I realized that I need to be able to access the battery packs easily, and for that it’s much easier to place them firewall forward or otherwise in/around the fuselage. (Having to take apart the wings, or build in a folding hatch, was not appealing to me).
I need to CAD this up, but the current thought is to place most of the batteries in front of the firewall, with the rest behind the main seat, as weight and balance dictates.
One of the super nice things about an electric plane is that the “fuel” doesn’t slosh around, or otherwise change the weight and balance. Which makes weight and balance calculations much easier, as well as allowing me to better optimize weight distribution. Of course, this nicety is countered by the fact that I’m always running at the heaviest fuel load.
I’m still figuring this out, and once I have this figured out, I’m sure my battery system will change to suit this.
I’m aware of the existence of a standard for electric airplane charging, but I’m unaware of it’s contents.
So, instead, I’m thinking of integrating an automotive EV charger. These are designed for ~400V battery systems, so I should be able to get one to work with mine.
I still haven’t ruled out working with actual electrical engineers to design/build my own.
Ideally, though, I’d be able to integrate an aircraft charger.
Current thought is two Emrax 228 motors in a stack configuration.
The stack configuration is for redundancy and power reasons.
- if one motor (or motor controller) dies, then the other can pick up the slack, with a lower max-power.
- this reduces the strain on each motor, which should improve their longevity
- For my desired voltage (400V), it’s much easier to find motor controllers that are rated for the lower power each motor will require.
Still researching this. Ideally, these’ll be air-cooled controllers that are rated for 100 A continuous at ~400V.
While I’m not going to slap solar cells on the plane, I do want to build a folding solar array that can be stored in the plane.
sunelec is a place where you can buy PALLETS of solar panels for fairly cheap.
Out of scope things that won’t be on the plane, at least, not initially.
For making ground operations much more efficient, I’ve considered placing ebike motors in main gear of the airplane. The thought was to aid in taxiing (don’t use the propeller to move), takeoff (use motors + propeller to get up to speed), and landing (regenerative breaking). However, for reasons of simplicity, I’m not going to do that.
I still might build motors into the wheels, but not hook them up to anything, though.
TL;DR: It’s not worth it. Yet.
For the planes I’m considering, I have about 130 square feet total wing area. With the most efficient solar cells available on the market, I expect to get approximately 25 watts per square foot, or about 3 kilowatts for the entire wing. For reasons, I expect to only be able to utilize at most 2/3rds of the total wing area. Reducing this down to 2 kilowatts at most (realistically, closer to only 1). This is not useful whatsoever for extending the duration of flight (It would add on the order of 10 minutes total duration), which means that it’s only useful for charging, either to supplement grid power (that’ll be a fun challenge), or when grid power is not available.
There’s other things I can do to increase the amount of solar, e.g. covering most of the fuselage & tail, but that’s not really worth doing.
Additionally, just adding solar cells on top of the wings will affect the aerodynamics, potentially in a way I don’t want it to.
Instead, I’m considering building a folding array that I can set up next to the plane and use to charge it. This’ll have a convenience and weight penalty compared to directly mounting the cells, but I’ll have much more surface area available, and it won’t interfere with the aerodynamics of the plane.
This plane is going to be based in LA. The batteries won’t overheat from use (air-cooled), though they do need some cooling to protect them while the plane sits outside in summer. Heating won’t be required (see: the model 3 lacks a battery heater), but even if it does, then I can utilize the same environmental cooling system to heat as well as cool.