If you only want three speeds, off, half, full, off and full are easy and don't require any work. For half, I think you're over-engineering your options here. Why not just do the wiring such that you can have the pack in full series, or half turned off? A simple switch that moves a wire from touching a lead to the motor in the half position, can then switch it over to touch the next battery terminal in the full position. It will be a little jerky, but you're only doing this once a flight, so it seems like that wouldn't be a problem. I think a transformer or resistor is too much work/a waste of energy. Just installing a lever that will pull a lead from the rest of the pack to a motor terminal should work fine.I'm an EV owner and aircraft pilot and I've built/fly 2 aircraft. I have a new long-term project to build an electric launch sailplane. So while I can't dispute the crackpot claims, I will live.
A self-launch glider had exactly two speeds... taxi/cruise-sustain (50%) & full (takeoff). I don't really need an elegant brushed aluminum digital controller with fuzzballs, just two speeds (off, low and closed).
The first and simple answer is good old Ohms law of resistance. The target system is 120v, 40ah SkyEnergy cells, 5C continuous, 10C max. This is connected to a Warp7 motor, I want to get about 20kw static (~18kw in the air, higher RPM). I did my own math to size a $30 digikey ceramic resistor. Can anyone here do it too to check me, I really don't want a fire.
The second option might be to run a transformer to chop the voltage. I need someone who didn't take EE-for-dummies in college like me for this one, as I don't understand how the lower voltage won't just over-current the line and again... catch fire! Did I mention that pilots don't like fire!?
So regardless of the resistor or transformer (if that can work), I will connect two big fat contactors to an "off-low-high" 3-way toggle switch.
But I have absolutely no credentials, not even EE for dummies, so take that with a grain of salt.