Off the wall question...

bruceme said:

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.

Thanks,

-Bruce

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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.

But I have absolutely no credentials, not even EE for dummies, so take that with a grain of salt.

I converted a push mower a while ago to electric, recently upgraded it to LiFePo4 pack. Since I had no need for speed control I went without controller, just a contactor.

My calculation of max current started with the motor rating. The motor is rated at 2100W at 48V, so the current was expected to be in 45Amp area. Like you said, it only takes a second to spin up, so initial dead short is not an issue. The size and the mass of the blade determine max RPM without load, this was outside of my control since I used existing blade. The motor is still in one piece, so I guess it worked out well for my mower.

When my motor stalls in tall grass the current shoots up and I already blew 60Amp fuse, which I replaced by 80Amp fuse. Since then I don't let it stall, but I am comfortable knowing that it would not exceed 80Amps , which is 2C for my 40AH cells.

So, in your case, what is the motor power rating at given pack voltage? This would tell you max cruising current. The propeller size will limit RPMs, I guess you might need to experiment in this area to get best RPM for best motor efficiency.

As for cells holding up to specs, there is a video showing CALB cell at 4C and it ain't pretty . Heat buildup inside the case is the main problem, this is what destroys it. The 4C spec probably assumes the user will magically keep the cell cool enough.

Also, remember that max rate and max lifecycle are mutually exclusive, so running at max C rate means minimum lifecycle. Something to consider when figuring long term cost or cost per mile.

piotrsko said:

if you run some of the constant speed props, motor overspeed shouldn't be an issue after the prop is set up.

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CS is really only useful if you need power at two very different speeds, like take-off and fast cruise. I won't have a fast cruise, so I'll just setup a fixed pitch climb configuration. But I will likely get a ground adjustable prop, so I can tweak it to match the motor.

piotrsko said:

I am guessing you'll need some sort of reduction drive because stuff gets silly when it goes sonic although Les King and IRV Culver says that isn't all that bad.

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You need gearing when the engine/motor gets power at speeds that are incompatible with the chosen propellor diameter. Diameter = Max rpm. 72" = 2700 RPM, 65" = 3000, 56 = 3500... etc. It's not tough math, tip speed shouldn't exceed 0.8mach.

So find a motor with known power curves and match it to a prop. Adjust blades to achieve the desired static (ground run-up) RPM (typically 200-300 rpm bellow climb or target maximum RPM)

piotrsko said:

How far do you need to taxi?? most everyone I know pushes theirs to the end of the runway, sometimes using a ATV.

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Yeah, I can do that on my Silent-IN self launch. I really wanted it to cruise around on bad gliding days. But given the complexity of this parallel/serial thing, maybe not. It really requires a 200amp DPDT switch, if you happen to have one of those, let me know. I'm not sure I know how good I feel about using single pull solenoids(contactors) for this purpose... what if one doesn't fire or sticks?! cabooom! Thanks again for the input.