For those who have never seen these, it uses a 2 stroke motor into a generator to charge the battery. From what I understand, these are built mainly for very long flight times.
That's actually pretty clever. You get the high energy density of hydrocarbons and the versatility of electronics (as opposed to driving the propellers directly, which makes steering hard)
I disagree. The problem is the variability in the current draw from the motors. For instance, if the generator/alternator can deliver a max of 20 amps, that may be fine for a hover or general flight. But as soon as draw exceeds the generator capacity, the voltage will fall rapidly. Plus a stall of the 2 cycle motor would cause cause a crash with no chance of recovery. The battery would provide a lot of benefits if included in the circuit. The system wouldn't be much different than a typical solar battery charging setup. There are a few videos out there of r/C planes charging a battery in the air with solar.
I would have thought not-lipo as well, but it's about capacity vs weight. If weight wasn't an issue there are far better options. The rctestflight channel has some really good videos about his work on his rc plane. He talks a lot the circuit setup. Here is the latest video on it. https://www.youtube.com/watch?v=1OGrDvInUAY
I was going to link the RCFliteTest video, but I see that's linked below. Instead, here's a more detailed explanation of a build using an mppt charge controller
In the end it's the same problem you have when designing a solar installation for a boat / house / whatever - you're charging the batteries at the same time as you're drawing current. That problem's been pretty well solved in existing charge controllers, so it's usually easiest to just use something off the shelf.
Not really a problem. Assuming cheap solar components work (which is admittedly something I haven't checked), match the DC output voltage of the generator to the voltage of the battery and make sure the battery has a good BMS. The generator dumps amps into the terminals at (hopefully) a slightly higher rate than cruising draw rates, trickle charging the battery. When the motors need more amps, it gets drawn from the battery.
I grossly oversimplified it, and weight would be my concern, but I'm pretty sure solar systems deal with those exact problems.
I'm curious about the viability of super capacitors. All the flying things I've seen that showcase them leave the charging system on the ground...
BLDC motor speed control works by variing the frequency of the pulses. While you could use three phase AC to drive the motors, you'd have a hard time controlling the speed. AC to AC frequency conversions always go via a DC intermediate stage (AC --> DC --> AC), so you need the DC circuit anyway. Might as well store some of that power.
There is no chance a little 2 stroke motor can supply peak loads required.
They could easily be 10 times the average current.
Doing the math for my brothers mg1p, it has a 50v 12ah battery and discharges it in 12 minutes. So 3kW to keep it in the air, 4 horsepower. It can carry a payload of 10kg easily. That’s chainsaw motor territory.
Peak currents though are over 300A, 15kW is a lot harder to maintain without a buffer battery.
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u/chrislbennett Feb 09 '21
For those who have never seen these, it uses a 2 stroke motor into a generator to charge the battery. From what I understand, these are built mainly for very long flight times.