This may be the most amazing thing we've ever posted here. DIY Drones member Brad Hughey built an electric multicopter capable of carrying a person (him), and then actually tried to fly it in his driveway (without a helmet!). Let's just say it didn't end well. But he's figured out what went wrong and he's going to give it another go.
In an email to me, he explains:
History was indeed made on August 10th, 2011 when the Revelation PoC prototype crashed unceremoniously in my driveway. It did briefly leave contact with the Earth, and one could argue that you have to fly in order to crash, but I do not have the audacity to declare a success out of this debacle. A root cause analysis has determined that multiple Magically Obliterating Smoke and Fire Emitting Transistor (MOSFET) failures are to blame. If you listen real closely, you can hear the power rail line inductance ringing (a bit of electronics levity). I wasn't laughing at the time, but an important lesson is finally learned; MOSFETs fail shorted (full throttle). One failure in the back started the pitch forward, then three in the front failed, catapulting me down the drive perilously close to a parked car, missing a rotor strike by mere inches.
The resolution isn't great due to the use of USB instead of FireWire to copy it off of the camcorder. That said, I'd rather this didn't go "viral", as it is a bit embarrassing. Such is the nature of invention. I proffer it mainly as a veracity enhancer; this effort is real and very close to success.
It is interesting to note that half the array out of ground effect managed to push the whole craft with me in it dragging against the asphalt for almost 20 feet before I managed to shut everything off. The power is certainly there. It's all a matter of control now, and the first thing to do next is make the power MOSFET stage for each thrust unit "bullet-proof".
The damage isn't as bad as it looks. The real work involves a total redesign of the power stage including FUSES for each thrust unit. There are much better MOSFETs around now, considering this iteration is seven years old.
New changes frantically being applied include:
- Higher current and more modern MOSFET devices
- A resistor-capacitor snubber network across every MOSFET to help mitigate ringing overvoltages
- Transient voltage suppressors (zener diode-based technology) across every MOSFET
- A complete rewiring to minimize power rail inductance
- FUSES on each motor as a fail-safe
- Larger decoupling capacitors on the outrigger thrust units
We're a couple weeks away from another run at it.
Yours in Daring Invention Progress,
Brad
Comments
Greetings again.
@everyone_taking_about_sandbags: I confess that part of the mission here is the achievement of the first manned electric VTOL aircraft flight. The danger is a bit over-stated, as no significant altitude or groundspeed is to be attempted. The root cause of the "sticking throttle" problem shall NEVER happen again, and arguably, should never have happened in the first place.
@Tom in ON: I question the true efficacy of autorotation as a fail-safe in typical flight modes (i.e. low and slow), as many a Robinson R22 pilot and V-22 proponent can attest, but the idea here is a highly available lifting platform through the use of redundancy. There should be no single points of failure, and there won't be. My ultimate answer to the parochial autorotation critics will be the incorporation of a Ballistic Recovery System chute as a fail-safe. No other rotary wing craft can incorporate such a device. Oh, and after watching the Sikorsky early test videos, I DO need wider landing gear.
@Martin Szymanski: You're correct that mitigation of supply rail inductance is at the root of the failure issue, so I'm increasing the wire size and rerouting. I have 3900mF lytics at the FETs now to help shunt some of the ringing, and at the longest runs I'm upping it to 10K. Adding ferrites methinks would exacerbate the parasitic inductance problem, although I do have them in combination with some rather expensive metal film capacitors at the motor brushes to help suppress commutation noise.
@Jack Crossfire: The MOSFETs failed because I did not properly apply them. Mea culpa, and I don't have access to a DSO or EMC EMI/RFI testing facility capable of capturing aperiodic noise events. So a bit of using The Force and best design practice art will fix it. Ultimately, I believe you're correct about the brushless "DC" motors but I beg to differ on the propeller choice. There are a myriad of definite aerodynamic differences between a model airplane propeller and a VTOL aircraft rotor, especially at the Reynolds number ranges in question. A thorough discussion of static thrust in extreme ground effect is the stuff of which PhD theses are made. We can take this discussion offline, if you like, or make a different thread.
@Everyone_else: Thank you very much for your feedback and support.
Sounds like a lot of unnecessary engineering. Manned, electric planes fly all the time, with brushless motors. Just use a ton of ESC's, brushless motors, & GWS propellers & be done with it. The back EMF, brush wear, & inefficiency of what you're trying to do is going to kill you. You're going to have MOSFETs blowing up all the time & motors suddenly failing. The sustained power output & RPM of flight is a lot higher than what cordless drills & battle bots have to handle.
heh that looks scary. I wonder what was color of the pants after Brad got it stopped :)
I wonder if that could be made as fully mechanical in way that you have mechanical swash plate that takes care mixing speeds/pitches on props.
At the risk of sounding like a Monday morning armchair quarterback, I would also extend the legs wider out. Could help during uneven thrust take-off and landing.
Man, that's a great project but consider safety first :) yours and others. Such an engineous guy, can for sure get sandbags or a crash test dummy to replace yourself during first tests as well as an RC controller and anchors to the ground to avoid having this "beast" go wild. I'm sure if it was me, I would have to face a witch hunt. But keep on, that looks great :)
I am not sure of the speed controllers being used. Having to work with variable freq drives they generate a lot of noise on the line leads. I have found that lowering the PWM freq to 2 Khz helps reduce spiking on long leads with distributed capactance to the motor end your situation is different. The easiest solution is to use torroids for smoothing at each line connection to the speed controller but this would add weight. Getting the resistances down to the power rail including the internal battery resistance will help. Good luck and be safe. You are not the first experimenter that tested their formula first hand.
Brad. With not having the auto-rotation option as with helicopters if all systems shut down, are you planning for some redundancy built-in to the power grid that would isolate a failure? Also, would adding more blades than required for lift allow for some room for indiviual motor/power failures and cushion the landing? Too much additional weight?
Good thing the Wright Brothers didn't have to listen to some of the comments in this thread.
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