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,
Keep 'em coming.
Good Luck Brad, I guess coming/hovering out of the home/car garage was part of the deal else you would have tested it else were :). Keep it posted and Helmet pls , we want to hear about flight/s from you .. Tnx for the feed backs too,
I read that Burt Rutan has also decide brushed motors will be on his electric design, less probability of catastrophic failures.
To address the technical questions popping up:
The MOSFETs were IR 1405s and 3907zs, which are being replaced by 4030s, all of which were obtained from reputable sources. Technically, I am not employing "best practices" running the gate lines out that far without opto isolators and having as much as 12 feet of wire between the power source and the MOSFET drains. But it was a compromise of using N-channel devices in a low-side controller while keeping complexity to a minimum...and yes, they are BRUSHED motors. The venerable DeWalt 319080 cordless hammerdrill motors can take a beating like no hobby-class polyphase motor can at a price that's reasonable (that's why the combat robot guys swear by them). I am on a budget here.:-)
The APM is driving 4 standard brushed motor ESCs for the control zones (hey, why redesign the wheel?) and the PWM output is taken right off the gates of the ESCs and distributed to the thrust unit MOSFETs via IR 4427 gate drive chips, with each MOSFET gate having its own channel.
The power source is comprised of 36 matching individual 3800 mAh 18C lithium polymer batteries of 10 cells each, yielding a 34 volt supply rail under load. Given this capacity, I am estimating an out of ground effect hover (the least efficient flight mode) time of approximately 6 minutes. And no, this is not a commercially-acceptable duration. Production units will most certainly have a Rotax-like motor-generator set with a battery backup. Lithium phosphate cells are a likely candidate for this application because they can dump their charge very quickly.
The rotors themselves are entirely my doing. Each blade is 23 inches long. The lower, counter-clockwise blades were hand-made using carbon fiber layups with a balsa filler. They are the originals, and their Clark-Y airfoil section, chosen because of the flat, easy-to-fabricate underside, work the best IGE for some reason I have yet to fully understand. The upper, clockwise blades are Martin Hepperle's MH-114 profile fabricated from polystyrene sections on an aluminum spar. They are the 4th generation of blades used, and should be markedly superior to the Clark-Y blades just on lift vs. drag coefficients alone...but they're not, offering quite frustratingly similar performance (although much more consistent from unit to unit by virtue of not being hand-fabricated).
The ground effect aerodynamics are both extremely complex and totally uncharted territory. Regular helicopter pilots merely drop a little collective pitch IGE to take advantage of the increase in virtual angle of attack. For reasons of minimal complexity and concomitant expense, I have avoided the temptation to incorporate variable pitch blades.
Thank you all for your support.
But the important question is: Will the kit be available through diydrones.com/store ?
I have to tell you Brad, I'd take it up for a flight. Well done indeed :)
(Just wear a proper full-face helmet next time, and you'll be my hero)
Thank you for the background information, it does provide a different perspective to this event that may help some of us understand the risks taken and the reason for the outcome.
Speaking only for myself, putting the humorous comments aside, I think most of us were a bit concerned about the residential driveway as the location for the test and not wearing a helmet. Not knowing what other factors were leading up to your test, i.e. probably the ease of just rolling out of the garage, with such history of conducting your prior tests, one was certainly distracted from the real merits of your invention\project.
I would be interested hearing and seeing more (safe) developments. I heard that not all MOFSETs are created equal, some from Asia may not be the same quality as from US or Europe. My question: how much flight time do you expect from the copter and what batteries are you using?
Thank you and wish you good luck on your project.
This is seriously cool and about as reckless as what you seen in experimental aircraft videos from the 30's and 40's. I would recommend that you load it up with sandbags and fly R/C until you have a proven stable design.
There is no reason to remake the mistakes of the past when we have easily available technology to avoid them.
https://www.youtube.com/watch?v=eDAKtW6Vwnc - 1:15 in this video is a good example of what to avoid.
Thank you, Chris, for such a warm welcome.
A little elaboration seems to be in order. What you are seeing is a hiccup on the way to the culmination of a 7-year effort to make affordable personal electric aviation a reality. The technology is covered under USP 7,699,260 and other patent(s) which may be pending. This is not the first test by any means and let's just say the previous ones were less...dramatic. The apparent risks were therefore of far less magnitude than if you believe this was the first ever throttle-up. The unexpected happened; I've never experienced multiple MOSFET failures on this magnitude before. Also, this is a 36-rotor proof-of-concept prototype and a configuration like this would never be offered for sale.
As J. Gordon Leishman, Professor of Rotorcraft Aerodynamics at the University of Maryland says, "The (ground) effect has long been recognized but the aerodynamics are still not fully understood." Believe me when I tell you, there is quite a bit going on IGE with multiple rotors that does not lend itself to simple blade element theory and casual vector addition. Having sat in that seat for a dozen prior tests, I need to be there. It is my design, no one else has ever flown anything remotely like it, and I do hold a PPSEL certificate. I am the most qualified to sit in that seat. One of my advisers, a 100+ mission Vietnam Cobra Gunship Pilot veteran seems to concur. The goal in this test was to establish a hover IGE at about a 3 foot altitude and nothing more.
@Kunckles904 - One of the first design requirements was indeed that it was able to be fit on a 4' X 8' sheet of plywood when disassembled because that's how big the trailer is. At least six open-field tests were held previously.
@Randy - All prior tests were done with a manual electronic control (hard-wired joystick and no position or inertial feedback). This version uses the exact same ArduPilot Mega and Rev-H "Oil Pan" with Mission Planner code as is currently available on DIYDrones, which is where it was purchased. It does indeed have R/C capability as a result. I chose not to test it that way because I need to feel how the craft responds, and I am a regular pilot with very little R/C experience.
@jcrubino Yes, there are now large enough brushless motors for a nice 12-rotor design, which is what production versions will sport.
Thank you everyone for your support and positive comments.