Another Moller Skycar :-)
The BRS will have to be a bit different from the ones currently used – otherwise there will be a big altitude gap (of a few hundred feet) where the failure of a single motor will be fatal. The BRS will have to essentially have zero-zero capability. These things don’t autorotate and need all motors and ESCs to be running to be controllable (especially if it’s done by fixed pitch fans using motor speed for control). (Even my RC helicopters are capable of autorotation – and I’ve had to do it a couple of times, once due to the failure of the main gear on my T-Rex 600, and once due to a chafed power supply wire to the receiver that caused the helicopter to go into “failsafe” (cut motor power) when the servos all moving at once caused the receiver to see a big voltage drop on a T-Rex 500).
I believe there’s no BRS system – depending on the redundancy in the motors to maintain controllability.
There are 8 motors and I’m guessing there’s a considerable margin to allow for acceleration and individual failures, so the failure of any single motor shouldn’t be a problem. Hexacopters can withstand failure of several motors, provided you’re careful about which you choose to fail. This is effectively an octacopter and should be even more resilient.
On another site it was stated that the craft could crash safely even if 3 of the 4 arms were inoperable. The only way I can think of for it do do this would be to yaw rapidly (by differentially powering the contra-rotating props on the same arm) and wobble like a coin spinning to rest on a table. It doesn’t sound very comfortable – and frankly I suspect that spinning fast enough to make it work could be fatal, even without an impact – but I can see how it could work at a lower speed if you had only two opposing arms, so even the total loss of a single arm need not be fatal.
The Skycar – as I recall – was stymied by the lack of reciprocating engines with sufficient power to weight ratio. Electric motors beat even turbine engines hands-down so I think this leaves the range problem.
How would a brushless DC motor (essentially a permanent magnet 3 phase synchronous motor) fail ? There is nothing touching except the bearings. What is the MTBF of such a motor? Several million hours? The controller could fail, but MTBF of that is probably just as high. The reliability of these things are at least an order of magnitude better than a Lycoming with a CS propeller, and essentially maintenance free. That motor bolted to a solid fixed pitch propeller – nothing can go wrong, unless you physically damage it.
Lightning strike nearby – EMF causes the MOSFET bridges to all go closed-circuit and the controllers all melt simultaneously. Perhaps? The hobbyist motors can shed magnets and do occasionally fail, but the controllers seem to be the weak link in general.
What surprises me a little about it is that the propellers seem quite conventional. Usually you use either a lightweight propeller that you can speed or slow quickly, or a collective-pitch propeller that can have heavier blades and runs at a constant speed but has a faster response at the expense of complexity. This seems to use heavy-looking fixed pitch propellers.
There are many failure modes in high power electronics – not least because of the thermal cycling involved. A 3 phase AC motor can indeed be extremely reliable but the electronics won’t be anywhere near as good.
LeSving wrote:
How would a brushless DC motor (essentially a permanent magnet 3 phase synchronous motor) fail ? There is nothing touching except the bearings.
Depends on the voltage and speed but three potential issues are: stator insulation breakdown due to heat/time, fatigue and failure of the rotor magnet retention bond (particularly for cheapy commercial motors), and dirt ingress or other factors blocking the cooling air flow. Those cause PM motor failures.
That said, as Peter says if you take the batteries out of the equation the bigger issue with electric propeller drive is the power conversion electronics, which have a relatively short MTBF given challenging weight limitations. It’s an area where technology is moving quickly because it needs to, reliability is not currently all that great at high power densities. Relatively speaking the motor is a no brainer.
As Silvaire says, motor winding insulation does break down. I have a TB20 landing gear pump (a cheap and dirty, possibly Chinese, motor from “Parker”) here which has some kind of a winding short. Before that I sent off a motor for an air blower for repair – a single phase capacitor start motor, brushless. Should have never failed. And winding shorts are a large % of the business of any motor rewind firm. Unfortunately enamelled copper is not that great a solution, but there isn’t much else out there and hasn’t been for well over 100 years.
nothing can go wrong, unless you physically damage it.
And this kind of thinking is why there is a need for certification. Not exclusively, of course, but safety assessment on the basis of belief isn’t everybody’s cup of tea…
