X8 vs. flat octo redundancy comparison.

[DucktileMedia]

If you or anyone is going to get an X8 make sure it is Bart's XY8. it has so many advantages over a typical X8 it's really not even funny.

You get easier orientation, no props in the F.O.V., packs up easier, with his custom camera mount you can actually point the camera upward without seeing props!, you can use it as a quad or octo, has good redundancy AND you would be supporting a small business started by the guy who brings us this forum.


As for the dual operator gimbal, I will let you know how it works out. Certainly not having a coaxial is probably ideal for the 360 setups. BUT, in my limited experience, the camera is usually pointed down a bit anyways and as long as you arent doing 360 pans while pitching forward more than 10-20 deg. you are probably fine. This is the one area of question I have for the X8 config. But certainly as a single operator setup (without a 360 gimbal) it is the bees knees.

 

[DennyR]

how is a two motor weight any different from two motors on two adjacent arms? i'm sorry but that's bunky.

Given that a coax is less efficient than one slightly larger motor that can deliver that same lift, it has less moment of inertia. Two motors on two arms is more efficient than a coax. and has a better distribution of lift and torque reaction. In order to yaw the aircraft in a stable manor the props should be working independently. A quad is the most efficient and a hexa should be the most stable. If you need to lift Okto payloads I would buy a TDR HELI.

 

[Bartman]

Given that a coax is less efficient than one slightly larger motor that can deliver that same lift, it has less moment of inertia. Two motors on two arms is more efficient than a coax. and has a better distribution of lift and torque reaction. In order to yaw the aircraft in a stable manor the props should be working independently. A quad is the most efficient and a hexa should be the most stable. If you need to lift Okto payloads I would buy a TDR HELI.

keep grinding that axe Denny. mankind has learned over a century of powered flight that malfunctions happen regardless of how carefully things are designed or assembled. to ignore that is foolish, imho. especially where people or property are at risk of being hurt/damaged

 

[Bartman]

Just to add to the voodoo the X8 has a double weight inertia at the end of the arm which is a destabilizing factor, so it is not all perfect. I still believe that a quad can do the job as good as anything if done right. Hexa is better in high wind yaw response. I don't fear the redundancy factor anymore. Fact is that there are several ways to get the job done if the build quality is right. IMHO. It's all about what is important to you. Prop protection, endurance, weight,etc.etc.

to clarify, your reference to inertial forces is inaccurate in the context of a comparison between flat 8's and coaxial quads

 

[DennyR]

Well I'm not trying to be critical of your design Bart I hope you can do well with it, This was a discussion about the merits of various configerations. However if you're really trying to be convincing over safety shouldn't those props. have guards around them and get the weight down a bit with better efficiency and materials. We are only putting forward ideas.

As they used to say, when the flag drops the bullsh.t stops. Get that thing flying and lets see what it can do.

 

[Bartman]

denny,
it's flying for hire multiple times each week and doing very well, thank you. in fact, my web shop would have been open weeks ago but i'm busy with the aerial media stuff and haven't been able to get the shop open. i'm also trying to incorporate the Hoverfly system and my one Hoverfly XY8 threw a prop and i'm rebuilding it with the newly finished Hoverfly specific frame plates. in due time.....

sorry if i sounded too defensive but your reference to inertia and moments was incorrect for the thread topic.

aerial designs have never been about specifics. sure you can peddle power your way across the English Channel but that's about all you'll do with that craft. safety, reliability, efficiency, budget considerations, maintenance considerations, etc. all must be balanced into any final design and to argue the merits of only one factor while turning a blind eye to the others isn't good design.

 

[DucktileMedia]

Switching track a little. What controller is it that spins the props in opposite directions on an X8 for all adjacent props? It seems DJI, MK and HF all tops rotate one way, the lowers the opposite. Someone stated they were surprised by this but I havent seen it any other way. My Y6 is the same and it flies wonderfully.

I love knowing the technicals behind as much of this stuff as i can. but i think at the end of the day, to really understand it, you need an aeronautical engineering degree. Sometimes the real world examples are worth more than they are on paper. i think since I have the components here to put together a Bart'o copter I am gonna give it a go. Certainly has more perks than negatives without question. This comparison is like comparing a military tank to an armored van. Certainly the tank could take a direct grenade hit but for driving in the city an armored van is more than enough to keep the solicitors away. Maybe a bad analogy but you get my drift.

I praise Bart for taking the initiative to even try out this non-standard configuration to see if it works. And after seeing numerous videos of his, some flying in 35kts of wind, I am highly impressed by how smoothly it flies and how much it can lift while still being able to fit in the back of a honda. I certainly plan on sharing my results after I get my XY8 going.

 

[jes1111]

Just for the record, the OpenPilot CopterControl can have the motors spinning in any combination you want. I agree though that all top one way and all bottom the other way gets the job done - and certainly makes prop sourcing a little easier.

 

[Bartman]

.............and certainly makes prop sourcing a little easier.

bingo!

 

[DucktileMedia]

Sorry, I'm confused by why your props would be different. 4 RH top, 4 LH bottoms would be the same either way right?

 

[jes1111]

Not if you have different pitch for the lower props - then you'd need to find two different pitch props in both CW and CCW: difficult!

 

[Bartman]

Y6uri,
like Jess said, if you had two CW and two CCW on top you'd need 2 12x5's in each rotation and then 2 12x7's of each rotation on the bottom making for four different props total, 2 of each.

when i started down the coaxial quad road i saw this as a problem so i figured i'd test the "can't have them all spin the same way on top or bottom" myth and made the tops all spin CW and the bottoms CCW. it flew fine, as well as my MK Okto II so that made that decision for me. the big plus is that once you buy the more expensive CW props for the tops, they're protected up there and less likely to get damaged.

on the bottoms you can either run the same pitch or increase pitch to compensate for the spiraling airflow that serves to make the prop appear to have less pitch. i didn't see big efficiency gains or big stability gains by going to the 12x7's on bottom, it flew great with the 12x5's on top and bottom. what i did see by going to the 12x7's on bottom was a lot more stability in descending flight. i was able to come down faster while maintaining perfectly stable flight. i can't come crashing down but i can come down faster than other stuff i've flown and the quad doesn't have the wobble you see in other multi's that are descending.

 

[DucktileMedia]

Got it. makes sense now. Not used to thinking this way yet as my Y6 has Graupner 11x5's top/bottom and works great.

 

[aForcefulThrust]

Ok maybe a silly question and a little off topic, but why is Bart's XY8 called an xY8? How is it different from an X8?

 

[DennyR]

My thinking on the idea of having top one way and bottom the other is that when you yaw left the response is going to be different to right -Right?

One set will be more dominant than the other, I expect. Yaw response is very critical of any prop differences. It could try to rise in one direction and drop in the other.

 

[jes1111]

My thinking on the idea of having top one way and bottom the other is that when you yaw left the response is going to be different to right -Right?

One set will be more dominant than the other, I expect. Yaw response is very critical of any prop differences. It could try to rise in one direction and drop in the other.

In theory having higher pitch on the bottom should result in equal thrust from top and bottom. In practise a precise balance is probably impossible to achieve. But I think the pertinent point is that this is only considering a single moment in time. In practise the yaw is being controlled by a closed loop feedback running at several hundred Hz, so I'm guessing that any small dominance/bias will be smoothed out, as it were, depending on the performance of the FC.

 

[Bartman]

The higher pitch props are slightly heavier due to a thicker hub. I thought this might be an issue but it hasn't been. The best test of yaw authority is to cob the throttle and watch what happens...straight up is a good test result and that's what i've seen so that box has been checked.

The xy format is a quad with 120the degrees across the front and rear arms. It flies like any other quad...is very compact...and has a wide open field of view like a y-6 but with better survivability (twice demonstrated) having 8 motors. Soon to be available through usdrones.us.

 

[Dewster]

Remember that in all cases (currently), the communication between the FC and the ESCs (and therefore the motors) is strictly one way, that is to say: the FC is never aware of whether its commands to the ESCs are being obeyed or not, and never aware of a motor stopping, a prop breaking, or whatever. In technical terms, that side of it is "open loop", i.e. there is no feedback - there is no path/mechanism for such feedback. Furthermore there is no code that says "if one motor stops then speed up this motor or slow down that one - such code, if it existed, would be called "adaptive", i.e. able to adapt to new/changed/different circumstances.

However, the other side of an FC - the gyros, accels, etc. - IS closed loop - i.e. the code is constantly trying to bring AttitudeActual and AttitudeDesired together by the only means available to it: varying the throttle setting of each of the motors. So whether or not it continues to fly with a motor/prop out depends entirely on the number of surviving motors involved, their positions and their relative contributions to the lift/control (as determined by the mixing table). It's therefore not even possible to generalise and say that all 8-motor MRs will survive the loss of a motor/prop. Too much will depend on the precise mixing set-up, the algorithm controlling the flight stabilisation, the attitude/inertia of the craft at the moment of failure, the reaction/input of the pilot, the CG balance of the craft, the prevailing wind, etc... etc... etc.

Nevertheless, many 8-motor craft (and, indeed many 6-motor craft) can and do survive such an event. What one can say is that the more motors you have, the better the chance of surviving the loss of one of them. Further, this "possible redundancy" starts at 6 motors, i.e. 4 motors will never survive, but 6-motored layouts tend to be more sensitive to the variables in play at the time (as compared to eight motors).

But... (there's always a "but", isn't there?)... statistically, a "failed motor" is probably the least common crash cause of all. From my observation across all the forums, broken props and self-disconnecting wires (along with poor soldering) are the leading causes (after pilot error, of course), followed closely by battery failure. It's easy enough to guard against the connection-linked failures (if you're aware of the importance). Avoiding cheapo battery packs reduces the risk of that cause. Good props (and a ruthless exchange policy if any damage/impact is experienced/observed) reduces that risk. Btw, one significant likelihood of a broken prop is that the pieces will take out an adjacent/nearby prop - pretty much negating all the redundancy possibilities.

My own conclusion, attempting to balance all these factors, is that an X8 represents the best layout possible for carrying a big/heavy/expensive camera, not because it has better redundancy than a flat-8 but simply because it's more compact/manageable.

However... ... the next generation of FC/ESC will have closed-loop feedback. The only one I know of right now is the forthcoming OpenPilot Revolution which, when combined with the (forthcoming) OP-ESC will know if a motor is obeying its commands (and should even be able to deduce that a prop is broken). With such feedback it will be possible to write adaptive code that, whilst not able to change the basic physics of the problem, should greatly increase the likelihood of a safe, controlled landing following a component failure.

I vote best answer. I had a quad once (Draganflyer Ti Pro) back in the day... I had one rotor fold onto itself and it was gone. I could do nothing but watch it flip,spin, and tumble out of the sky. I saw a hexa lose a prop. It had a hard landing, but it wasn't a total loss.

 

[homer911]

Going back to the original topic of redundancy, how many of you have had hands on experience with one or more motors failing on on Octo? And what were the results? It look like there are not that many!!


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[DucktileMedia]

My Xaircraft V8(V for value) had at least one prop break (big surprise) which led to at least 2 or 3 props breaking mid air. It came down fast and pretty hard but i some how was able to assist its direction from over the water and landed on its skids upright.

I also didnt tighten a prop nut on my hex(V config) and at 2-3' off the ground the right prop flew up in the air. I was able to give it full left cyclic and it came down nice and smooth. I think had it been a lot higher it might have developed an unrecoverable drift which would have landed hard.