iFlight XL7 Octo-X8 Conversion

By Jayembee67 on Mar 26, 2020

14  846  21

We know whose fault this is... So after finally besting the Y4 conversion and getting it to, you know, actually fly, I felt emboldened to try something even more poorly considered. I was going to turn the Y-Knot Y4 into a Y6, but I still need parts, so decided to build an X8 instead.

I hit up the spares bins, and sacrificed three benched quads for this, two for the motors, and one for the XL7 frame. I had spare XL7 rotor arms, so whilst i really wanted to build an 8" machine with 7" props, I also wanted to spend as little money as possible on this fool's errand, and so went with what I had. And so it's just 6" props unfortunately. I had to hunt down a suitable FC, and In the end I managed to find a GEPRC BetaFlight flight controller with 8 motor inputs. Such FCs are not easy to come by; once you are in OctoLand, it all seems to be PixHawk hardware, and I have no idea what that is about.

I just doubled up the arms, drilled a bolt hole near the motors, printed a little spacer to keep the arms apart. I only used 2 bolts per motor so that I could alternate the upper and lower. Making the space for 4 bolts on each would have been tedious, and it's not like the bolts are going to unscrew and go anywhere, so hopefully I am good there.

I had wanted to build 5S, and theoretically it is, but those old EMAX motors, even at 1500kv, are only rated for 4S. If it flies in an interesting manner, I might splash the cash for another set of 6S motors to match those that came off one of the sacrificed machines, and do the upgrade. But for now it's just 4S. But it is dual-4S.

I did some reading before diving in, and more than one X8 build I have seen uses dual battery setups, which makes sense given what you need to spin. So I decided to do the same, and to do a variant on Andy Shen's pigtail splicing. I stripped the insulation and plaited the two strands together with stripped multi-core, and slathered it with solder. Again, hopefully I am good there. But the machine powers up with a battery in either XT60, and doesn't burst into flames with when I connect 2.

I stacked 2 filtered PDBs, and the pads were a bit small for all the tin that was needed, so it's pretty ugly. But I much prefer the idea of spreading that current load around. Also, whilst they were small, it did give me enough pads to attach all those motor leads. Man, the leads. I thought hexes were bad, but 8 ESCs sure result in a lot of wire. And signal ground wires? What the heck are you ever supposed to do with those?. Luckily the GEPRC FC had JST ports for camera, VTX and beeper, which helped reduce the soldering load and general clutter a bit.

When I powered it up to do the final configuration, it didn't work. Of course it didn't work, they never do. But this was odd; motors 5 and 6 just didn't show up at all, but all the others were visible and span just fine. So I took everything apart to check and redo my soldering just in case. But 5 and 6 were still AWOL. I swapped about ESCs, but 5 and 6 stayed missing, so it was the board. I dug about in the CLI, but I'm not really very good with that stuff and my results were inconclusive.

In the end, I was looking at articles about motor pins when I came across a comment that mapping to other pins requires those pins to have DMA support (??) if you wanted to use DSHOT. Hrm, interesting... And so, in good QA fashion, I ran the experiment, dropped the ESCs down to OneShot and suddenly I had all 8 motors. But this is really poor show on GEPRC's part; it's a new board, maybe mine was faulty, but to offer 8 motor pads and then to only fully support 6 is a bit rubbish. Still, I got the thing to work, which I suppose is what really matters.

And it does indeed hover around the back garden just fine; very stable, nicely responsive, it makes quite a big noise and it sure pushes a lot of air. But then it needs to because it's stupidly heavy; 1207g with 2x1300mAh 4S and a GoPro. Yeesh. Still, it's very crisp, even with OneShot, but the proof of the pudding, as they say, will come at the flying field. I expect it to be a lumbering beast that eats batteries for no particularly thrilling reason, but I would be happy to be proven otherwise.

If nothing else, it's a deal more interesting than the benched quads that offered themselves up to make this possible. And as much as I love the techno thrills of the sleek HD screamers, I do have a huge soft spot for bonkers and obscure flying machines. With these, it's more the owning than the flying, but what artifacts they are.

UPDATE 2020-03-29
So after my initial tests, and on some good advice, I removed the tri-blades, replaced them with bi-blades, and removed the 2x4S batteries and just ran 1x5S. Now we seem to be getting somewhere. Still big and heavy, but the power-to-weight feels much better, I even got some sloppy-but-not-horrendous Acro out of it. It certainly gets along, and all those props grip the air, giving it an interesting on-rails feel. Plus it's still stupid loud. I'm rather liking this ridiculous machine now:



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HK-AERIAL VTOL   Mar 29, 2020  

I am so glad more and more examples (of quad+ machines) like this are popping up. I got one of these OTW in a month. The Thicc's are 7", but I think the APC 6045 is gonna be the prop I build around (Andy and his crew built them for carrying big cameras... mine will not be for that =] I like a "chopper"). I can't wait to hear your opinion of your rig's thrust... I bet it'll really scream. Thanks for sharing Jay... Killer

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Jayembee67   Apr 18, 2020 

So a question for you about the up-coming Thicc build! I presume you are doing the full set of 8 counter-rotating, rather than 4 sets of synced props like the inmitable Kwadkenstein, yes? If so, what FC are you going to use? I have run into problems with two 8-input FCs only supporting DSHOT for 6 motors (HGLRC and iFlight), and so having to fall back to Oneshot / Multishot (and ESC synching - tedious). If you manage to find a FC that does DSHOT on all 8 motor inputs, I'd love to know. I got a tip the other day for an H7 board, but it's AIO for individual ESCs, and I want something I can plug into stacked 4-in-1 ESCs.

HK-AERIAL VTOL   Apr 18, 2020 

You mean S1-S8 output instead of 4 signals split... right? That is how I did my micro (all 8), and how I feel most comfortable. That said... I am the biggest, dumbest sap for Matek, and I am so fortunate that they have a few default 8 DSHOT output options. It's timely that you inquire that to me right now, because... I really, really prefer the F722-STD. The thing about that is that it is the one F722 series that does not actually have S7 and S8 as such, but they are defaulted for use on PWM and TX2. I need S8 on TX3 (so I can stack my RC), and so I have that final Q into Matek Support. Now... if you like and are good at tiny solder points the F722 Mini just straight up has a 4-in-1 out + S5-S8. I did use it on my micro, but dang... tight AF for an old man like me. The SE is an AIO... and maybe that would be more well suited if I was gonna use individual ESCs, but I am using 2 T-Motor F45A V2's. The SE just doesn't fit this build BUT... it too has S1-S8 dedicated. In the very worst case I will use the Mini with the 30MM adapter. If I can push S8 to TX3... the STD.

Edit: Matek Sampson says no... I cannot use TX3 for S8. I am guessing I will decide to go with the F722 Mini. I guess I will just use my highest strength magnifying bar (on my loupe) and have a slug of Bulliet to keep my hand steady.

Jayembee67   Jul 13, 2020 

Funny - I just ordered the F722 Mini for a 3" X8 project. Maybe I'd better order some bourbon too...? ;-)

kwadkenstine   Mar 26, 2020  

Extra cool . I dont run 8 motor outputs , only 4 and i just spin the motors the same way . i have no tuning issues and yes it seems almoste pointless given the mass of the thing , But they are a bundle o fun to fly. If its wired like mine it should fly on 1 or 2 bats . even 2 big honkin 3s 2200mha lipos will give good results.
Please post a flight or your opinions.

Jayembee67   Mar 27, 2020 

You are such a bad, or is it good, influence, it's hard to say, it really is! ;-)
But yeah, an intersting project and I suspect your approach is actually better as far as power and speed go. I've only had the chance to put it up twice so far, and in bad winds so sub-optimal testing, but I haven't been so impressed.
It's big and heavy and hovers at half throttle, which is more than I would have liked. It's definitely not an acro machine, I tried and it was an unhappy experience for all concerned; just too much mass and not quite enough power. It is quite fast, but feels more like a cruiser, and I am getting 8+ minutes out of 2x1300mAh, which is acceptable. What is not, however, is how much rattle and shake is coming through into the video, not jello, just tics and bounces, rough road stuff from all the dirty air (I presume). I dropped down to bi-blades today for a second test flight and that worked out better, but it was far too windy to really be able to tell. But if this is going to be a cruising cinema rig, it needs to be smoother. BF 4.1.x and tuning to come, I think.
But it does sound kind of amazing. A throaty roar you can clearly hear at the far end of the field. It's impressive. A bit like a flying leaf blower.
Still, I think the issue is the power to weight ratio is off, too small, too heavy, and the counter-rotating props are eating too much power; if it's that loud, that's means a lot of wasted energy. I suspect 8" frame with 7" bi-blades with motors spinning together, exactly what you have in fact, would be a way better setup. I mean, I watched the vid from your machine, and that thing is terrifying, whereas this, so far, is merely lumbering.
But the sound is cool.
When I get a chance to fly in decent conditions, I'll post footage!

kwadkenstine   Mar 27, 2020 

There is a vid of my 5" version . it runs 2205 2500kv motors and its not quite as quick as my racers with 2306 2450 xings , but its still a handfull. yes i think the counter rotating props add vibration and they do not help eachother spool up from a stall or low rpm event , motors are verry forgiving regarding voltage ratings , they should be fine on even 6s, keep hacking at it , all knowlege is good

Jayembee67   Mar 29, 2020 

And, perhaps not unexpectedly, you were quite right; 1x5S got me sorted! It flew a lot better, and the motors didn't burst into flames or anything, so I think we are good. Much smoother with bi-blades, and some decent poke with the 5S. And I do like the way all those props grip the air and give it an on-rails feel in the straights. Good stuff actually, I'm liking this now!

unionglz   Mar 27, 2020  

whats the benefits of a build like this.....it looks AWESOME

Jayembee67   Mar 27, 2020 

Thank you. And, well, so far at least, the look is the most awesome aspect of it. Oh, and how it sounds, it is slightly ridiculous how loud it is. Bordering on epic. But anyhow...

So as I understand it, the benefits should be lift, stability, flight time, and size. The really big camera rigs are often octocopters to get enough lift to drag around huge cinema gear, and all those rotors help keep things stable. And all that lift means you can carry a lot of battery too, and so stay up a long time. But octos are big to space out all those props, so this X8 config keeps the size small but still, theoretically, gives at least some of the benefits of an octo. Theoretically...

So far the experience has been a bit mediocre, but I have barely had a chance to fly it, so hopefully I can iron out the tuning issues I am seeing.

kwadkenstine   Mar 27, 2020 

Hi the kv on the motors may be holding it back . try a single 5s

Jayembee67   Mar 27, 2020 

"The engines cannae taek it, Captain"... 4S rated motors unfortunately, old gear. But I do have more motors coming, so I might swap them out and try a more pokey battery. Or just build a much bigger machine. :-)

Jodie Froster   Mar 26, 2020  

Really enjoyed the write up, seems like a cool adventure

Jayembee67   Mar 27, 2020 

Thank you! Yes, it has been an interesting project, and whilst the end result is not currently wowing me, I am pleased that I built it, it's quite the machine. And the noise it makes, good lord, the noise.

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