KUNHD3-X8 Caddx Vista 3" X8 Octocopter

By Jayembee67 on Jul 26, 2020

25  1,265  20

This is mostly @kwadekenstein's fault, of course... Having seen a couple of 3" X8 octocopters here, I almost felt obliged to give it a go myself. I could have gone the Shendrones route, but really preferred the idea of taking a more DIY approach - and having managed to pull off a couple of large scale X8 kit-bashing builds, I began to trawl the interwebs in search of a suitable frame.

I was looking for something with a dual stack configuration as there were going to be two ESCs, and I wanted to use Caddx Vista which whilst smaller than the DJI Air Unit, is hardly slim. In the end I happened across this Phisital KUN HD3 (yeah, I don't know, your guess is as good as mine) 3" frame on Amazon, and unexpectedly it looked like it would fit the bill. It has dual stack spacing and is actually designed to hold a full sized Air Unit on a little shelf, so I presumed there would be plenty of vertical height to get everything in there. And so ordered a couple of the frames...

(...This is the real problem with X8 machines; the doubling up of the frame, motors, ESCs... they are not cheap and cheerful builds, even using obscure import frames and middling motors. Ack, my poor bank balance. Any road up)

When the parts started to arrive I started to do layout and printing, and spent a week on that aspect of the build, by far and away the majority of the build time. But a build like this needs a lot of parts, and I tend to have to do a lot of trial-and-error printing and fitting, all of which takes aaaaaages. I did shortcut the bottom plate spacer though; I drew around one of the plates, scanned it and created an SVG, and then imported that into TinkerCAD. It wasn't very precise, but it was a lot quicker and easier than attempting to build that shape by hand in TC. And it printed right first time. So that is a technique I shall use more in the future.

Once I had everything printed well enough to pass muster, I set to with the actual soldering iron work, something I like less and less as my eyesight gets poorer and my hands shakier (Lesson for today kids: Don't Get Any Older!) particularly with 20x20 hardware. The ESCs are fine, nice big chunky pads, but those little FCs are getting less fun with each build. Still, having said that, the Matek F277 uses the edge-groove pads (I am sure there is a proper name for them) and that made soldering up the FC surprisingly easy. The 4 surface pads for motors 5-8 were an exciting challenge, but overall, the FC was a good deal less awful than I had feared.

Amazingly everything worked first time, this is pretty much unheard of with a normal build, let alone something nominally twice as complex. And this FC does indeed support DSHOT on all 8 signal pads, most excellent. So I got to build a proper contra-rotating X8 with proper DSHOT control. The only issue I have is that I used iFlight Succex ESCs which I was expecting to be BLHeli_32, but when I plugged in, they were BLHeli_S (maybe I was sent the wrong ones? Maybe I'm an idiot? Likely the latter...), so I can't try RPM filtering on 8 motors yet. Although that might be horribly ill-advised anyhow.

Regardless, it hovers, does all the video sending and receiving one expects, and was buzzed around the back garden successfully LoS. As with my other contra-rotating X8 machines, it makes a heck of a noise in flight, but it also looks like it has that locked-in thing going on. Hopefully I shall get to test it properly soon and find out for sure.

UPDATE 2020-08-06
I built this machine with the intention that it would be a Hero7 Hauler - and it does fit that bill, but with the H7 on board, it's a bit too heavy, a bit too power hungry, and bit too sluggish. Which is a shame, you could feel it champing at the bit, but when you opened it up, the battery drained fast. I was watching the throttle and the battery voltage all the time, which wasn't that much fun. I know I intended a small cruiser, but it had more to offer.

So I took the H7 off and replaced it with the much inferior but still stabilised, and most importantly much lighter, Insta360 GO just to see what it would do. And that's much more like it. I can now get 3:30-4:00min of vigorous flying out of a 1300mAh 4S pack, and the machine flies really well - all those props grip the air very nicely; you can throw it about in a very agreeable manner.

The Insta360 GO footage is low res, low frame rate and entirely mediocre looking, but the quality of the stabilisation and the CinemaScope-y aspect are great, so the trade-off for the weight saving is not a wash. I imagine this is how this setup will be flown going forwards. Because it's really good fun now!



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DS84   Nov 06, 2020  

sick bro great skill and willingness to do something new!!!

Jayembee67   Nov 06, 2020 

Thank you! And even better that the end result flies really, really well! Which was a little unexpected, but I'll definitely take it... :-)

SlightlyScratchedCorolla   Oct 02, 2020  

Hey love your build a lot!
I'm gathering parts for a 3" x8, and I have zero experience in anything more than 4 motors. Google didn't help me much, I'm a little unsure about how I wire the second esc. Is it only motor 1, 2, 3, 4 to the s5, 6, 7, 8 pads on the fc, or should I also connect a ground wire? And I've seen couple people saying 8 bi-directional dshot could be ill-advised, is it because too much processing burden for the fc?

Jayembee67   Oct 02, 2020 

Thank you!
And excellent, yes the 3" X8 has definitely become a thing, but with good reason if this one is anything to go by, it's enormous fun to fly!

So, obviously the 1st ESC is just plugged into the FC socket, which gives you power, gnd, current (probably), and the m1-4. And for the 2nd ESC you don't need the power or current, but yes you DO need the gnd to complete the signal circuit back to the ESC from the FC. And then m1-4 leads solder directly to s5-8.

I have had issues with FCs having s1-8 pads, but only 6 of the connections supporting DSHOT - 2 of them weren't digital, which meant I had to drop all of the motors back to MultiShot or OneShot. The Matek boards don't have this issue, s1-8 all support DSHOT.

Unfortunately I haven't had the chance to test out RPM filtering with this build, as I foolishly ordered BLHeli_S ESCs, and had them installed before I realised my mistake. But it could be an issue, even an F7 processor might not have enough headroom for all that filter number crunching, I too have heard the same, but I really don't know. Maybe you can try it and let us all know! :-)

But I look forwards to seeing your build! What are you doing for the frame? Big Baby? Or DIY-ing it?

SlightlyScratchedCorolla   Oct 02, 2020 

Thank you so much for this detailed explanation! This really cleared my wiring plan.

Yes I'm doing the big baby, with a lumenier f7 that is also supposed to do 8 dshot straight up, that's why I'm tempted to try rpm filtering, but I figure i'd have to turn the gyro filter way down so I'm pretty torn (but to be honest there is no freaking way I don't try it at some point). I also want to put 4 blade on top motors and 3 blade bottom to maximize grip and make it queiter but idk if this is a good idea either, maybe it'll mess up the coaxial motors too much.

Will post it once built and definitely credit your help! btw awesome flight video and location, and I know it has a lot to do with your stick control but damn this thing moves so crisp across all axis.

Jayembee67   Oct 03, 2020 

Well, good, I await the results of your brave filtering experiments! :) And yes, I am always very nervous about mixing props, but that is based on no actual information, just that "it doesn't seem right", but that's hardly a scientific approach. One thing I have read is that with contra-rotating props, the back prop helps 'untwist' the air from the front prop, giving a smoother thrust, but whether that has any real effect for a 3" quad is hard to say.

But yes, the thing is basically a ball of props, so it really grips tight and you can throw it about in a very agreeable manner!

Ooghee   Aug 23, 2020  

your builds are over the top! love it!

Jayembee67   Aug 23, 2020 

Thank you! :-)
Well, I do like the challenges building brings, frustrating though it can be, I love that you can create these bonkers machines of all shapes and sizes and almost unexpectedly they still fly (often really well), and I am resigned to never being much of a pilot, so I might as well put my energies into building lovely robots! ;-)

Jodie Froster   Jul 27, 2020  

This thing is stellar!

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Jodie Froster   Jul 28, 2020 

Why would you not fly it?

Jayembee67   Jul 28, 2020 

Actually, after testing it yesterday, I have shifted my opinion and now think I might be flying this quite a lot! It was really more fun than I had expected! :-D If I can just get a bit more battery life out of it... I put on some less aggressive props today, I might get to run some further tests this afternoon if I am lucky. I also just reprinted the GoPro mount - I had originally set it to 25º but the machine wants more of an angle than that, so 30º it is. More news when I have it to share.

Jayembee67   Aug 19, 2020 

So I just wanted to let you know, having taken the GoPro off, put an Insta360 GO on, and put a whole pile of packs through it, this little X8 machine is absolutely one of my favorite machines of all time. I thought it was a bit of a novelty build, but without the weight of the Hero7, it's an absolute acro star, it really is. If this bites the dust, I will definitely build another one!

erikschmidt   Aug 19, 2020  

cant believe fit all that in there.

Jayembee67   Aug 19, 2020 

I know, but it's quite a tall frame, so there was actually plenty of vertical height (well, 'plenty' as far as a 3" build goes). It was a surprisingly easy build, considering all the components. And it flies really well too! :-)

DoubleJackOnTap   Jul 27, 2020  

Wow! Very nice. Im super impressed with the cable management.

Jayembee67   Jul 27, 2020 

Thank you! Yes, there was a lot of wire to wrangle, but ultimately a fun and satisfying build. And it flies too, which is always a plus! ;-)

kwadkenstine   Jul 27, 2020  

Good work , Im impressed!!!

Jayembee67   Jul 27, 2020 

Thank you! Although we shall have to see how well it actually flies, of course; although it does fly, which is encouraging. But yes, it came out pretty neatly and has a nicely chunky aspect - I kind of reminds me of one of those very colourful jumping spiders; kind of squat, sort of ugly-cute, and a little hydrocephalic.

kwadkenstine   Jul 26, 2020  

Its so cute

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