Why coax?

[crayfellow]

Hi guys,
Can someone explain to me why coaxial configurations have become so popular? We are looking at designing a platform for imaging that will need to be as stable as possible for an industrial/agricultural application, so I am considering options.

It seems for general AP that having props on the bottom would force you to mount the camera lower, likewise forcing CG lower and adding complexity to the overall design, but is that somehow compensated for?

In this application, the imager would be pointed down, so props in view is not relevant.

My guess is that all things being equal a coax octo has an advantage over a flat octo since it gives identical redundancy with half the booms, and likewise, a weight savings. Does it also tend to be more stable since each pair of props are leveled together? Any other benefits/detriments?

 

[SleepyC]

If you do a search there are a lot of opinions but here are the facts.

1) Coax = smaller. To run powerful motors and 18" props on a Hex it would need to be around 1150 - 1200mm And octo I think 1300mm.
You can run 18" props on a 700mm Coax. Easy to transport compared to a GIANT octo.

2) Coax handle wind better. Instead of a giant plate (flat octo) getting hit by the wind, you have 4 smaller plates with a lot of space in between.

3) Less props in camera view (not your concern but a fact)

4) Coax do lose a bit in efficiency. Some say as much as 10% others say as little as 3%, I personally find around a 5% loss. BUT- this is "tuning stand" measurements. Personally I think a flat octo on normal conditions (meaning some wind) has to fight the wind more than an X8 so to me it feels like a wash.

 

[crayfellow]

Really great insight, thanks @SleepyC . So the compromise is losing a bit of thrust efficiency in exchange for the gain in portability (another kind of efficiency). You are right, I am looking for objective truth to discern an optimal design.

And as you infer, when props are in view, these are spread apart further so it may be irrelevant that they are lower relative to the camera.

 

[maxwelltub]

I think efficiency loss is hardly noticeable in my experience. I use different props on top and bottom which I find works way better

 

[crayfellow]

I think efficiency loss is hardly noticeable in my experience. I use different props on top and bottom which I find works way better

What ratio do you use for top to bottom prop sizes? How do you square that in ecalc, just use the average?


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[maxwelltub]

I don't use ecalc I bench test or guestimate from factory specs. I use an inch longer on bottom, but others may do it differently

 

[crayfellow]

OK, thanks. I am looking forward to experimenting and A/B testing with a flat hex carrying a similar payload. Since I'm already familiar with KDE3520 400kv's, what do you think of a coax octo with those?

 

[violetwolf]

Everything above is right on.

However for agriculture/mapping you want as long a flight time as possible to fly grids. This generally means large/huge props and slow turning motors. You'll get better efficiency but it'll be more of a floater. For agriculture your not as worried about stability and heavy lift as camera types tend to be lighter and you're generally shooting stills.

A large (huge) quad may be preferred. Check out Foxtech's "Devourer 130" for an example of what I mean. Some prefer planes as well.

 

[crayfellow]

Thanks, that is good insight. I've seen a few of those really big ones posted on rcg.


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[Vermiform]

Awesome topic! THIS is why I come here.

 

[Pumpkinguy]

I respect sleepy a lot and always value his opinion but I've talked to other guys I respect equally and they claim the ability of a coax to buck wind better than a flat rig is bs. These are guys that have experience with both flat and coax.
I believe the jury is still out and there are just too many variables to make an apples to apples comparison.
It just makes sense to me that a wider wheel base will be more stable than a short one and that reserve power and the ability to apply that power quickly has more to do with stability than the configuration.

@crayfellow if you have the motors I'd say go for it. If not, consider other brands as well. I believe @SleepyC is just trying some 3520 avrotos. They have a good reputation and probably as good or better than the kde's at a better price, specially when you add the esc's. Avroto website says they use german bearings. My kde motors had NMB (china) bearings in them. This is something I feel is important to consider.

 

[Stacky]

I respect sleepy a lot and always value his opinion but I've talked to other guys I respect equally and they claim the ability of a coax to buck wind better than a flat rig is bs. These are guys that have experience with both flat and coax.
I believe the jury is still out and there are just too many variables to make an apples to apples comparison.
It just makes sense to me that a wider wheel base will be more stable than a short one and that reserve power and the ability to apply that power quickly has more to do with stability than the configuration.

@crayfellow if you have the motors I'd say go for it. If not, consider other brands as well. I believe @SleepyC is just trying some 3520 avrotos. They have a good reputation and probably as good or better than the kde's at a better price, specially when you add the esc's. Avroto website says they use german bearings. My kde motors had NMB (china) bearings in them. This is something I feel is important to consider.

I have both a flat and a coax rig. I have flown both in the same winds. In my experience there is a clear difference in performance on windy days, the coax wins hands down, its not even close. I built my first X8 back in feb 2011.
However the biggest plus of a coax for me over a flat setup is I find orientation easier with a coax and I find mine to be more nimble.
With respect to efficiency I find it to be a bigger jump than mentioned here. I get between 10%-15% longer flight times using the same batteries for a flat set up. My flat setup and my coax all use the same motors, props, esc's and FC. In reality there isnt a huge weight saving. CF arms weigh little and the saving is small with having 4 versus 8 arms. Included in that is a small saving in motor wiring. I have tried the different pitch props on the coax and it makes a small difference but not something really worth worrying about for me.
However, clear and absolutely better performance in the wind with my X8 over a flat setup using the same motors, esc's, props and FC.

 

[violetwolf]

That's good to hear Stacky. I'm planning to build a coax in fall myself for all the reasons you and Sleepy are talking about. I'm in Vancouver and we get a strong sea breeze pretty much continuously...

It makes sense that coax's are more stable if only because they present a smaller over-all disc.

I'm flying a hex with a 5D mkll right now, but I'm planning to fly a more expensive camera soon so I'm going to need redundancy

 

[crayfellow]

What do you guys feel are the ideal X8 frames, from a value standpoint for professional applications?

I have a need for portability so this weighs heavy on my design, but for others this may be less of an issue.

I have a QuadFrame hex and really like its folding mechanism; no bolts need to be removed, but there are 4 per arm to loosen/tighten each time. Here is their foldable X8.

There is the Hexacrafter X8, which I especially like because it is US-made and obviously very good quality. I think the folding is done by removing 2 bolts per arm, is that true? If so, I can see that being a challenge for me as I'll essentially need to fold/unfold every time I have a flight/demo.

Vulcan has a 'latch' mechanism for folding, right? Any comments (or better yet, real world professional experience) on that?

Any other options in this realm?

 

[Rattler]

The efficacy is in how the propellers handle the air that they produce. In an 8 X design like in a helicopter you have rotor wash from the upper propeller that is why you use different size propellers to make it more efficient use of how they push the air. It's not in the design of the disk as to it rather its flat or not.

 

[crayfellow]

The efficacy is in how the propellers handle the air that they produce. In an 8 X design like in a helicopter you have rotor wash from the upper propeller that is why you use different size propellers to make it more efficient use of how they push the air. It's not in the design of the disk as to it rather its flat or not.

are you suggesting it is better than a flat layout? if so, in what way? and what ratio do you suggest for upper/lower prop? Cheers.

 

[SleepyC]

I believe @SleepyC is just trying some 3520 avrotos. They have a good reputation and probably as good or better than the kde's at a better price, specially when you add the esc's. Avroto website says they use german bearings. My kde motors had NMB (china) bearings in them. This is something I feel is important to consider.

I have 3 rigs with Avroto's GREAT motors. So smooth, operate cool and fall right within the listed specs.
Also Jim from MontoRC is a great guy to deal with and really stands behind his product. Each one of his motors is hand tested, and boxed locally, so you are getting a GOOD motor.

My Hexacrafter X8 is finished, and has flown, I was waiting on some heat sinks for the ESC's as I wanted to make sure I was going to get some nice dependability and these should drop the esc's temp by 10 - 15 deg.

I'll get pics up later today of the rig... It's seriously my favorite at the moment.

 

[Carapau]

Vulcan locking system is excellent. Very quick and very secure with a locking latch to ensure the arms cant fold even if the main cams come loose. My Vulcan is a work horse and I cant praise the air frame enough.

 

[Motopreserve]

Great thread!

I'm looking at a folding X8 as well, partially because of redundancy other advantages mentioned in this thread, but primarily due to space constraints. My hex has been great - but I had to haul it in the bed of my truck (with a fairly elaborate "suspension system" for peace of mind).

But now that I'm back in the city - I need to be able to transport a craft in the small 'extra cab' behind the seats of a Tacoma. I'm not comfortable with it in the bed for extended periods of time, and there's not enough room inside the truck for a hex.

 

[crayfellow]

You will laugh, but I haul my hex in the back seat of the Golf; he gets a seatbelt and everything!