To Coax or not to Coax

[Cheshirecat]

Hi

What are considered the Pros and Cons for and against Coaxial layouts compared with Flat arrangements for Multirotors?
This would be in the 1.5 - 2KG payload class.


Am i right in thinking with a Coax you can loose a motor without gravity regaining the upper hand?
What happens if you loose a motor with a flat layout say a 6 or 8 prop design.
How does stability in the air compare with a good autopilot like the Wookong M say and do they have different abilities with wind conditions etc..

I am thinking about safety for bystanders as well as the expense of breaking things and plaftform stability.


Thanks in advance....

 

[Bowley]

I flew my Hoverfly X650 V8 for its maiden flight with 7 motors running and I never knew it, I could see a delay in the motor and it seems a little slower but it was spinning of course, feathering in the wash of the prop above. It flew perfectly, next flight I was noticing the same so I thought I better check it out, turns out one of the motors wires had broken at the weak spot just after the bullet connector solder connection. Not sure what would have happened if It had a significant payload or if the failure had occurred in flight but its given me a little confidence.

 

[matwelli]

redundancy, but i believe a standard octo would also handle redundancy ?

heres my tests, Y6 1 and 2 props missing, it worked OK, but i completely removed the props, on a coax, as stated by bowley, if its a lower motor cutting out, it will free spin and greatly reduce the thrust available from the top motor, i did some tests on an X8, and it was a real handfull

the main reasons for coax are , reduced frame/compact , less props in shot, and in theory much better flying in wind due to less prop area/higher effective disc loading so that wind gusts have less area to push against

 

[Cheshirecat]

Thanks guy's for the updates, that helps a lot.
Impressive redundancy on your Y6, So no specific disadvantages to a Coax then, does it just come down to costs? Are Coax frames more expensive??

 

[kloner]

i really really like the xy8 i'm flying. thing is rock solid smooth even in wind and even up at speed

 

[Bowley]

Thanks guy's for the updates, that helps a lot.
Impressive redundancy on your Y6, So no specific disadvantages to a Coax then, does it just come down to costs? Are Coax frames more expensive??

Not really. probably a little cheaper, as you have half the booms.
Have a look at Vulcan Skyhook frames if you are undecided, as you can just change the configuration if you dont like what have initially built.

I do also absolutely love my little X650 X8. all quiet and smooth and lovely. makes me warm and fuzzy watching it fly. lol
theres some nice coax builds on the droidworx build thread on this forum.

 

[Bartman]

Thanks guy's for the updates, that helps a lot.
Impressive redundancy on your Y6, So no specific disadvantages to a Coax then, does it just come down to costs? Are Coax frames more expensive??

cheshire

if you're only going to be flying 10 or 12" props then it's not such a big deal to fly a flat Oktokopter. Overall, they're big and with bigger props they get really big, as in you'll-need-a-Suburban big. They fly about the same although I think the coaxial format handles wind better.

For me, the deciding factor was that I like to shoot straight ahead and in a more dynamic fashion. With real estate photography it really helps to be able to shoot buildings with the camera as close to level as possible. With video, shooting straight ahead and being able to fly the helicopter makes the footage more realistic and more like you're moving through space. With a big flat oktokopter it's hard to do this as the props/motors will come down into your field of view both because of the long arms and because they're straight out in front of the camera. I've gone one step further with my preference for 8 motor coaxial helicopters by squishing the X format into what I call my XY8 format.

There's an issue with efficiency where the flat okto with each prop outside of the others' wash will use less energy to lift the same weight but then there's the fact that the coaxial has a much lower parts count so there's some offset there. I've experimented with propeller combinations to find what is most efficient but I'm still not ready to draw any conclusions other than to say you can't have the same diameter and pitch on the bottoms as you do on the tops. I'm not a fan of the top/bottom speed differential strategy for balancing the top and bottom prop outputs as I prefer to compensate with higher pitch or lager diameter on the bottom.

Each camp has their own arguments and if you're just starting out then the best choice is whatever you're most comfortable with. Once you've got all the major components together and flying, it isn't such a huge deal to reconfigure them into another layout.

Bart

 

[Benjamin Rowland]

Hey Bart,

Just looking for a bit more detail here. How come you don't like top/bottom motor speed differential strategy?

Thanks,
Ben

 

[Steve Townsend]

I have only just recently had correspondence with Droidworx about this.

Indeed on their site in their FAQ's they suggest the bottom props are 7% faster and the pitch of these props is increased. I am interested in building an X4 Lite and just trying the find the best combo, the balance between bigger slower props more flight time and smaller faster ones greater stability in the wind. The testing they have done, revealing the above result was on a XM frame I think, but they are planning coaxial testing on the Skyjib frames very soon.

Steve.

 

[Bartman]

Hey Bart,

Just looking for a bit more detail here. How come you don't like top/bottom motor speed differential strategy?

Thanks,
Ben

Ben,

I just get the feeling with the bottoms that they're like driving on ice compared to the tops when you've got the same diameter/pitch top and bottom. I've got this test stand all ready to go and I've finally calibrated my sensors and stuff but I haven't had time to really run it.

i think two things are happening with the bottom props. first, the incoming airflow on the bottoms having motion and direction compared to what the tops are seeing reduces the effective pitch of the props. second, i think the airflow closest to the hubs is very turbulent as it's trying to get past the motors and motor mounts before encountering the lower props. because of this I think the inner area of the bottom prop isn't as effective. like i said, i'm still trying to get the testing process up and running but i'm leaning towards a rule of thumb of either plus one on the lower pitch or plus one on the lower diameter. i wouldn't order anything based on that but that's the most recent hypothesis i'm pursuing. another improvement would be an aerodynamic shroud of some sort between the motors but for the serious working rig, i still think simple is better and very small improvements don't justify the effort especially if they make field repairs more difficult.

Bart

 

[Bartman]

one of the things i did see is that at the mid range of lift for these props and motors, the amp draw and watts are about the same. for the XOAR 12x5, 12x6, and 12x7 they all use about the same amount of power to make 800 grams but each one makes about 100 grams more than the next at full throttle. the test stand has been interesting to say the least.

 

[matwelli]

wort testing is distance between the props vertically. I don't have great numbers but when testing the overlapping props the further apart the better

Sent from my GT-I9300T using Tapatalk 2

 

[Cheshirecat]

Great info, Thanks for all the replies.
I am doing some more reading with all this info in mind.

Thanks again everyone.

 

[crcr]

wort testing is distance between the props vertically. I don't have great numbers but when testing the overlapping props the further apart the better

Sent from my GT-I9300T using Tapatalk 2

Thanks Mat. I have been trying to find this out for the last week. Do you know what is the optimum distance between top and bottom props? I'm thinking 14, 15 and 16' props if it makes a difference.

Thanks
Craig

 

[Cheshirecat]

Great feedback all, thanks for tha detail Bartman.
I am starting to lean towards Coax after watching videos and reading about wind stability. i want to look at using the Zenmuse as it looking awesome and prepared to budget it in, only problem with that is they don't make one for the 5D MKII (Yet) but then the new Mirrorless SLR are impressive.
Anyway i am commited now as i have just ordered a DJI phantom to be my flight trainer, should be here tommorow. My poor wife would like me to find cheaper hobbies i am sure, far too much spent on high end Canon cameras and lenses, and now this.
I can't wait....

 

[koolkiwikat]

Matt, You should try a slo blow fuse to test redundancy...when it is flying, then you/us will see what happens, Rusty tried if a couple years back on RCG

 

[flytofly]

Bart what are your thoughts about using a 14x4.7" prop on top and a 13x6.5" prop on the bottom? The bottom have a larger pitch than top. I chose this combo because it's all I have in stock

 

[flytofly]

Bump

Bart what are your thoughts about using a 14x4.7" prop on top and a 13x6.5" prop on the bottom? The bottom have a larger pitch than top. I chose this combo because it's all I have in stock

 

[Steve Townsend]

I had been looking in to this based on the FAQ on the Droidworx site which I quote

"The coaxial setups are generally slightly less efficient – even though they do offer some increased prop efficiency for the bottom set of props which, being aircraft props, are designed for and are happier in moving air, which they get under the wash of the top prop. With careful prop selection – a slightly higher pitch on the bottom set – you could pretty much match the efficiency of the standard flat plane configuration.[FONT=Helvetica Neue, Helvetica, Arial, sans-serif]Incidentally, if you are using the same prop top and bottom and if your flight controller allows you to alter the running speed of your engines, you can adjust the bottom engine running speed to run slightly faster – the most optimum figure for the bottom set is + 7% faster."[/FONT]

When I asked them directly for more information they hadn't done much testing but they do have some scheduled soon since coaxials are becoming more popular. When I have asked other [FONT=Helvetica Neue, Helvetica, Arial, sans-serif]knowledgable folk about this subject they said don't bother, use the same top and bottom. All I can say it is probably worth a try, and also the same size props top and bottom just with the pitch slightly increased.[/FONT]

 

[Lanzar]

Advise from Kopterworx.

DJI does not support different prop sizes except iff you change mixed to your own.
MK is perfecr with different pitch size. Bottom usually need +1 pitch. 13x5 upper and 13x6 lover.

But you can always check with amp meter on how much upper and lower motors draw and try to match this.

We like it hammer style.