Custom Multirotor for payload and flight time

[Yellow]

So I';ve got not so much a problem, but more an exercise in what';s possible.

I';m working with some new clients and they want to use multirotors for lighting effects, my S900 is fantastic, and I love how I can modify the payload and how reliable/stable it is.
I don';t need anything more right now, but I';d like to be able to go back to them on the next job and say what I think would be possible with somehting new.

So what I';d like to know is how big a payload (let';s say max 5kg but more likely around 3kg) could fly with perhaps 20min flight time? I know these numbers need tweaking, I';d just like to know what would be possible with a multirotor that';s made mostly with fairly easy to source parts ( I don';t mind printing or cutting parts for it at our workshop - just last thing I want is an out of the box kit I can';t fix) I';m pretty happy building whatever large multirotor you come up with, and if I get stuck all you wonderful people will be there to help I';m sure.

The payload will be a mixture of hard mounted, and like below on a cable, so stability is pretty crucial to cope with pendulum effect - the s900 dealt with it fantastically, even in the wind.

So what are the things I need to aim for in this frame?

Edit: Forgot to mention, as you may have guessed with the  payload, the 7kg limit isn';t really an issue for us here

[Revs]

Just bashed some numbers in the calc.

Assuming we';re talking a 5kg hex lifting 5kg, so 10kg total. That';s 1.7kg per motor, so you need about 3kg lift per motor

6x sunnysky X4110S 460kv motors running 6s with 15x5.5 props use 12A each lifting 1.7kg

12A x 6 = 72A in a hover

72Ah / 3 = 24Ah

So you';d need 2x 12000mAh 6s, and that';s not allowing for leaving 20% in the battery! So lets say 2x 15000mAh 6s or 4x 7500mAh 6s.

Multistar 6s 8000mAh 10c batteries weigh 956g each, so your hex would need to be ~2kg without the batteries to pull this off.

[Yellow]

It doesn';t have to be a hex, would an X8 be more efficient?

Thanks for doing the calcs though, still getting my head around it

[Revs]

Probably not I don';t think. With more motors you';s have smaller props so less efficient. Plus a oct would probably be heavier than a hex. X8';s are even less efficient due to the bottom prop spinning in the prop-wash from the top prop.

A quad might be better, but you';d have to have something like 18" blades.

I hope this isn';t flying over people...?

[PaulJC]

I guess airframe size plays a part in this too, you could have a big octo, could lift loads. but physically it would be massive, a hex is a nice middle ground or an X8 can have a fair amount of lift for a reasonable physical size... Plenty of options..

[Yellow]

Size shouldn';t matter too much, its only ever going to be operating 150m away from people anyway, possibly 100 if I get round to applying for more of an exemption, but I can';t see a use for that right now.

I was liking the look of the cortex I saw someone mention it a while ago, but if a quad with long arms and big props would be best I see no reason why can';t use that, just need some sort of folding/detatching mechanism for transport

[Revs]

BTW if you want me to explain any of the calcs, just ask. It';s not an exact science but you can get near enough as long as you have motor/prop data, which Sunnysky nicely provide on their site... http://www.rcsunnysky.com/html/2014/xsseries_0922/27.html

[Yellow]

BTW if you want me to explain any of the calcs, just ask. It';s not an exact science but you can get near enough as long as you have motor/prop data, which Sunnysky nicely provide on their site... http://www.rcsunnysky.com/html/2014/xsseries_0922/27.html

That would be great, I mostly understand it, I';d just like to know why you choose to run at 12A or is that just a good average?

I was looking at these motors with the 18" props - more thrust, and same efficiency as the ones you mentionned at 12A http://www.rcsunnysky.com/html/2014/mseries_0925/30.html

Say I was to make it an X8 with those 18" props, any idea how much the thrust would suffer being in the wash of the others? If its really significant, I';ll look at maybe a hex with some nifty design to avoid being too big and not 100% overhang like a Y6

[Revs]

12A is just what each motor should draw when lifting 1.7kg, according to the chart...



..look at the DJI1555 box ^^

[Yellow]

I get that, but if it were carrying 2200, it would draw 18, how would this be controlled? Or does it just lift what it has to?

[Revs]

Yes, 18A at 2.2kg, controlled with the throttle in manual mode. The FC should compensate in loiter mode, it just ups the throttle until the baro says it';s off the ground.

Also, I';m sure you know but 10kg flying weight is over the frames max limit. Not sure if you';d be insured or indeed whether the frame would actually take it.

[Yellow]

10kg is certainly over the s900';s capability, that';s why I';m looking for a new build. Spotted that D130 from another thread, might use that As a base, do you know much about that?