Ok, so just a thought, 6 motors drawing let's say 18 amps each is 6x18=108 Amps, now unless you have one heck of a battery, even a 3000mah rated pack with 30C is only 90amps meaning you're killing your pack or must have good 4000-5000mah pack rated still a 25-30C. So that could be one problem (BTW, most battery connectors aren't rated for 100+amps).
Problem #2 is that you are using 1200kv motors when you should be using something less than 900kv (spec motors are 850 or 880kv )so you have quite a mismatch of propellor load. Basically you have very high rpm motors trying to drive props intended to spin slow and move lots of air. This means your motor will draw max amps, not reach the intended rpm range and further, your battery and wiring may not be up to the task you have presented. You either need props with less pitch (like a 3 or 3.5) or less diameter (may 8 or 6 inch) to match the motors. Both of those are less than optimal again because you chose less than optimal motors.
Sorry for your bad luck but you made some wrong choices in hardware.
full prop size: 10x4.5
copter weight. with lipos 1500g
battery type: 2x 3S 5700 mA 30c (11.5V)
motor type emax 1200KV
distance motor to motor 650mm
Distance to motor or arm length only affects stabiltiy, not lift.
So it sounds like the battery is just too heavy. Use one battery which I already proved is enough current, but again, you have the wrong motors and props so likely it's going to be marginal even with one battery.
Had you gotten the right motors, then maybe it would work better, but 1200kv is completely wrong for 10x4.5 props.
Think of it this way, with 2 heavy batteries you basically have a large truck trying to go up a steep hill (the props 4.5 pitch), and the 1200kv motors are like being in 5th gear. The motors cannot even get into their RPM band to make power and are just wasting it as heat.
ok... with one battery no lift offf. so what should i do: change the motors or change the props to a pitch of 3.
you need to go to smaller diameter. see what you can find in a 8 inch. not sure if you can plug your info into a motor calc prog but you might try.
Let's go back to the basics of a heli. Rotor disk area is the amount of area added together of each rotor, so in your current case PI (3.14)x10=31.4in2 per prop x6 each = 188.4 in2
Divide the total area by the total weight of the machine and you get a factor called disk loading. You want that to be a really low number because that's how much weight every square inch of prop must lift. It doesn't take a rocket scientist to figure out that if you go to smaller props and the weight remains the same, you have a MUCH higher disk loading.
So if your current configuration cannot build enough lift and your are using 10 inchs props, it's HIGHLY unlikely smaller props is going to help the situation since we know the disk loading will be higher.
Your only real options are same size props with less pitch (they are also less efficient but it's the only give we can make), or replace the motors. We are hoping that it's just the props are lugging the motors so bad that lesser pitch props will let them edge into the RPM band and by keeping the disk loading the same, we will get enough fgin in thrust to fly.
Again, by your choices, it sounds like you either wanted heavy lift, or long endurance (over 10Ah of battery), but because the motors are so far out of spec, you may not ever (even after changing the props) be able to fly with the second battery.
Just did some calcs over here using this: http://adamone.rchomepage.com/calc_motor.htm
The results do not look good for a 1200kv motor
Static thrust is the number we care about. Not 100% sure what your max motor amperage is since I don't ahve the exact specs but I capped it at 20A to be safe.
Per motor the 10x4.5 prop makes 563g of thrust and 53.7% efficiency
A 8x4.5 makes 510g at a better 65.8% efficiency
A 6x4.5 makes a 265g but at 70.1% efficiency
Going the other way 12x4.5 is 482g but 40.4% efficient
11x4.7 that DIY drones has is 540g and 46.3% efficient
I tried going lower pitch but got even worse thrust figures.
Switching the motor to 850kv and a 12x4.5 prop is 590g @ 58.5%
11x4.7 grey prop is 579g @ 62.7%
Again, this ballpark figures since I don't have your exact motor specs.
Exactly what motors do you have? Are they emax 2822 1200kv?
yes emax 2822
If that's the motor than no way is this thing going to get off the ground.
Plugging in the detail into the calculate and the fact they list an 8 inch prop.
is a 1.4 ounce, 1300 KV
The staic thrust with the 8x4.0 propeller they suggest is 459g thrust
10x4.5 prop your using is 521g but 14.1amps
Further, down the page they list props for a 3cell
Recommended props when using 3 cell Lipo batteries:
The GWS EP9050 produces 30 ounces of thrust at 17 amps. Use with caution
The GWS EP8040 produces about 24 ounces of thrust at 12 amps. Best all around prop for this motor.
The TP 8x4E produces 25 ounces of thrust at 14 amps.
The GWS HD8060 produces 22 ounces of thrust at 17 amps. Use with caution
The GWS EP7035 produces 15 ounces of thrust at 7 amps.
The TP 7x6E produces 19 ounces of thrust at 12 amps.
The highest static thrust figure for that motor is 24oz =680g.
The 850kv motor speced for the hexa build (I know yours is a Y6) is
Notice with the 10x4.5 is makes 1095 g of thrust.
The larger 880kv https://store.diydrones.com/Motor_AC2830_358_880Kv_p/ac-0004-05.htm
Makes 1380g thrust with the 12x4.5 prop.
Simply put, you need to get better motors.
Your battery system is excellent, but the rest of your build is way off. I hope you got 25 or 30A ESCs because the 850 burns 18A and the larger 880 burns 20A. I always give myself a safety margin so higher amp ESC is better.