Mass and lift calculation - help!

[asomaro]

I never was much good at calculations.... so if one of you clever people could help, I'd appreciate it!

I need to work out what mass / weight could be lifted by 6 motors, each motor will spin a 10x4.5 prop and each motor is rated at 930kv. I've put a watt meter on the motors and at 90% throttle they read 133 watts each (pulling just under 13 amps each).

Obviously this is a hexacopter that I'm building from scratch so I just need to have some idea of what it will be capable of lifting. The current target for the all up weight is 2.40kg and I'm almost spot on so far. I need to understand the limits of the weight so that I know what camera I can play with!

Any help would be much appreciated

Cheers,

[trillian]

You really need thrust figures to plug in to the equation to find anything meaningful. There are some guidelines around about watts per kilo for fixed-wing but it would only have the vaguest of implications for a multi-rotor copter. There may be similar rules of thumb available for that though. Very roughly, if you have a total of 798 watts (133 X 6) to work with, hmmm it's hard to say. I can only compare with a similar weight heli. A 500 weighs about 2kg ready to fly but is probably more efficient in terms of lift.

My rough guess is you're probably OK with those figures but that's just a guess. If you had thrust numbers you could calculate it exactly.

[Toolman]

The problem you have is that the motor calculators out there arn't much use for multirotors because you need to calculate for around 50-70% throttle to allow for manouvering, plus you are flying effectively in stationary air where as they generally allow for the speed the model is flying at.

I did some testing with motors & props using a digital scale to measure actual thrust.
I did test a 900kv motor on 3S using a 10x3.8 APC slowfly prop, I was looking at lift vs current so didn't go above 15A which gave 650g thrust. I think that was about 85% throttle. From memory I think full throttle was just over 750g.

Your 10x4.5 prop would give about the same result, as interestingly enough the thrust is almost completely dependant on dia not pitch. The pitch just effects the current draw.

If you allow 650g per motor that would still allow a bit of margin for contol.
Hope this helps
Tim.

[asomaro]

Interesting - I think I'm going to set up a test rig and see what a single motor will actually lift. and then maybe work it backwards to see how it compares to the "theory" - all good fun!

Cheers,