I'm planning to build a hexacopter and trying to find some motors which will provide good efficiency. The up in air weight will be around 3.6 kg(600-900g frame, 6x150g for motor + esc, 800g battery, 500g gimbal, 500g camera) give or take a few hundred grams. As far as I know, the key to long flight times is large props and low KV motors, given that all else remains constant. I've been looking at the rctimer HP4215 and sunnysky x4108 which provide about 600g thrust at around 3A according to test data. In my head that should give you 3600g thrust at 18A, giving a hoover time of 16min with a 5000mAh. In theory anyway. What I'm wondering is if using a motor like this would be overpowering the copter, giving control issues? It seems like the motor would be working at it's lower range when hovering. How about if I try to fly it without gimbal and camera, loosing another 1000g? On the other hand it seems to me that the key to long flight times is having a motor which provide alot of thrust with low current. From the test data I've seen, this seems to be related to high power motors with low KV. Am I missing something?
As a side note: for motors which lack test data but have test data for a higher KV motor of the same type, can I assume that the low KV motor will provide the same lift as the higher KV motor at same or less current?
also, for maximum efficiancy, don't forget to use high voltage, IE 6S.
This lets you send twice the power to the motors using the same Current, if you compare 3S and 6S and have ESC's and motors that tolerate both. The 6S motor market is quite small though.
I am flying an Octo Monster, 8x3536/9 motors (910kv-370W) at 4S. It can probably carry a lot of weight, but it really drains the batteries. For my self, I would choose a smaller setup next time, probably something like 250W motors to carry a Nex5-camera and allt he equipment I need. But that's just a gut feeling.
The HP4215's look very nice at 6S, giving you lots of power if you need it. You might get away with smaller motors though, but I would say that this is a nice, safe choice.
As mentioned high efficiency = high voltage with low kv and large propeller.
Motor size/kv should be selected so that hover with payload is at or slightly below the RPM efficiency peak of the motor. This all sounds easy on paper, but can be very hard to achieve in reality. Data on motors from manufacturers usually is lacking or just plain wrong. Using eCalc you can get and idea of how it will work, but the only way to be sure is to test different motor and propeller combinations.
I've wasted considerable time with eCalc without getting wiser. I'm a bit at loss as how eCalc calculate flight time as it seems that mixed flight time sometimes is close to the full throttle flight time and sometimes close to the hover time. I think that it might actually be more useful to get a graph of the flight time vs. battery capacity f.ex.
My biggest concern is that the high power motors with large propellers will run on a very low RPM in hover, atleast if I don't fly with full payload and thus the APM would have a hard time controlling it. Is this a valid concern?
To some degree, but a normal 15" propeller setup should not be a problem. A common setup for large "efficient" hexa or octo copters is 6S, ~300kv motor and 15x55 propeller. Alternatively 4S and ~500kv motors.
Having a more "powerful" motor is very subjective. Making the motor bigger does not automatically make it more powerful. Larger motors means that it will handle more watts, but they also have a lower kv. So unless you scale the voltage and propeller size to match the motor you hardly get any lift at all since the motor isn't capable of spinning fast enough. So you have to look at it the other way around. How small can the motor be and still be able to drive the propeller size/voltage you want to the rpm you need without overheating.
Read this thread.
Wow, that's a long read. +1 for effort, but I was thinking more about off the shelf components and get good flight times, not going for records. To recap my understanding of that thread: Keep weight down, use large propellers, low KV motors, batteries with high energy density, keep max current low so you can use batteries with low C rating which are lighter, higher S is not necessarily more efficient(not that I understood why...), quads are always more efficient than hexa because of the extra frame and motor weight (though Y6 might be a good compromise with a chance of getting your expensive camera and gimbal down in one piece in case a motor fails).
I found this table interesting:
Note the efficiency curve. I suspect it's similar in shape for other motors too. So basically I should be looking for the smallest, lightest motor that will provide enough lift for the weight I'm after in it's low throttle range as that's the throttle where it's most efficient?