This is a compressed little story of an endurance test today with a new hexa multirotor. Some time ago I noticed a post from Ecosynth made here, which attracted my attention. It outlined the use of an octocopter with 30 minutes of flight time for the purposes of ecological analysis and 3D modeling. On the same website I found a concept design that someone specified for a hexa which was cheaper, lighter and offered slightly more endurance. This hexa is almost a 1:1 realization of that design.
The result above shows it easily achieves 30m30s in hover with 168g payload, reducing the battery from full to nominal voltage. Running the battery flat to 3.4V per cell or so I guess I could have taken out some extra 5-9 minutes. The calculations show it should be possible slapping another battery on for 53 minutes total, but in practice I'm noticing the signal going to 3 motors is getting a bit high already, so I'm not sure if there really are power reserves to do that. With 500g payload the duration is reduced by some 6 minutes or so (theoretically).
Next thing to do is see how endurance keeps up when the vehicle is in actual flight. At slow speeds like 5 m/s I don't expect too much reduction in endurance, it may actually improve slightly.
I dove into the subject of multirotor endurance and found some really good information here: http://www.rcgroups.com/forums/showthread.php?t=1880665
There are three main design pillars for multirotors: agility, weight and endurance. Optimizing towards one or two will definitely remove capabilities in the other. The choices for propellers, motors, frame, battery should be made on the basis of this understanding. There's a tool called e-calc , which was instrumental in verifying the design.
The objective of this vehicle is to map areas up to 1km2 for 2D and 3D modeling and perhaps some video surveillance assistance. All the electronics are 3DR, drive system from rctimer.
Comments
@Saad: the larger rctimer ones are these: http://www.rctimer.com/product_922.html (the "TM1515" ones). The ones that work are these: http://www.rctimer.com/product_747.html, yet have a thrust difference of 20-30g. AUW would have been about 2kg, relatively low with pwm outputs in the 1400 ranges, sometimes dipping towards 1350.
@Para: thanks. I think in the end I need to make similar changes to the motors I bought once the rest gets here. Haven't been able to draw more than 4.11A on my test stand and generate more than 590g on any propeller, but it's close to a table, which could interferere with airflow. You were on 4S?
Confirmed!
Gerard, I made a new test. I corrected the tilt of the motor/prop combination. I measured from the distance from a leveled surface to the tips of the props in all directions. In most cases the left side was 0.5 - 1 cm higher compared to the right side resulting in a continuos push in CCW direction. I corrected it and analyzed the logs. You find the results here. I have to double check it (tomorrow). However, it seems to be part of the problem.
I have been flying a quad based on RCTimer 5010-360Kv from april 2012 until last february. At the time, the only suitable props for my 4S packs were the $85 a pair 15x6 CF. Pretty costly, but I was surprised that every time I had to order a pair, RCTimer would upgrade my shipping from Airmail to DHL.
Back in the day, 6 ah nanotechs would yeld 24 minutes of flight, surpassing the 18-ish minutes I could get from faster motors on different quads. Then I put together two 6.6 ah 2S A-Spec boat packs and I got 30 minutes of flight time with a fixed Hero2. Never had the patience to just hover in a spot and time it.
My build was 1050g and the pack added 650g for 1,7 Kg AUW. With a 425g load on each motor the total current consumption in loiter was 13.6A and it didn't get above 22A in flight. On a test stand, the motor/prop combination would top at 9.5 Amps draw and 985g pull.
In the air it handled like a truck. Slow to accelerate, slow to recover from a fast descent and a handful to control in gusting wind. I had a hard time balancing the props. I ended up pairing them to motor bells, marking the position and holes used and balancing both the best I could. Still it vibrated a lot. I went from 10x10 alu square tube to 12x10 CF tube then to 12x8 CF tube which strengthened the frame, then put 4 silicone dampeners in the middle and tied the FC to the LiPo pack to isolate and add some inertia. Flew well, but jello kept creeping in the footage during maneouvers.
Then last winter the brushless gimbal craze started and I deemed the 5010s the best target for a rewind, which rendered the quad motorless.
Gerard,
Can you clarify what exactly you meant by "larger rctimer props". Also, what All-Up-Weight did you have when you had these flips?
Regards,
Saad
Sorry Gerard, I forgot we have such a strange motor numbering layout for the hex, it is clearly a difference based on CW / CCW.
Which still makes no sense at all, but it is hard to envision the motors being responsible, they are wound symmetrically on armature winding lathes and the parts are symmetrical and ball bearings don't care which direction they are turning.
So it pretty much seems to leave the props and from what you have done so far including switching motors around, the problem follows the direction of rotation but not the individual motor.
Still you have now used 2 different sets of props and the same problem existed virtually identically with both of them which makes it hard to believe it is simply props.
And it is difficult to believe that local frame misalignments could cause such a distinctive and repeatable pattern on all CW / CCW motors.
Hopefully my motors will be getting here soon.
I am thinking of trying the GemFan CF paddle props and am strongly considering the Tarot 650 Quadcopter as a good frame for research (can handle up to 17" props).
It's going to take quite a while for all that stuff to get here from China though and in the meantime I wish you great luck in getting to the bottom of this.
Right now,there simply doesn't seem to be any straightforward logical explanation for the results you are getting.
Some more testing today. I'm so stuffed up from this thing that I'm glad it's weekend. So just before leaving for a nice sunny beach with a beer.
- Replaced motor #5 with a spare one. That was the one with apparently more friction and stopping ahead of time. Outside, this time in rather heavy windy conditions, spun up the rotors and slowed them down. All stopped at roughly the same time now. So that issue is fixed.
- Verified ESC throttle settings. All motors spun up and had max throttle about the same.
- Set declination to a fixed value and turned off "auto-learning".
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On these larger props I didn't get a lot of stability. 23g vs. 16g and this makes a huge difference in wind. The hexa flipped twice. So I think it can't handle the torque.
In the end roughly the same graph, but the exact value distribution now looks different.
The next test I'm going to do will put the original props back on (it's pointless with these larger ones) and then I'll pinpoint the CG. It's off by 0.5-1cm to the back right now. The CG offset has been a constant so far.
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The larger rctimer props should not be used. It feels on the boundary of loss of control, no snappy movements, a hint of oscillation and the two flips. I think the motors can't handle the torque at these low rpm's. Maybe the large battery and more load improves that as the rpm/torque graph goes up, but then I'd still be concerned about descents. So slim props are better to increase stability for low kv motors. With the larger props the hexa really travels on the wind.
2,4,5 are CCW. 1,3,6 are CW. The first couple of screens further up this thread are the slim propellers. The last graph 4 comments up is from another type of propeller I purchased from rctimer which has a broader middle bit. The result... still the difference. I measured thrust on the slim props and there was a substantial difference, averaging 30g per prop and there was also quite a bit of variation in the thrust measured (20g). I also measured the replacement props and they had less variation during the test and ended up around 580-590g thrust per prop.
First I'll work out the #4 cutting out (pwm wire).Then to rule out absolutely everything I need to verify what throttle limits the ESC's are set to.
Then I can try to replace my #5 motor, which is running a bit heavier from the others. I wouldn't expect this particular pattern if it did though, more like lower throttle on the opposites and slightly higher throttle on side motors to compensate.
If by then it's flyable, I'll use it for a couple of days this way. Soon as the new motors/props arrive I'll slap those on and rerun these tests, see if anything changed.
Wait, he sees motor to motor variation, and it's not simply CW vs. CCW? Sounds like not all the motors are wound the same.
Gerard it still seems very strange to see such a difference in PWM output to one batch of 3 motors from other 3 motors.
And I notice they are not simply CW / CCW ones, because 2 pairs are next to each other on the same output band.
It doesn't seem reasonable to me that you have 2 sets of 3 motors that each match in throttle requirement and yet are so mismatched with each other.
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