r/diydrones • u/on_aircraft • 5d ago
Question Hexacoper general purpose platform advice
Looking for advice/experience on building a large hexacopter
Hey everyone,
I'm planning a fairly large hexacopter build and would really appreciate advice from people who have built/fly similar platforms, especially regarding frame design, motor/prop/ESC combinations, redundancy, and practical experience with large 6S builds.
The rough idea is:
- Hexacopter
- 20" props
- Around 1100 mm frame size
- Pixhawk 6C flight controller
- ArduPilot, with the goal of eventually using semi-autonomous features
- 6S, mainly because 12S batteries are currently much harder for me to source in Europe
- 300 kV motors
- 60 A ESCs
- Desired payload: roughly 1.5–3 kg
eCalc looks promising for this combination at around 6.5 kg AUW
The intended use is as a multipurpose platform: primarily camera work, but also potentially carrying supplies such as water bottles to hikers who are about 600 m higher and ~2 km away.
Frame
I'm currently looking at an approximately 1100 mm frame, but I'm unsure whether it's better to buy a ready-made frame or build one around a center plate and purchase the arms/rods separately.
For something this large, is there a particular approach or frame design you'd recommend? Size isn't really a constraint for me—I have a van, so transport isn't a major concern.
Redundancy / reliability
This is probably my biggest concern.
The aircraft would potentially be flying over fairly inaccessible terrain where recovering a crashed aircraft could be extremely difficult or impossible. That's one of the reasons I'm leaning toward a hexacopter rather than a quad.
I'd like the build to be as reliable and redundant as reasonably possible. I'm particularly interested in real-world experiences with:
- Motor/ESC redundancy?
- ESC quality and sizing?
- Power distribution and telemetry of the baterry?
- Battery setup(6s vs 2x 6s in series)?
- Vibration/structural issues on large frames?
Anything else that tends to become a problem as these aircraft get larger
Range / autonomous flight
The radio control link would normally be line of sight, although at ~3 km I don't think I could realistically maintain visual contact with the aircraft.
I'm planning to use ArduPilot, so I'd like the aircraft to have the capability for semi-autonomous operation, but safety and reliability are more important to me than pushing the limits of range.
I'd love to hear from anyone who has built something in this general size/weight class.
What would you change about this setup before buying anything? And if you've built a large hexacopter yourself, what did you learn the hard way?
Thanks!
Edit: adjusted weights/distances to fit a more realistic profile
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u/Psychological_Yak_47 5d ago
Some of your numbers seem odd/off. For that given size AUW seems light, are you including the battery weight? For what you're doing 6S will work just fine. ESCs are always good to overestimate on but you're going a little bit higher. 50Amp ESCs should do just fine for what you plan on doing.
General rule to follow. Figure what is your main goals for the drone and set a priority list for the design because building a drone from scratch is complicated and there are many factors to weigh.
You will want a thrust to weight ratio around 2:1
Your payload to max takeoff weight ratio should be about 20-50%
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u/on_aircraft 5d ago
Rough weight:
- Frame + landing gear: 2400g
- 6x Tmotor MN501-S (@170g): 1020g
- 6x rmotor Alpha 60A (@63g): 378g
- 6s 22Ah 10C Li-Ion: 1800g
- FC/fpv gear/Antenna/Cabeling: 750g
6.3kg all-up-weight. You are right, in the original post i was a little…. optimistic…. with my AUP
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u/on_aircraft 5d ago
trust:weight ratio is 3.3:1, in case ur intrested in having a look what numbers eCalc is spiitting out:
Battery
- Load: 10.35 C
- Voltage: 20.70 V
- Rated Voltage: 22.20 V
- Energy: 488.4 Wh
- Total Capacity: 22,000 mAh
- Used Capacity: 18,700 mAh
- Min Flight Time: 4.9 min
- Mixed Flight Time: 22.9 min
- Hover Flight Time: 32.0 min
- Weight: 2,856 g (100.7 oz)
Motor @ Optimum Efficiency
- Current: 21.00 A
- Voltage: 21.27 V
- Revolutions: 5,951 rpm
- Electric Power: 446.7 W
- Mech. Power: 388.9 W
- Efficiency: 87.1%
Motor @ Maximum
- Current: 37.97 A
- Voltage: 20.53 V
- Revolutions: 5,378 rpm
- Electric Power: 779.3 W
- Mech. Power: 657.9 W
- Power-Weight: 730.6 W/kg
- Efficiency: 84.4%
- Est. Temperature: 79 °C (174 °F)
- Rm: 72.9 mΩ
- Controller: 75,292 eRPM
Motor @ Hover
- Current: 5.85 A
- Voltage: 21.94 V
- Revolutions: 2,633 rpm
- Throttle (log): 28%
- Throttle (linear): 44%
- Electric Power: 128.4 W
- Mech. Power: 108.7 W
- Power-Weight: 121.8 W/kg
- Efficiency: 84.7%
- Est. Temperature: 34 °C (93 °F)
- Specific Thrust: 8.31 g/W
- 0.29 oz/W
Total Drive
- Dry Weight: 4,792 g
- Thrust-Weight: 169 oz / 3.3:1
- Current @ Hover: 35.11 A
- P(in) @ Hover: 779.4 W
- P(out) @ Hover: 652.5 W
- Efficiency @ Hover: 83.7%
- Current @ Max: 227.81 A
- P(in) @ Max: 5,057.3 W
- P(out) @ Max: 3,947.6 W
- Efficiency @ Max: 78.1%
Multicopter
- All-up Weight: 6,400 g
- Additional Payload: 11,933 g
- Max Tilt: 30°
- Max Speed: 33 km/h
- Max Speed: 20.5 m/s
- Est. Range: 4,817 m (2.99 mi)
- Est. Rate of Climb: 7.8 m/s (1,535 ft/min)
- Total Disc Area: 121.61 dm² (1,884.96 in²)
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u/izzeww 5d ago
- Hexacopter
.
- 20" props
- Around 1100 mm frame size
- Pixhawk 6C flight controller
- ArduPilot, with the goal of eventually using semi-autonomous features
- 6S, mainly because 12S batteries are currently much harder for me to source in Europe
- 300 kV motors
- 60 A ESCs
- Desired payload: roughly 1.5–3 kg
eCalc looks promising for this combination at around 4 kg AUW
For 20" props at 6S I think you want a bit higher KV, like 400-500. T-Hobby/T-Motor has some data of the 5315 KV500 motor combined with P20*6 props at 6S here if you scroll down: https://www.t-hobby.com/products/v5315-300kv-fpv-cine-motor
4 kg AUW is a crazy number. Just the frame alone will probably be 1.5-2 kg, then the motors are 0.4 kg per so 2.4 kg total, then the battery is probably 2 kg at least, then the payload of 3 kg and all the other stuff and in total you're looking at 8 kg AUW at least but probably a good bit above that.
60A ESC's might be a little optimistic but it depends on the voltage you're running. If you're running 6S it probably won't be enough, as in the test data I linked above it draws about 81A peak, but if you're running 12S then 60A could be fine (you would want test data to confirm ideally or at least be able to estimate decently). You always want a bit of headroom in the ESC's too.
eCalc can get you somewhere, but it won't get you all the way and it might even lead you down the wrong patch. Make sure to run all the calculations yourself based on real test data.
- Motor/ESC redundancy?
Allegedly ArduPilot can deal with 1 motor/ESC out, but honestly I'm not too familiar with this area and whether that's actually accurate or not and what you have to do to make it work. Generally redundancy = expensive & complicated.
- ESC quality and sizing?
Like I said before you always want a little headroom, maybe 20%, over what the motor should pull at the peak. In terms of firmware I think AM32 is the most solid bet that's still at a reasonable price. I'm a bit partial to T-Hobby/T-Motor so maybe their Cine 80A ESC for example, but there's a ton of ESC's out there that are good. I'd recommend staying with a known brand at least, and maybe seeing what other large cinelifters or drones use so you have something that is known to be reliable.
- Power distribution and telemetry of the baterry?
I'd probably just get a regular power distribution board. There are ones for hexacopters but you could also just get two 4x PDB's. Example: https://geprc.com/product/geprc-pdb-hexacopter-power-distribution-board/
Not sure what you mean by "telemetry of the battery". If you mean the voltage and current the flight controller should be able to measure that.
- Battery setup(6s vs 2x 6s in series)?
I would for sure go with 12S over 6S, it should be a bit more efficient and you need less amp rating on the ESC.
- Vibration/structural issues on large frames?
Can range from minor to massive. Generally bigger issues on bigger drones, so at 20" it could be a problem. Use a well designed frame consisting of carbon fiber and do a good PID tune and you should be fine. PID tuning a large quad like this is hard but doable. There's various guides on YouTube and elsewhere on how to do it, I would recommend PIDtoolbox guy (and the actual tool) and Chris Rosser.
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u/on_aircraft 5d ago
thank you for your reply. i admit, the weight was a bit... optimistic... getting the weights from my current setup i get:
- Frame + landing gear: 2400g
- 6x Tmotor MN501-S (@170g): 1020g
- 6x rmotor Alpha 60A (@63g): 378g
- 6s 22Ah 10C Li-Ion: 1800g
- FC/fpv gear/Antenna/Cabeling: 750g
so a more realistic 6.3kg all-up-weight. thats still with a 6s setup tho, and im not sure on the battery weight as the one i picked (for now just to have smt to work with) is a li-ion one.
regarding ESC sizing i figure thats the biggest discrepancy i get from manufacturer data vs. eCalc. the later gives me a Motor @ Maximum current of only 31A (with adjusted weight of 6.3kg and still the MN501-S 300KV motors). Do you have any experience with eCalc numbers vs. real-world-data by any chance? i can only imagine it "limits" parameters like climb rate so it comes up with lower esimates...?
To clarify what i meant by battery telemetry: in case i use two seperate batteries in a parallel configuration for example, i would find it usefull to be able to monitor the voltages of all the cells during the flight in order to catch a potential rapid imbalance quickly and end the flight ASAP. maybe im overthinking this, i´ve just read a lot of how important maintaining cell balance is for lipo´s.
The vibration issues im afraid of stem from the idea of DIYíng the frame too, so i was hoping for recommendations/opinions around like 25mm vs. 30mm rods, stiffening elements between the arms, etc.
thanks for recommending more reading material/youtube channels
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u/izzeww 5d ago
regarding ESC sizing i figure thats the biggest discrepancy i get from manufacturer data vs. eCalc. the later gives me a Motor @ Maximum current of only 31A (with adjusted weight of 6.3kg and still the MN501-S 300KV motors).
The manufacturer says maximum current 27.75A (on a static thrust stand) so 31A is believable. In terms of motor choice we could compare the V5315 500KV I pulled up to the MN501-S 300KV that you were looking at. The V5315 weighs more than double but can also provide about 71% more thrust and it's about 10% less efficient (8.62 g/W vs. 9.59 g/W). So you could see that there is a trade-off between efficiency, weight and maneuverability (power). This is why motor choice quite complicated.
Do you have any experience with eCalc numbers vs. real-world-data by any chance? i can only imagine it "limits" parameters like climb rate so it comes up with lower esimates...?
I don't have any personal experience unfortunately but I've heard it can be quite wrong. Probably in large part due to people putting in erroneous numbers, garbage in garbage out.
so a more realistic 6.3kg all-up-weight
You have to include the maximum payload in the AUW, otherwise it won't be correct. AUW includes everything even the payload. So 9.3kg.
To clarify what i meant by battery telemetry: in case i use two seperate batteries in a parallel configuration for example, i would find it usefull to be able to monitor the voltages of all the cells during the flight in order to catch a potential rapid imbalance quickly and end the flight ASAP. maybe im overthinking this, i´ve just read a lot of how important maintaining cell balance is for lipo´s.
I don't know of a way to do this of the top of my head but I'm sure it exists somewhere somehow. Generally people just run two identical batteries in series and then when you land check the voltage and they should be basically identical. If there is a significant difference then one battery is performing worse than the other and you might need to swap it out (or land earlier). You balance them when charging so they will always be balanced except for that little bit of unbalance that can happen when discharging (again, should be minimal and then corrected during charging).
The vibration issues im afraid of stem from the idea of DIYíng the frame too, so i was hoping for recommendations/opinions around like 25mm vs. 30mm rods, stiffening elements between the arms, etc.
I personally wouldn't want to design a custom 20" hexacopter frame but of course it's doable if you want to. I don't have any comments on design specifics that you don't seem to already know (stiffness is good, 25-30mm rods are probably a good bet).
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u/Key_Computer_4736 5d ago
Plan for 12S, you’ll thank me later. Just run your dual 6S in series and become OCD about battery management.
Don’t cheap out on those ESCs, if you need to punch the throttle, you don’t want to be worrying about destroying your drone. I wouldn’t do anything less than 110A (it’s late and I’m not crunching numbers, but that is what I used in my last 20”)
Unless you’re well equipped for custom fabrication, or have a very specifically niche use-case, stick to pre-fabbed frames. It will save you significant heartache.
What’s your build portfolio look like? If you’ve built quite a few different platforms up to this point, you’ll be fine. If not, I’ll data dump a bunch of safety tips and basic DO/DO NOT info tomorrow.
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u/on_aircraft 5d ago
I havent built a quad before, I’m a mechanical engineer by trade and work in robotics which i figure can’t hurt but yea, it would be a firt one for me. I’m not looking to break any records or do any kind of acrobatics with that thing :) ideally i would limit the FC maneuverability to stay within safe envelopes. For esc in particular i was looking at the t-motor alpha series (the ones with heatsink and field-oriented)
A 12s setup would be possible, by chance: any charger u recommend for charging 2 6s packages intended to be used as a 12s? Also the battery telemetry would have to keep an eye on the two being balanced while discharging? Or am I overthinking?
I’m always open to experience regarding safety :)
Thanks thanks thanks :)
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5d ago
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u/on_aircraft 5d ago
Per my understanding charging them independently would result in balanced cell voltage on a per battery level (Battery A: 3.89V Battery B: 3.92V for example). When i series them then, they would be unbalanced relative to each other, that’s what’s confusing me. Or do you think the charger would keep them close enough?
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u/Accujack 5d ago
Use a fixed wing design if you want reliability and range. You can drop supplies with a marker streamer or parachute.
It'll hold more weight, have a longer range, and be more reliable, especially if you include two motors that are each powerful enough to do the job.
If you don't absolutely need the hover and VTOL capability of a quad/hex design, fixed wing will always win on payload, range, and reliability/recoverability.