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My Octo Doesn’t Lift Enough or Fly Long Enough – What Can I Do?

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Hello Everyone :)

My goal is to build an octocopter that will be able to lift a 15lb to 25lb payload and fly for at least 15 minutes (hopefully more).

The problem I’m having is with reading the specs of the components.

Some are only in Chinese, some are inaccurate, and there doesn’t seem to be a strong standard to make an apple to apple comparison.

So I built what I thought would be a heavy lifter, only to find out that it isn’t quite up to the task.

Here are the components I’m using (won’t include Naza and other components non-essential to flight time and lift):

* Hobbyking X930 Glass Fiber Octocopter Frame 895mm (Heavily Modified)

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=26455

* (8) NTM Prop Drive Series 35-36A 910Kv / 350W

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=14847

* (8) HobbyKing F-30A 30A SimonK Brushless ESC w/ 3A 5V UBEC

* (8) 11X4.5 Props

* Turnigy nano-tech 6000mah 3S 25~50C Lipo Pack

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=20681

** Weight of the craft: 8.5lb

** Weight with 1 Battery: 9.5lb

** Weight with 2 Batteries: 10.5lb

** Weight with 3 Batteries: 11.5lb

*** Flight time with 1 battery: 6.5 min

*** Flight time with 2 batteries: 12.5 min

*** Flight time with 3 batteries: 11.5 min

As you can see, the weight of the 3rd battery (approx. 1lb) nullifies the benefit of the additional 6000mAh and actually reduces the flight time.

Now this one is a big problem for me because my camera and gimbal weigh 5 or 5.5lb:

*** Flight time with 2 batteries and a 5lb payload : 3.5 – 4.2 min.

-----

One thing that I noticed in hind sight is that my battery is only 25C rated, and my motors max out at 38 amps. HobbyKing sold me the motors and their listing suggested 30amp ESC.

My tests showed that the max amps that I could draw with a 12” prop was 23.6 amps and with an 11” prop the max was 20.1 amps.

Because of my tests I felt comfortable with the 30 amp ESCs, but I’m wondering if the battery’s 25C rating is responsible for the low max amp readings or that the HK specs are just off??

I have some questions:

-- Would a higher C rating battery actually give me better (higher amperage readings) test results?

-- Would a higher C rating give me more lifting capacity?

-- Would a higher C rating give me more flight time?

-----

**** Here is the million dollar question… What do I have to do to upgrade the above octocopter to lift 8lb and fly for at least 15 min?

-- I like these motors, they’re smooth and I feel I can trust them… Is there anything I can do to get more lifting capacity and flight time out of them?

-- What motors would give me the results I’m looking for?

-- Would more cells make a difference?

Any advice and feedback is greatly appreciated!!!

Anthony

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Your run time of 3.5 minutes is low since the 15 lb model should achieve about 6-8 minute flights with two parallel 3S/6AH packs. But the exact time will depend on how you fly it.

A heavy lifter using 3S power is not the most efficient solution due to the large currents. I'm not a big copter builder so can't say for sure what the sweet spot is, but I suspect that a carefully chosen motor/prop setup using a 5S or 6S pack would be a more likely solution. Best to check to see what the other big builds are using.

I've used several of the 2200mAH/45C rated Turnigy Nano's on a small 450 class Quad. From my observations they are nothing special because overall performance dropped sooner than I expected. BTW, from my experience the stated C ratings on some packs are too optimistic, so it is best to choose higher C ratings than you think you need to compensate for this.

To answer your questions:

-- Would a higher C rating battery actually give me better (higher amperage readings) test results?
Yes. Voltage sag under load is less, so current is higher.

-- Would a higher C rating give me more lifting capacity?
Yes. Voltage sag is less, so motor wattage will be higher.

-- Would a higher C rating give me more flight time?
Yes because the voltage sag will be less so LVC will occur later. But your 15 minute requirement is going to be a serious challenge with a 3S power system on a 15 lb Octo. You will find that as more batteries are added the weight burden causes a diminishing return.

-- I like these motors, they’re smooth and I feel I can trust them… Is there anything I can do to get more lifting capacity and flight time out of them?
As mentioned, prop choice is extremely important. There are free R/C motor calc programs you can use to simulate/experiment with different props to find the best compromise for your needs.

-- What motors would give me the results I’m looking for?
I don't have experience with building big heavy lifting octos. But very low kv motors seem to be the usual solution for situations like yours.

-- Would more cells make a difference?
Yes. But this will affect your motor and prop choices.

My overall advise is that you need to completely rethink your motor/prop/cell count situation due to the payload weights you require.

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Thank you so much RC-Cam Mentor!

BTW an update to my flight times.

I made some adjustments to my payload and copter and did another test flight... this time in high wind.

Using the 2 batts mentioned before, and lifting a 5lb weight, I got 6min 35sec before I felt the power waivering.

I was in high wind, so that must have taken some time off of my flight, and I still had power in the batteries, but I had to use more power to maintain positive control and the wind was quite strong, so I landed.

You are spot on with your calculations!!

I find it difficult to get enough data to use the calc programs. Some vendors are good about publishing their specs, but others aren't , and without the right data the calc apps just error out.

I'm building my second octocopter (the one this threads is about is my first copter build ever and the new one is naturally my second build ever... so I'm new to this). It will have 500mm arms giving me plenty of room for bigger props (present octo has 12" props on it now, and I just got my 13" props, but that is the largest prop I can put on this airframe due to the close proximity of the motors to one another).

Based on my experience with the first one and your good recommendations, I will be getting the right combination of motor, ESC, Props, and battery.

As you suggested, I'll have to see what other big builds are using. I haven't really found any specs on anything bigger than an 800, and my new one will be bigger than that.

Thanks Again!!!!!

Anthony

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