I am currently building a custom X8 coaxial octocopter designed for stable aerial cinematography. I am using a mix of legacy parts (motors found on a shelf) and modern control systems. Before the maiden flight, I would highly appreciate an expert sanity check regarding the electrical limits, thermal behavior under heavy load, and overall airworthiness at an estimated takeoff weight of ~6.0 kg (carrying a Panasonic GH5 and a lightweight 3-axis brushless gimbal).
Aerodynamics (Pitch Staggering): To optimize coaxial efficiency and balance yaw torque, the bottom propellers utilize a 5.5" pitch, while the top ones use 4.5". This directly addresses the accelerated downwash from the upper layer, ensuring the lower blades retain a functional angle of attack.
The 6S + 14" Propeller Dilemma: Running a 14" prop on a 770KV motor with full 6S voltage ($25.2\text{V}$) would pull over 40A at full throttle and burn the windings. By hard-limiting the maximum motor output to 60% in ArduPilot/iNav, the effective average voltage is clamped to roughly $13.5\text{V}$ (simulating a 4S system). This keeps the peak current draw per motor within a safe 13A to 15A window, staying well under the motor’s 350W power limitation.
The Real Challenge: 6.0 kg Takeoff Weight
Hover State: To lift 6.0 kg, each of the 8 motors must consistently generate ~750g of thrust. According to the motor spec sheet, they produce up to 2200g max thrust. 750g puts them in an efficient RPM range, drawing an estimated 6A to 7A per motor. Total hover current will be around 48A to 56A.
Dynamic Peaks: When punching out or countering wind gusts up to the 60% electronic limit, individual motor draw will rise to ~14A. Across 8 motors, the total system draw peaks at ~112A. Split across the 3P battery configuration, each Li-Ion cell must deliver ~37.3A.
My Main Concerns for the Community:
While high-drain 21700 cells can handle a 37A peak pulse, the internal resistance of Li-Ion chemistry will cause a massive Voltage Sag under sudden load spikes (potentially dropping the pack from 25.2V down to under 20.0V instantly).
Given that this drone will only be used for smooth, non-aggressive cinematic tracking shots, do you think this setup is reliable, or am I pushing the Li-Ion pack and the heavily restricted 6S Teillast (partial load) ESC setup too close to the edge?
I never clamp any of my builds to prevent burnout of the motors. Basically, for short gusts of wind, where you would comensate with higher power input, passing above the maximum capacity of the motors does not do any damage. I think for a few seconds 50 to 70 % above maximum power rating is absolutely OK. I routinely do that, and also design the copters in such a way that a full throttle would exceed the nominal power by 100 %. What is not OK is to have too hot motors, and if you run them hot for a long time, then they will get damaged, and often it occurs well belov advertized maximum power. Caveat, lately the chinese advertize insane powers for small motors. But this power rating is exactly this punch rating…
As to the battery sag, that is OK, ardupilot most probably will compensate well for it. Just an example, I was testing my VTOL, 5S (15..21 V), and I could fly it even with at 11 volts (with a huge current whgich peaked at 150 A).
6S3P for 40 amps will most probably heat the pack too much. You most probably will have a problem with that. But this battery is only just above 10 % of the copter weight. Usually you should aim for about 30 % battery weight. So I definitively would increase the total battery weight (double it).
Freewheeling ESCs if designed for plane are a no no for me. I had a VTOL based on YEP ESCs and it did not go well.
what batteries are you planning to use?
I have 6s3p packs made of EVE 40PL Tenpower 50XG.
The EVE battery can deliver 70A peak, the Tenpower can deliver 90A - according to the datasheet.
So far I have not seen anything that would rise a red flag with these packs. I’m using the packs in a 14inch quadcopter. TOW: 3.5-4kg
My previous copter (5.1inch) was using the same EVE cell, in a 4s2p config. At full throttle the quad drained 100Amps from the pack.
In both drones, the packs were getting warm after heavy use.
I have removed the second almost identical topic. Keep relevant discussion here. Also, please avoid blind copy paste of LLM/AI output. No one really wants to just talk to a robot with you as the proxy.
Going from 4 kg to 6 kg will really heat up the pack, especially the interior cells. It is always better to make two small batteries to improve cooling than using a big one.
