I tested some high KV motors with 6S and the result is pretty interesting. Generally it’s recommended to use low KV motors around 1700KV for 6S and 5″ propellers. But when I first heard the idea of using high KV motors on 6S I was a bit skeptical so I decided to find out for myself :)
Low KV on 6S
I recently built a 6S Mini Quad and it’s great fun to fly! (Build log and videos)
The idea is to choose a low KV motor for 6S, so the “theoretical maximum RPM” is similar to a 4S mini quad with 2500KV motors.
Despite of the similar max RPM, the performance of my 6S quad really stands out. It has great acceleration and punch, yet there is so much low end resolution which makes it noticeably “easier to handle”. It’s extremely responsive and puts much less stress on the batteries.
I wrote an article about the differences between 6S and 4S in more detail.
High KV on 6S
So what happens if we use higher KV motors with 6S? Well, It’s basically the same as using higher KV motors with 4S.
Higher RPM, more power, and faster!!
But your quad will be pulling a lot more current at full throttle, it’s going to be really bad for your components, especially your battery, ESC and motor.
Motor RPM and current draw has an interesting relationship, they are NOT proportional! Amp draw increases more rapidly than RPM. This means you will lose efficiency in high throttle.
Setting up a Throttle Curve
In order to use high KV motors, we will have to use a throttle curve to lower the top of the throttle. The way I see it is, not only it lowers current, you also get better resolution on the throttle range that is meaningful to you when racing.
Here is a tutorial on setting up throttle curve in the Taranis.
Another cool thing with high KV on 6S is you can remove the throttle curve and run 4S or 5S! :)
So here is my curve for the 6S setup.
I basically started from 100% (-100 to 100) and worked my way down by monitoring current at full throttle. At 70% (-100 to 40), the amp draw drops to about 65A-70A max which is about the same as my previous low KV setup (1720KV).
As an interesting observation, the 1720KV motor has a max RPM of 43344, which is about 73% of the 2350KV of 59220. :)
Once setup, you should verify if the throttle channel is actually outputting the value you want at full throttle.
Update (May 2019): Betaflight 4.0 now has two features for adjusting the power of your quad too. These allows you to scale throttle, which effectively lets you turn down the throttle input.
2350KV on 6S
I simply swapped out the old 1720KV motors with these 2305 2350KV, which are rated for 2S to 6S.
Here is a video of my test flight.
Somehow these 2350KV motors are extremely noisy, even when I run 4S, so I am pretty sure there is something wrong with them. Apart from the noise the quad still handles great and it was enough for me to complete this test :)
Are High KV Motors Better for 6S?
I think it depends.
If you want to keep the same efficiency, and have the same amp draw as running lower KV motors, you can use a throttle curve to limit throttle. But then the flight performance really just feels like running lower KV motors… Maybe I haven’t flown it enough or tested enough motors to tell the subtle differences, but that’s what I experienced so far.
In my opinion, the biggest advantage is you can “unleash” the power of higher KV motors whenever you want by removing the throttle curve, or slightly raise the limit. This can be flexible for the different types of flying and race. And you can also run 4S and 5S on the same setup which is great :)
Lastly I think lower KV motors are more efficient and give me slightly longer flight time for similar flying.
Next, 2750KV on 6S!
No, No, No!
The quad flies terrible on these high KV motors (RCINPower Mark1 2306 2750KV). There is so much oscillation at mid/high throttle I can’t get rid of. I tried adding more capacitors, more FC and motor soft mounting, nothing seem to help. I even swap out the frame for a more sturdy one. Same issue.
I don’t blame these motors, they have excellent build quality, and they fly great on 4S, just not the case on 6S.
Maybe Pro racers can benefit from the extra power of higher KV motors, but for casual pilots I really don’t see the point and cannot recommend it. I will stay with lower KV motors for 6S for now.
Anyway it’s been a fun experiment, and I learn a few things along the way such as using throttle curve to limit current draw :)