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Everyone should be buying high kv motors for 4s, and 6s - Lets Discuss

In Blog

A discussion about this blog post is available on our facebook group here.

NOTE. Both my suggestions are now implemented in BF 4.0
Auto PIDS based on CELL COUNT
AUTO LIMIT MOTORS for 5s/6s

PROS

  • Can easily swap between 4s, 5s, and 6s batteries
  • Can use the same motors on all quads whether setup for solely 4s/6s etc
  • Shops can hold more stock of a smaller range as a 6s variant kv is not required to stock.

CONS

  • Have to remember to change rate/pid profile  NOW IN BF 4.0+!
  • You need quality components to begin with that cant take 6s voltage spikes and will last on 6s voltage 
  • Slightly less efficient?
  • You may require HIGHER voltage CAP as higher electrical spikes will occur. A 35v cap did not cut the mustard for my builds.
  • Only works up to a certain point. Eg. You can't run 2207 2700kv motors on a X Class build due to mounting, thrust, prop size etc.

Why not buy all high kv motors, simply limit the throttle when you plug in and then you can run both 4s and 6s rigs?

I recently did some research when I was building my new Quad Racer Nimbus.

The low kv motors I wanted weren't available so after some discussion with John from Quad Racer, I decided to run the new Black Bird 2700kv Motors and do some research as to how I could make it work.

If you simply limit your throttle to say 70% on your remote there is a problem.

You are only limiting the radio rc input. If you have a crash or hit a gate, and your quad needs to correct itself, it will use full power on a motor if it needs to, and thus your 2700kv motor will be spinning at "4000kv" (64800rpm) on 6s and things will blow up.

A solution put forward was a throttle scale discussed in more detail here, however, that DOES NOT WORK, and is simply the same as limiting throttle on your remote. If you want to do that please see below under "Throttle Limit / Throttle Scale Solution"

1. Custom Motor Mix

A solution that works is running a custom Motor Mix which I found from KWOD.co issue 1 (page 12-14) written by Luke Godeassi.

for a 67% limit you can apply the following cli commands

mixer custom
mmix reset
mmix 0 0.67 -0.67 0.67 -0.67
mmix 1 0.67 -0.67 -0.67 0.67
mmix 2 0.67 0.67 0.67 0.67
mmix 3 0.67 0.67 -0.67 -0.67

The only downside to this is you can't switch between 4s/6s on the fly.

2. Throttle Limit / Throttle Scale Solution

Select the rate profile that you will run 6s on, and goto CLI and type this

set throttle_limit_type = scale
set throttle_limit_percent = 85
save

For my setup I wanted to hit around 1800-1850kv, so i set mine at set throttle_limit_percent = 67 which gave me a kv of 1809

You also want to set up different PIDs for 6s, so you can also do this with a stick command as follows. I have my set up on profile 3.

My CLI looks like the following for this setup of both rates

(BTW 0 =" rate profile 1", 2= "rate profile 3")

rateprofile 0

set roll_rc_rate = 130
set pitch_rc_rate = 130
set yaw_rc_rate = 150
set roll_expo = 10
set pitch_expo = 10
set yaw_expo = 10
set roll_srate = 50
set pitch_srate = 40
set yaw_srate = 60
set tpa_rate = 30

rateprofile 2

set roll_rc_rate = 130
set pitch_rc_rate = 130
set yaw_rc_rate = 150
set roll_expo = 10
set pitch_expo = 10
set yaw_expo = 10
set roll_srate = 50
set pitch_srate = 40
set yaw_srate = 60
set tpa_rate = 20
set throttle_limit_type = SCALE
set throttle_limit_percent = 67

3. Future Solution please Betaflight? - EDIT, BOTH THESE NOW IMPLEMENTED IN BF 4.0!

What we need is a combination of the above 1 & 2.

We need a GUI value in Beta flight that you can simply type in, eg. 0.67 and it will limit the motor mix and throttle to get the proper result. - EDIT. NOW IN BF 4.0!

A further implementation would be to automatically switch profiles when you plug your battery in. Beta flight already knows if you have plugged in a 4s, or 6s, why not have the option to auto switch profiles depending on the battery? - NOW IN BF 4.0+!

I have suggested both these options as feature requestes in betaflight here and here.

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