Is it OK to treat an OctoPlane as a QuadPlane?

RCnorm · March 22, 2025, 11:50am

I am now building an OctoPlane like this;

When I designed it I did not know that this is classified as Octo H and I planned to treat it as a QuadPlane with X frame type. My idea was to combine motor 1 with 3, 2 with 4, 5 with7 and 6 with 8 by connecting their ESC signal lines with Y harnesses. Motors 1&3 and 6&8 are CW and motors 2&4 and 5&7 are CCW. Thus, Motors 2&4 are connected with output 5, 5&7 are with output 6, 1&3 are with output 7 and 6&8 are with output 8.
For yaw control augmentation I made all motors are tilted outward by 2 deg.

Is this my design OK?

dkemxr · March 22, 2025, 2:29pm

Octo-H VTOL has the same Motor layout and direction as Copter Octo-H

And, suggest you have an ESC per motor.

Seen · March 22, 2025, 8:44pm

Hello, for this layout I recommend using scripting, You can setup the 6DoF motor configuration with attitude_control:set_forward_enable(true) for the two forward propelling motors in the picture as well to provide better yawing, These two scripts might help you understand.

github.com

ArduPilot/ardupilot/blob/master/libraries/AP_Scripting/examples/Motors_6DoF.lua

-- This script loads the 6DoF mixer matrix for a 6 motor frame
-- https://youtu.be/QUDhnYvH66k

Motors_6DoF:add_motor(0,  0.003285,  0.470445,  0.031489,  0.885866,  0.563927,  0.031026, true, 1)
Motors_6DoF:add_motor(1, -0.373686, -0.267196,  0.092861,  0.789066, -0.223271,  0.495176, true, 2)
Motors_6DoF:add_motor(2,  0.370400, -0.203249, -0.053720,  0.841080, -0.340656, -0.526202, true, 3)
Motors_6DoF:add_motor(3, -0.146980, -0.216342, -0.338296,  0.001577,  0.900822, -0.460986, true, 4)
Motors_6DoF:add_motor(4, -0.205533, -0.035715,  0.359267, -0.048371,  0.010753, -1.013924, true, 5)
Motors_6DoF:add_motor(5,  0.143881, -0.227449,  0.375220, -0.046098,  0.811593,  0.431718, true, 6)

assert(Motors_6DoF:init(6),'unable to setup 6 motors')

motors:set_frame_string("6DoF example")

github.com

ArduPilot/ardupilot/blob/master/libraries/AP_Scripting/examples/6DoF_roll_pitch.lua

-- A script for controlling the roll and pitch of 6DoF vehicles
-- The script sets the target roll and pitch to the current value when arming or when E-stop is removed
-- this allows the vehicle to be placed upside-down on the ground and to take off as normal
-- its is also possible to setup a switch with option 300 to use either 4 or 6DoF control

-- make sure the vehicle is capable of 6DoF control
assert(param:get('FRAME_CLASS') == 16, "script requires a 6DoF vehicle")

local e_stop = rc:find_channel_for_option(31)
local sw = rc:find_channel_for_option(300)
local motors_spinning = false

function update() -- this is the loop which periodically runs

  -- motor state
  local current_motors_spinning = false

  if arming:is_armed() then
    -- if armed then motors are spinning
    current_motors_spinning = true

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RCnorm · March 23, 2025, 12:06am

Thank you for your suggestion.
I know the standard way to fly my OcotPlane is treat it as an Octo-H.
But I have designed it with propellers tilted outward like X frame type by mistake.
So that is why I had an idea to treat it as a Quad-X by combining motors. Each motor has its own ESC but the signal lines for ESC1 and ESC3 are connected by Y harness and plugged with same Output-7. Other motors are combined same way.

Does this my idea work well? Or it is not an good idea, is it possible to treat it as an Octo-X, i.e. Q_FRAME_CLASS=3 (Octa) and Q_FRAME_TYPE=1 (X frame) and all motors rotational direction are revesed from those of Octo-H?

3

RCnorm · March 23, 2025, 12:15am

Thank you for your recomendation. But I am a new commer for this field and it is very difficult to use the script. Also my plane has 8 VTOL motors (not 6) and 2 forward pushing motors.

My current concern is that is it OK to treat it as a Quad-x by combining two motors. If it is not recommended, is it OK to treat as a Octo-X by all the motor rotational derections are reversed from that of Octo-H?

Seen · March 23, 2025, 12:21am

Hi, 6DoF does not mean 6 motors. Also, I just realized this is ArduPlane, not ArduCopter. I’m not very familiar with ArduPlane, so I don’t think I can be of much help here.

Allister · March 23, 2025, 12:26am

I think using a y-cable to two motors is a bad idea. I would take @dkemxr 's suggstion and go with Octo-H, even with the motors angled out. I can’t put my finger on it, but I just have this feeling if you try to run it as a quad as you have laid out you won’t be able to tune or control it well. It might fly, but I feel going to the Octo-H will actually allow you get proper performance out it. The code is there, you don’t have to re-invent anything.

RCnorm · March 23, 2025, 12:40am

Thank you your comment.
How about to treat it as Octo-X as I described above?

Allister · March 23, 2025, 12:44am

Why are you reluctant to use the configuration that is meant for your airframe?

RCnorm · March 23, 2025, 12:46am

Because I had made outward motor tilt that is bad direction for a H frame.

Allister · March 23, 2025, 12:49am

It may not be ideal, but it will be better than trying to use a configuration that is for a completely different airframe.

RCnorm · March 23, 2025, 12:57am

Thank you for your comments.
I will think again which is better; take your advise or redesign it with innner motor tilting.