OCTA-QUAD X8 Coaxial Quad Design Notes for Arducopter - RC Groups
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Sep 06, 2017, 02:42 PM
Wisconsin
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OCTA-QUAD X8 Coaxial Quad Design Notes for Arducopter



A quad with two motors per arm is a Coaxial Quad also know as an X8 and Octa-Quad. An X8 is inherently less efficient than a flat octacopter because the bottom propeller is blanketed or covered by the top propeller. Hence building an X8 is almost entirely for increased reliability. If one motor fails you have a second motor on each arm. An X8 is also more compact than a flat Octacopter. An Octo with same propeller size as an X8 would have to be much much bigger in diameter. An X8 can carry more weight than a flat quad, of the same prop size, but at the price of efficiency. The efficiency penalty is in the order of 15% but building an X8 is a black art. There are so many variables that getting it right with your available motors and props is difficult.


The top propeller increases the speed of the air moving down. This means that if the lower motor were running at the same speed it would just be idling. Since the air it "sees" from above is already moving as fast as it can accelerate air.

So the basic theory is that the bottom motor must run faster than the top motor. We can generally do that two ways. Increase the pitch of the lower propeller and or increase the KV of the lower motor which increases it's speed. General guess is that the bottom motor should run about 10-20% faster than the top motor. The problem is that it is difficult to get motors of any particular size or propellers of a particular length that are only 10-20% different than each other. It will depend greatly on why size aircraft you are wanting to make if components are readily available to meet these requirements. Building an X8 requires a certain amount of experimentation to find the optimal combination for your choosen size craft.

The next problem is that the top propeller creates a stream tube, a column of faster moving air below it. If the props on the bottom "stick out" , beyond the diameter of the stream tube created by the top motor, the tips of the bottom propeller will cause a lot of turbulence as the tips cut through the slower moving air outside the stream tube. So we make the lower propeller smaller than the top propeller to fit inside the stream tube of the top. I generally use a prop one size smaller. So 17" props on top and 16" props with a higher pitch on the bottom, for example.


Note that in the picture above both props "face" up.

The next problem is figuring out the motor order to use with ArduCopter. In the ArduCopter wiki there is a diagram showing the motor order for an X8 but it does not specify the "view".


The view is from the absolute top. That means that bottom motors rotation is relative to viewing from above. Hence when you install the motors on the lower motor mounts they will be upside down. Pointing down. So the rotation must be reversed from looking at it normally right-side up. Remember if you flip the quad over, to more easily work on it, that everything will be reversed. Also the propellers always have top surface upward. So you must install the propellers upside down on the lower motor relative to having the motor sitting in a normal upright position in front of you.

This is an excellent presentation by Robert Lefebvre at Dronecon:
DroneCon Talks: Robert Lefebvre, "Introduction to Rotorcraft Dynamics" (23 min 6 sec)

An X8 is a fun second build to extend your knowledge and build skills. Hope this helps a bit getting it all straight. If it doesn't help let me know and I'll edit it with your suggestions.
Last edited by mike_kelly; Sep 06, 2017 at 03:49 PM.
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Sep 28, 2017, 09:19 AM
scared to fly
audio is not very audible.
Nov 04, 2017, 11:47 PM
Wisconsin
Quote:
Originally Posted by anindyaswati
audio is not very audible.
Unfortunately!
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Dec 13, 2017, 07:44 AM
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lokanaft's Avatar
It's all good, but without change throttle output, coaxial not effective. I opened the task 1.5 years ago, but the developers did not pay attention to it: https://github.com/ArduPilot/ardupilot/issues/4130
Dec 13, 2017, 10:09 AM
Wisconsin
Quote:
Originally Posted by lokanaft
It's all good, but without change throttle output, coaxial not effective. I opened the task 1.5 years ago, but the developers did not pay attention to it: https://github.com/ArduPilot/ardupilot/issues/4130
It is easier to get the speed differential by increasing the kv or pitch of the bottom motor and prop. My X8's fly just fine.
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Dec 13, 2017, 12:52 PM
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lokanaft's Avatar
I have different motor and kv on top and bottom, but want use full possible optimisation.
And today I add 1.5kg and total weight became 5kg. And when I use yaw control - no any reaction! On standart quad turn by yaw produced by change speed opposite X motors, but here on octa quad, when we change speed opposite motors - nothing happens, they balance each other.
AC not ready for coaxial drones.
Dec 13, 2017, 07:38 PM
Wisconsin
Quote:
Originally Posted by lokanaft
I have different motor and kv on top and bottom, but want use full possible optimisation.
And today I add 1.5kg and total weight became 5kg. And when I use yaw control - no any reaction! On standart quad turn by yaw produced by change speed opposite X motors, but here on octa quad, when we change speed opposite motors - nothing happens, they balance each other.
AC not ready for coaxial drones.
OF course it is. You have the wrong props on the motors or the wrong rotations on the props. The props move in the same way as any quad. Nothing different. Yaw works fine.
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Yesterday, 12:31 AM
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lokanaft's Avatar
But without this 1.5kg it's fly fun.
Yesterday, 10:45 AM
Wisconsin
Quote:
Originally Posted by lokanaft
But without this 1.5kg it's fly fun.
As I said the Arducopter X8 uses the same motor differential to cause a yaw. The props on each arm turn in the same direction as if they were one prop and yaw is caused by changing the motor speed just as you have noted. No difference and nothing "cancelling out". 5kg is a heavy quad. So you would need the right motors and prop size to handle that load.
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