I plan to do what is called a "pull" to made changes to the custom code regarding custom shaped copters.
This change in code would only impacts irregular shaped quads, hexas, and octas when the pilot specifically opts to go custom versus the default (the standard X or +). You have a custom quad if the:
o aspect ratio <> 1 (length is different than width).
o rotors are non symmetric around the
- forward axis (y) going through the center of gravity (CG)
- sideways axis (x) going through the CG
- vertical axis (z) going through the CG
o has a front that is more open for a camera
o deviates from the pictures of a a + or X for the quad, hexa, or octa
o this includes ships described as spider, V, H, U, 88--88, C, etc.
o motor spin direction(s) are different than the pictures
o your CG is pushed somewhere else besides the centroid of the motors.
The advantages of going custom is that the motor factors will be tuned to the coordinate/spin system of your copter versus the coordinate/spin system of the regular copter. They will fly better. Pilots will probably not notice small deviations nor would they see significantly improved flight times. Large deviations might be noticed and provide noticeable changes in flight duration.
Please reply with the motor number (the out-pin number on the APM), coordinates of the motor, and rotation direction of each rotor. For example,
the owner of this copter would reply (motor number, x, y, CCW/CW):
o 1 (400, 200) CCW
o 2 (-250, -200) CCW
o 3 (-400, 200), CW
o 4 (250, -200 CW
[note: no need to tell us your units of measure just so long as you are consistent in measuring; say mm or inches]
Please note:
o The center of gravity of any quad spider or V is not necessarily where the bars cross. The bars typically cross behind the CG. .
o The CG is the center of the coordinates or (0,0) where x=0 and y=0
If you decide to participate by replying, the idea is that you will be able to access your custom motor factors without having to compile firmware. No promises at this point. First we see what's out there. But if you do reply, it's far more likely that your design will be implemented in the library.
If you have any questions or difficulties in doing this, let me know so I can help.
Replies
really nice test stand. i assume you have studied the flight modes? do any of those do what you want?
On Yaw Forces ... why are they non-intuitive?
The torque from Yaw definitely is not intuitive. one would think that there is a force and thus the farther it is away from the hub the more torque it delivers. Torque = Distance x Force. Right? or for your and my full understanding, since Force = mass x Velocity^2, thus torque = dmV^2
Roll and pitch of a rotor can be simplified to a single point of center of gravity (CG). Thus there is only one force arrow at each rotor. and it points up. The torque on the attitude (levelness) of the ship = dmV^2, where d is the distance from the roll or pitch axis.
If a prop is at the CG of the ship, then the rotational force (torque) on the ship for that centered prop = force x zero (the distance) = 0. Good enough. A prop located at the CG (middle) of the axis will not rotate that axis.
Let's verify that. If I stand on a teeter-totter at the very middle and let gravity apply a force on my mass, the teeter-totter doesn't rotate.
What about Yaw. If a rotor is placed at the very center of the ship and the rotor is powered up, what happens? Of course, the ship spins in the opposite direction of the rotor. But torque = force x distance! If distance = 0 then shouldn't the force = 0? So why does would the ship spin. Something is goofy.
The problem is that the force vector from a prop in the upward direction can be viewed as one arrow pointing up. But the torque from a spinning object like a prop can only be simplified into two masses and thus two force arrows, one on each blade of the prop.
Consider a two bladed prop (this one is easy). As it spins, each blade has the same mass but are continually going in opposite directions. As one blade is moving towards you, the other is moving away. Thus the direction of those forces are always opposite. And that is what makes the object want to spin. Spin the rotor clockwise and the ship will want to spin counter clockwise (for every force there is an opposite and ... ).
d is the distance from the CG of each blade to the CG of the ship. So when the rotor is placed at the CG of the ship and we are talking about an 8" prop, then the d to the center is about 4ish inches. The forces are opposite but on opposite sides of the CG of the ship so the forces add up (neg x neg = +). 4" + 4" = 8".
Now put the motor out from the center of CG 10". The outer force is at 14" and the inner (and opposite) force is at 6". Since both are on the same side of the CG, the forces subtract, giving a net distance of 8". Thus, put the motor where ever you want, and the difference, d, is always 8" in that case.
of course the spinning rotor (not the stator) and casing of the motor itself also contributes to the Yaw forces acting on your ship.
Hope that explains it. I call it a mind !&%$, but it's a four letter word and not nice.
I have build a V octocopter and I have a pixhawk.I can't find any setup in misson planner for it
So guess it is not supported? Or is it any motor mixer in latest version?or any links regarding
setup a v octocopter?
What is the aspect ratio of your copter (width to length)?
please see drawing
There are two ways that you are supported.
1) Choose Octa and then V (unless identical, there will be some coupling with Yaw)
2) Choose Octa and then custom program the V code to be your V (your ship and every other type of ship V, U, H, OO, etc. will behave perfectly)
Depending on what you answer below, how precise your piloting requirements, comfort with compiling ... will determine the best route for you to take.
My suggestion is to start by using the prop rotation as defined in the instructions for an octa-V. and if you do, then you can use the octa V standard in MP.
If you notice coupling between roll/pitch and yaw and don't like it, then you can consider going to creating your own custom code. I have a special manual on how to do that if you get interested. It isn't too difficult ... if i figured it out ... :-). Then you can go to an optimized spin direction and optimized motor factors (no coupling and faster yaw response).
Thank you Forrest very much for the help and the spreadsheet! I thought that only"ordinary" octocopter was supported and the V was only for V quad?
The prop directions I don't know what is best?
I have not done any compiling but maybe I would give it a try(depending how the supported(?) V octo behave).
And thank you for making this call for the motor mixer!
Are you OK with the props going in the following directions:
I'd be interested in learning more.
I'm working on an H frame type. (actually more of a capital letter I )
Square: 4
H or I : 3
X or + : 2*sqrt(1^2+1^2)=2*1.43=2.86 almost 3...
I like H frames because I need only 2 fixtures
And it allow placing equipment all along the frame, keeping the "com" at prop level
Manu
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