Let's talk geometry -- is 90deg X (or plus) frame the MOST stable?

[Hugh Hemington]

As I wait for parts, I'm thinking of the job the controller has to do, and thinking about how this job is made harder or easier by the geometry of the frame, which spaces the arms and motors.

Logically, it seems like a perfect 90deg. X (or plus) where the pivot point and motor spacing are identical is easiest to control, since power applied to one axis rotates the craft exactly on the axis of the opposing "cross" arm. And since the arc of rotation on all four arms is identical, as the quad is rotated, on that axis, no mixing is required to continue to produce the same correction. And inversely, the farther from the same vector the arms are, power applied to rotate in one axis also rotates to a degree in the other.

Following this logic it seems that the more the vectors described by each opposing arm deviate from parallel and/or the SAME vector, the harder the quad is to make stable with motor adjustments because the axis of rotation each produces is more oblique to frame-center.

Further, it seems logical that the chip on the controller board which measures tilt on any axis needs to be located as close as possible to the center of the vectors described by the opposing motors. (ideally also the opposing arms) That way, adjustments directed by the controller have a LINEAR effect on axial tilt.

I realize most controllers have settings for 'H' vs. 'X' vs. Plus etc. to compensate for this, but I'm assuming there is an ideal form for simplicity, and every other form will invariably introduce some error despite controller programming. In short, the less compensation the controller needs to do, the less impact it can have if it does it poorly.

It seems like the most problematic layout would be straight front and forked rear arms.

Right now, my frame is about an inch out of square. the motor centers are one inch different front to side, and the pivot points are also one inch wider than long. This will probably have little impact on flight, but there is still time to shorten the frame across the front, making both the pivot points and motor centers describe perfect squares, and bringing opposing arms onto the same vectors.

What are your thoughts?

 

[John Doecliff]

Yes it makes sense that the quad would be more stable but like you mentioned isn't that the same as saying the controller has to work less to keep it stable? The flight controller does all the work to keep it stable so a perfectly balanced and symmetrical quad would mean that it doesn't have to work as hard. There is the "dead cat" frame that has the front two arms spread out a little so as not to interfere with the camera. hexa and octos are probably more stable but I think they are favored because of there heavy lift capability's. I think quads are just barely "stable" enough and the microprocessor of the flight controller is what really makes it possible for them to actually fly. If you built a quad without a flight controller that was balanced and had 4 motors spinning exactly the same speed shouldn't it hover (or rise) but remain level? I had some problems with my quad oscillating violently. I thought it was when I was descending because I was falling into my own prop wash but I'm pretty sure my battery kept slipping off to one side and the quad was trying to keep it stable but the balance was just too far off. It was like a high speed wobble cause it seemed to have a hard time coming out of the oscillation. Now that I am paying better attention to securing my battery it seems to have gone away but we will see.