Motor shaft length stability problem

hey everyone, i have my Quadcopter with APM2.6 , and Turnigy 1000kv motors,

and been having a stability problem where the Quad hovered but kept trying to roll to the right with some  clockwise-yaw no matter what i do.

i re-calibrated the Accelerometre  several times (with and without the wizard ) and made sure of the controllers level with the quad on the ground, but it kept happening.

today i was looking at the rotors and saw that the shaft of motor #2 is about 20mm shorter than the others! (motor #2 is running counter-clock wise)

could that be the problem ?
or isnt the gyro is supposed to take care of how fast the motor is going to stabilize the Quad ?

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Replies

  • Unlikely in my opinion. Yaw problems are often due to a twisted arm resulting in a torque force. In the log this will look like motors 1 and 2 at different levels to 3 and 4 as it tries to fight it. It doesn't take much misalignment.

    • yes.. Thank you ! motors 3 and 4 have higher values than motors 1 and 2  in the graph. 1 and 2 are almost exactly like each other,  and 4 is a bit higher than 3.

      but how can i know which arm is twisted ? which way the twist is ?

      • The easiest way I have found eyeballing it is to look down the arm towards the centre of the craft with the propeller on that arm you are looking down across ways and try to eyeball the ends of the props and see how they align with the ends of the adjacent arm motors tips. Assuming your props are straight the ends should point at the same point of each adjacent motor.

        If 3 and 4 are higher it is trying to yaw the opposite way to the way those props are spinning i.e anti clockwise, which matches what you say about it yawing clockwise....its fighting but not winning. I would guess one or more motor will have a prop tip higher on the right (as you look down the arm) if my mental gymnastics are correct which would generate a clockwise torque.

        If there is a twist your copter should be more efficient once realigned too.

        • yes  the right side of the props of motors 1 and  3 is slightly higher than the left side.

          and i noticed that the shaft of motor 2 is shorter than the rest .

          what should i do ? is this hard to fix? should i change the frame?

          • Difficult to say with out seeing it. If its the round arm with clamps variety try adjusting the clamps at one end or other. If its box or a moulded plastic type one maybe a gentle twist to bend it a touch, or possibly washers under one side of the motor, but that could be tricky if you are using all 4 screws, could bend the motor base if too tight and the mount is stiff.

            If the motor outputs aren't miles apart from each other you could try tuning the yaw gains so it fights harder, but that will end up with bigger motor differences and isn't best for efficiency.

            For the motor shaft, when I used turnigy in the past I cut them down a bit with a dremel as I thought they were too long. Shorter is less likely to bend in a crash were my thoughts.

            • thank you a lot for your help and the great information .

              it drove me crazy to check every part , software and hardware.

              • No problem, get it sorted and enjoy flying :)

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