Chinook Helicopter

Chinook dual-rotor 'copter

Chinook dual-rotor ‘copter

The Chinook helicopter has two sets of rotors which rotate in the same direction, leading to 0 yaw.  A quadcopter takes the same approach – diagonally opposite blades rotate in the same direction, so both pairs result in 0 yaw.

So how does my bloomin’ quadcopter have yaw problems?  I’m bringing this up again as it was a problem in the PID testing; during the first few tests, although the 2 motors started at full hover speed, they slowed down dramatically during the test.

The cause was the yaw PID which I’d forgotten to take out, and once I did, the tests ran consistently at hover speed.

Yet physically there was not yaw to correct – only one diagonal pair of motors was operating and due to the step-ladder, there was no way the quad could rotate around its z axis.  And that suggests the yaw sensor is seriously duff.

Next time she’s flying outside, I think I’ll set the yaw rate PID gains to 0 and see what happens – something must have suggested to me I needed them in the first place?

11 thoughts on “Chinook Helicopter

  1. Gyros measure the angular momentum (hence the need for integrating to get the angle), so there will be a drift.is that what you are seeing?

    • Yes and no – there’s no rotation around the Z axis in the PID testing because of how tightly the non-spinning blades are strapped to the step ladder, yet the z-axis gyro says there is. That’s where the duff-sensor though comes from.

      And yet at the same time, I do see real physical yaw in live test flights if I disable the yaw PID – and that suggests there is also a physical factor causing yaw, though the point is that due to the layout of the blades, their rotation direction, their placement and the construction of the frame joining them, there (physically) should not be spin around the vertical axis, and yet there visibly is, and the z-axis PID is required to compensate. Without it, live flights pirouette; with it, the quad stays facing the same direction.

      So from the Z-N PID tuning, it seems the sensors are duff, as there absolutely could not have been physical yaw, yet in real flight, there is visible yaw unless there is a yaw PID driven by the z-gyro to compensate.

      So my problem isn’t how to manage the yaw, but why on earth is there yaw in the first place.

      Just weird!

      The other possible cause (assuming accurate frame, and working sensors) is 1 of the 4 motors is duff, yet again, I struggle to believe that since they are PWM controlled and have no choice but to spin at the correct rate.

      I’m at a complete loss.

      • Does the physical yaw you experience when disabling yaw PIDs match the measured yaw when suspended on the ladder? If not then it might be separate issues. Measured yaw could be sensor drift while physical yaw could be a slight mismatch between motors or ESCs somewhere.

        • That’s what it’s starting to look like – both physical yaw and sensor drift.

          Physically the quad shouldn’t have any yaw, so once I find the cause of that, then I can ignore the gyro yaw rate sensor and delete its PID which will make me very happy – it’s the dominant factor in any of my live flight testing and swamps any subtle problems in the diagnostic logs generated!

  2. The axes of the motors might not be in the same vertical plane. Any misalignment there can lead to yaw and require active correction with PIDs.

    • That’s the advantage of buying a kit from like the DJI F450 I’m using. Because of the way it’s built you’d expect all motors to be point straight up because of the way all the arms are connected. At the same time, the frame has sustained a lot of crashes so I will double check the arms for twists and breakages – thanks!

      • A quick visual inspection of the alignment of the arms with the body, and a physical inspection (twisting the arms looking for weak points) haven’t shown anything. However, that doesn’t rule out the motor alignments as the cause of the problem. I posted something about this on DIYdrones.com a while back, and motor orientation was the only decent answer I got back from there too. I think I just need to look harder.

    • z-axis gyro measures yaw rate – integrating it gives absolute yaw, but I don’t give a monkeys about that as long as the rate is 0. Yet despite gyro calibration when the quad is very stationary, the z-gyro seems to be putting out an output in flight.

      Certainly in my very early days of testing where I tethered the quadcopter to the ground with about 2m strings, she was definitely spinning around her z axis resulting in a spiral of twistest strings, and ultimately a violent crash.

      The yaw PID cured that problem, and yet it shouldn’t be necessary for just a vertical take-off. I am clearly missing something but am at a complete loss as to what!

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