Quadcopter Unstable With 15” Propellers

I have a 650mm quadcopter with an all up weight of 4 lbs. 10 ozs. Flight controller: APM 2.5. With 12 x 3.8” props the hover is very stable. Just a little bit of wobble near ground level. The frame is made by Dynam and has very short legs; the props are only 5.25” off the deck. Due to the props close proximity to the ground, prop wash may be the cause of the wobble. When I put 15 x 4” props on the quad it’s very unstable (especially below 3 feet) and difficult to fly. According to calculations the hover RPM with the 12 x 3.8” props is 4500 RPM. The hover RPM’s with the 15 x 4” props is 2900 RPM’s. Originally I thought that the problem was due to undersized motors. I replaced the original NTM 28-30, 750 Kv, 120 Watt motors with NTM 35-42, 1000 Kv, 450 Watt motors. The problem actually seems worse with the larger motors. I would really like to use the 15” props to improve efficiency. My suspicions are: 1. RPM with 15” props too low. 2. Low ground clearance causing too much turbulence. 3. Default gain settings for aileron and elevator channels not compatible with 15” props. Any suggestions?

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Replies

  • I believe the suggestion above to reduce your PID gains is good advice.  Larger props produce more thrust at a given RPM, so using larger props is somewhat analogous to increasing your control gains.

    Also, your change in motors made the problem worse because your new motors have higher Kv.  Because that means higher RPM at a given throttle setting, it is also analogous to increasing your control gains.  It would have been advisable to switch to lower Kv motors, if anything.  You might try switching back to your old motors, although I agree the power rating might be a concern.

  • Moderator
    What PID tuning have you done? This sounds far more simple to me.

    Larger prop (more thrust) so you get a wobble/oscillation. Your response, add more powerful motors, and it gets worse. Have you followed the PID tuning guides? Reduced P?
  • Hi, I think your props are simply too heavy (inertial momentum) for the motors to change their speed fast enough for stability. That happens with any copter from some point on, and no amount of adjustment will fix it.
    Regards, Soren
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