During tesing of motor-off recovery at safe altitude (60meters), the quad fell stright down, - propellers were spinning backwards, and motors stalled, not being able to turn in right direction.

At much too low altitude, the quad rolled, making 2 of the motors able to spin up, but it was too late then.

-motors should be kept on at all time when the APM is armed, stopping motors is asking for dangerous situations..

I created a issue here: https://github.com/diydrones/ardupilot/issues/431

Feel free to support it.

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  • I feel this is a problem with ESCs and should be solved at the source. We need ESCs that are specially designed for quads and can spin up even if they are rotating in reverse after a situation such as you describe. I think it would be dangerous if the props kept rotating in idle as there would be no quick way to do a quick shutdown in an emergency situation. It would also be nice if the ESCs had a PID controller to keep prop RPMs proportional to input signal and if they could report their status back to the APM if for whatever reason they couldn't maintain desired RPM. I know AutoQuad is working on an ESC designed especially for quads... I wonder if they have any solutions for this problem.

  • I think a feature which would be handy and along the same vein as this would be a "rapid descend" where the APM will bring it down as quickly as possible but without stalling the motors (tell-tale sign without feedback is uncommanded pitching or rolling)

    I often descend for the fun of it with the motors just about stalling. I bring it down as quick as possible and as and when they stall I give it a bit of throttle. From an APM PoV it would be tough because of the instability caused by falling through your own  prop wash.

  • 100KM

    The option used to be there to either spin motors when armed or not.  Option seems to have been removed somewhere along the way.

    Making the motors always spin when armed creates a whole lot of new problems.  You can't shut them down when things go horribly wrong, to reduce damage (both to copter and to people/property).  You have arming and disarming delay which may confuse people(hey why don't my motors start, oh now that I put my finger in they start), etc.

    Furthermore you have the variable pwm value of "stationary spin' for different copters.  An overpowered copter may take off when spinning at a stationary pwm, while a normal one does not.  For some esc's the stationary pwm might be insufficient to make the motors spin.  For esc's without external oscillator the stationary rpm will vary with temperature, etc etc

    The issue has been brought up  before on the developpers list.  But my opinion is that a pilot still needs some common sense and that a machine will never be able to replace that.

  • To recap and make certain we are not misinterpreting your test:

    • The quad was at altitude in Stabilized mode (hovering, ascending, ?)
    • You commanded full throttle off (!!) or did you command DISARM?(same throttle location)
    • The aircraft began to descend..(immediately -- probably pitching,rolling or a combination)
    • ...at some point...you commanded some value of throttle (half, full?) to arrest the descent
    • The quad was unable to spin up all the motors (allegedly) and recover to a level hover and crashed

    Based on this experiment, you are petitioning the developers to implement a flight mode that completely compensates for any pilot action of shutting down the motors in flight? In short, you are asking for a 'smart throttle' function.

    Interesting concept.

    Nylon or other plastic screws may not be a bad idea if they do not get too hot and soften or deform and then twist. In the case where the arm is aluminum, some heat sink action of the motor base and the arm may delay this possibility.


  • I like the idea of once the APM knows you are in the air it should prevent a full zero throttle in stabilize mode. If you really want to shut them off then go to ACRO. Full off shouldn't be allowed in stabilize like it's not in other modes like alt hold. But it should be able to descend faster than a full down in alt hold. Just keep them spinning at the minimum speed that prevents them from stopping.

  • strangely your motor cables look like they were cut by the props.

    Weren't they secured?

  • There should be a standard "motor on and slow spin" as soon as the APM is armed to avoid a total motor shutdown in mid air. I think mikrokopter offers that.

  • If you stop the motors mid-flight you're rolling the dice as to whether they will start up again.

    As you say, they reverse in freefall and the ESCs can't get back in sync to move them the right way again, they're not designed to be able to handle that case.

  • I am confused. Did you intentionally stop your motors in flight?

    Was this to test an added safety circuit or some other failsafe/rescue mechanism?

    Or are you stating that while flying, two of your motors stopped?

    It is unclear if you are reporting an induced result or a system failure?

    Please clarify. If a system failure, post your *.tlog file from the APM so that we may attempt to augur out the cause...perhaps.

    It is sad to see damage but let us hope you can learn what went wrong.


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