I've noticed on my motors and a lot of other motors that having the motors above the quadcopter means that the max lift capacity relies on the little C-Clip under the motor shaft which leads me to my question, would it not be better to mount the motors under the arms so the thrust will push the motor together rather than pull it apart when mounted above.

Would there be any negative effects from mounting the motors under the arms ?

I had one of the C-Clips pop off during a flight yesterday and would prefer this doesn't happen again is all :)

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  • Indeed, the magnets hold the motor together. I have flown many times with a motor that had the clip missing. No problems. One side benefit is, on the chance of a crash, the prop amd
  • I think you will find that that c clip isn't taking any of the weight, the top of the motor is held in place vertically by the magnets.

    • Well, I actually fly one of my R&D quads without c-clips all the time. The magnetic force is huge, especially when power is applied. Full power only moves the outer part about half a millimeter. Never have encountered a 'fly-away' of the prop, only one time when crashing. I guess it saved my motor/prop. Some other advantage I see:

      - the bearings are not stressed by axial forces. However, ball bearings are supposed to resist 'some' axial stress.

      - transport is very easy, just take off the outer part / prop

      - makes balancing very easy (at least for me)

      - when playing with firmware, just take the props off for maximum protection.

      - easy access to the bearings for oiling

      - because of the kind of flexible magnetic field, might even prevent some axial vibration from the prop to the frame. Not sure about this, I never tested/measured this.

    • I can agree with that.  It took a lot of force to pull the motor off when I removed the c-clip.  Can we assume the same magnetic force is in place when the motor is spinning?

  • Yes, they can. No negative effects.

    Google for "Aeryon SkyRanger".

    - blades are not closer to the ground - usual landing gear avoid contact to grass.

    - lifting points above CG (good stability).
    - doesn't interfer on cameras.
    - increased ground effect.
    - no aerodynamic pressure over the arms pushing the drone down.
    - no blade buffeting over the arms and no vortex shedding from the arms (less noise and less vibrations).
    Actually this design is all good, in my opinion. I think this tend to be the standard design of the next generation of drones. Press "print screen" ;D
    • I'm pretty sure that these points:

      - no aerodynamic pressure over the arms pushing the drone down.

      - lifting points above CG (good stability)

      are not valid (there are a lot of other discussions about this here). there is no force that can push the arms down, and it also doesn't matter if the cog is above or below (see pendulum fallacy)

      • Thanks for your feedback Christoph,

        I corrected my first statement in my response to Rick Boden above. Does it seem coherent for you now?

        About the lifting points, I compare the lifting point of a drone to the buoyancy force on a ship. It should be above the CG for stability. I'm not so sure if the pendulum rocket fallacy really applies to drones. There are three types of angular momentum equilibrium: stable, neutral and unstable. As far as I know, rockets are designed to be in neutral equilibrium, while ships and drones are designed to be in a stable condition.

        • The CG location doesn't matter as far as above or below the lifting points.  It's not analogous to buoyancy because resultant buoyancy is "always up" (in an inertial reference frame) whereas propellers generally push in one direction - the axial direction in which they are facing.  

          The only instability if the CG is high is caused by aerodynamics. It is not caused by the CG location.  Here's why:

           High CG is correlated to more material being above the prop plane.  If the vehicle is flying with a high speed, the drag caused by the material above the CG applies a moment that destabilizes the flight condition back toward a hover position.  Other than that, as Christoph mentioned, the pendulum/rocket fallacy certainly applies to multicopters.

        • Developer

          As far as I know there is no "broomstick" or pendulum effect to exploit in multicopters.

          The shortest answer that I have found, is that since multicopters are inherently unstable, the most efficient CG is a neutral one. In other words it takes the least energy to operate and maintain stability in flight, if the trust-line (CoR) and weight (CoG) of the copter are at the same spot.

          • I am far from being specialist on this, but sharing my thoughts, I'd say drones with neutral stability might have a better maneuverability. About taking less energy to maintain stability, I'd like to understand how, as I thought the system would need to apply corrections every instant to maintain stability.

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