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 :)
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.
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.
Yes Chris I do it all the time!
Slightly more efficient, we have noticed a 5-10 percent increase in flight times using this method and as you said the c clip popping off doesn't down the bird!
With current multicopter motors, there is really no difference at all between up or down, push or pull.
The bearings used in all multicopter motors at this point are a simple pressed "Conrad" design and they work just as well with the forces pulling up or down.
It shouldn't be that way, they should be using thrust bearings (which would work better with the forces in one direction than the other).
Thrust bearings intrinsically are designed for an up or a down load whereas Conrad bearings work better in a radial load.
But for the present, nobody has yet incorporated real thrust bearings in any commercial multicopter motor.
So up or down makes no difference at all.
In some of them the internal ventilation may be designed to work slightly better with the external airfow going one direction or the other, but for the most part the influence would be negligible.
Also as Scott said above, if you can accommodate it, having the motors mounted below the support arms generally results in a real 5 to 10 percent increase in flight duration due to considerably less negative influence on airflow to the props with the spar on top versus having the spar below the prop.
Here I had no negative effect: http://diydrones.com/forum/topics/new-quadcopter-zecke-q185-max
From aerodynamic point of view a pusher is better, The propeller flow is similar, but the motor is located in the region with less flow velocity. From handling point of view the tractor is better because of the distance to the ground.
No doubt this has been addressed, however I just wanted to point out that any idea that there is a "pendulum effect" from mounting propellers above the CG as opposed to below it is a fallacy. Because the thrust vector is fixed and gravity acts on the vehicle universally you dont have to worry about any stability compromises with having props under a boom vs over a boom.
EDIT: However you do have to worry about vertical distance away from the Center of Mass as this can introduce translation because of the motor torque :)
This is a really old post but I thought I'd chime in any way. There is not as much force on that little c-clip as you might think. We actually had a DJI 450 about a year ago that we flew for almost a week with out realizing that one of the clips was missing. We could actually pull the blade, housing and all right off the motor when it was powered down but it flew fine. This got us wondering so we went and had a chat with one of the engineers where I work who has done a lot with brushless motors. He was not as surprised as us. He told us the magnetic field is so string that there is no way they would pop off while energized. NOT saying I recommend trying this, but it was pretty interesting.
I did some tests on RCgroups about this subject specifically. It does not do much difference, but a pusher configuration can be slightly more efficiency from an aerodynamic point of view.
Vernon Barry said:
blades are closer to the ground thus may hit in grass. (also more subject to FOD)
Blades under means the lifting points are below CG, making it unstable (more so than normal, Blades up means the weight hangs below the blades in a more self stabilizing manner)
Blades down interferes with FPV camera views in some cases.
You do realize in a normal plane, the same thrust is always on that c-clip, thus by design, that's the way the motor is used normally. Further, in a good design, the bearing would be spring preloaded in the normal forward thrust, thus reversing that removes the spring preload and may actually cause problem in the bearing.
Again, not saying you cannot do it, but unless you are building upper and lower motors like a 8 motor quad or six motor Y, not the "prefered" orientation. People do use these motors as you have suggested and there haven't been reports of bearing failures, but as I said just from a thrust VS CG analysis, it would tend to de-stabilize the aircraft.