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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The effect is very similar to how a brushless camera gimbal works. The gimbal motors are fast but have have very little torque, so even if the CG is just slightly off they are unable to stabilize out all movements.
The same goes for multicopters. When they are balanced it takes less energy to start and stop a rotation. Meaning it also takes less energy (and is quicker) to maintain a still state.
Thank's for this explanation John, it's a good point :) I'm going to check my cuad.
Batteries are the problem.
I found mounting my batteries low on my landing gear caused the motors to work much harder in maintaining a stable flight in winds. Also hovering was fine in calm air but directional flight proved sluggish and battery life was shorter.
Just curious about the "no aerodynamic pressure pushing the arms down". Wouldn't the prop be sucking a huge quantity of air downward and what about that air hitting the arms and motor?
Hi Rick,
I understand your concern, and I would correct my statement to "much less aerodynamic pressure pushing the arms down" instead of "no aerodynamic pressure".
There is much less pressure because the flow velocity 1 inch upstream the rotor is much smaller than the flow velocity 1 inch downstream. You will be able to feel this when placing your hand above the rotor and below it. It is the same for a standard desk fan, or any kind of fan or propeller in an open environment (not inside a closed duct with constant section).
That sounds reasonable Marco. Thanks! I suppose one could even measure the difference with a pressure sensor or some similar tool.
Negative side affects aside, there's a roughly 5% increase in efficiency (depending on the exact size and shape of your arms) due to not blowing air over the arms. So you can get a roughly 5% increase in flight time, all other things being equal. Another small benefit is increased ground affect, so hovering in ground affect will use a little less power.
Stability will be a little less due to the lowered thrust line, but if you're going for aerobatics, this may be a benefit for you.
Probably the biggest consideration though, would be your landing gear and the danger of damaging the props on grass or other objects as others have mentioned.
Enjoy!
Negative effects:
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.
Yes
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