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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Yes

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.

Thanks for the info!

I am a bit concerned about CG too. do you think maybe it's just cheap motors that is my problem?

It just doesn't seem to make sense to have the weight of the quadcopter in flight hinging on those little c clips. Total flight weight is 1080 grams and although each c clip is responsible for 1/4 that weight i can only imagine during dynamic flight adjustments and takeoff/landing that those stresses would be multiplied.

Has this ever happened to you?

Only when the c-clip wasn't fully seated to begin with. Once, during testing. In a shipment of twenty motors, I had two where the c-clip wasn't fully seated.

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!

In planes the pusher type design is less efficient than a standard tractor design, and the arms are still going to be in the airstream, so I'm not sure you'd get any gains.

Well, we're talking multi-copters, so there isn't much of an airstream above the prop.  Props are the same, just below or above motor/frame mounting.  There's definately gains to be had, but it does put the prop in harms way on bad landings.

There's another very small advantage of downward pointing props - less vibrations in the frame.  Every rotation produces a power pulse from each blade that strikes the arm and causes a small "ping" to the airframe.  Under the frame prop mounting obviously will not cause this problem.

Good luck!

You, sir, have just inspired me to design my quadcopter as a pusher type.

is there any outcome of this? I'm about to do the same thing

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

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?

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)

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