I have a heavy lift hexa and due to adding a gimbal, am looking into mounting my dual 5200 MAH 6S batteries on top of the copter. I've considered mounting them on the bottom in between the gimbal and the copter, but this would require much additional modifying. Does anyone have any thoughts or knowledge as to how mounting these heavy batteries a couple of inches (5 cm or so) above the center plate of the hexa would affect stability? Thanks for your responses.
Mounting Batteries above centerplate
- Thread starter atpfilms
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iseeit
Day or Night...
The higher up the more you would be changing the CofG.
I have a pair of 8000mAh 6S on top of my SkyJib 8 540mm booms and it flies rock solid.
GPS hold barely moves at all (WKM).
I just cutout two trapezoids from a piece of carbon (or anything for that matter) and tie rap one on each side of the center plate, on the top side of the booms.
There out of the way and still close enough to reach everything that needs power.
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Thanks for your response iseeit. Hmmm. I see, you have them mounted on the arms. I have thought about that as well. I am talking about mounting them directly OVER the centerplate. The COG could remain the same with proper positioning of the batteries. They would just be mounted higher - which is what I worry about. Maybe I'm better off mounting them lower and changing the COG??
Wondered about this too. Currently building a 1000 using a frame that was originally designed for top-side batteries. I'm busily working on some sort of bottom-side battery tray arrangement as I really don't think top-side is the best choice. To my way of thinking, if there's going to be a disymmetry in the CG along the vertical axis, for the most stability it should be shifted *below* the rotor plane, not above. Has anyone ever done a A/B comparison of both places ?
SMP
Member
One of the guys on here (Webheadfred) does a lot of building tweaking and tuning and I recall seeing a thread where he ended up redoing his build as the batteries mounted above simply created too much instability. Makes sense really. Heavy stuff over the top wants to swing down.
We too use boom mounts on our skyjib and use photohigher batt trays for the X8.
We too use boom mounts on our skyjib and use photohigher batt trays for the X8.
R_Lefebvre
Arducopter Developer
It is counter intuitive, but if you cannot have the mass centered directly between the motors, it is better to have it ABOVE the motors. Putting it below the motors actually leads to instability. This is called "The Pendulum Rocket Falacy" and you can read up on more if you're interested.
OK, interesting. I read up on "The Pendulum Rocket Falacy". I could be completely wrong here, but it seems to me that the "Pendulum Rocket Falacy" basically states that it doesn't matter if you mount them above or below. The Falacy is that if weight is mounted above the force, gravity will keep the rest of the object aligned - like a pendulum. This doesn't work though because unlike a pendulum, the tip of a rocket or the top of a copter in this case are not a fixed point. This is interesting..."Even a Goddard-type rocket, with the engine at the front, will fly correctly if fitted with fins or another means of control" So basically, its all about your flight controller. After talking to an engineer, he basically stated, if I mounted them above the center plate, the copter would probably be a bit less stable than if I mounted them on the booms, but it would probably be a bit more responsive.
PeterLester
Member
Seems that the original IMU concept was developed back in the late 60's for the Saturn V rocket gimbal system as a replacement to conventional rocket fins which created excessive drag. So if the IMU can keep the bottom of a long thin rocket balanced (as if balanced on the head of a pin) then it would seem that it could also keep the heavy batteries balanced on the top of a multirotor where the center of gravity is higher than the IMU.
Another IMU example would the the Apollo LEM - Luner excursion module where retro-rockets where uses instead of propellers to balance the LEM as it touched down on the moon's surface. However 99% of the upward thrust came from the rocket motor mounted directly underneat the LEM......
Now..... if someone on this forum could just put a big rocket motor on the bottom of their MR and test it out to see if it will actually work for MRs!!!:highly_amused:
Another IMU example would the the Apollo LEM - Luner excursion module where retro-rockets where uses instead of propellers to balance the LEM as it touched down on the moon's surface. However 99% of the upward thrust came from the rocket motor mounted directly underneat the LEM......
Now..... if someone on this forum could just put a big rocket motor on the bottom of their MR and test it out to see if it will actually work for MRs!!!:highly_amused:
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Wow! Truly facinating. The engines-on-pylons thing in the Scott Manley video throws more rocks into the equation as it more closley mimics a multirotor. No matter how "cool" your frame design is, there will be *some* flex on the booms. Would it be enough to ellicit the effect the Manley video describes, dunno?? Unfortunately, running the battery leads up from a low-side power distro board location brings them nearer to the mag compass sensor. Now, the potential for a motor current induced mag-field to interfere with the compass is far greater. (I ran into this with the last smaller frame.) If I end up using a WK-M, this probably wouldn't matter as the mag is out-on-a-stick, anyway. That said, I've see my frame fly truely divinely with top-side packs, which are now back on the design roster for this one. Great stuff, *this* is the reason I like multis over other RC aircraft.
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