THE PROPER WAY TO DO YOUR QUAD CENTRE OF GRAVITY (CG)

[bunnygirl80]

Hi guys,

Well, the TBS build is now complete and I';m sooo excited and looking forward to getting her out for a spin!  ::)

I think the critical thing in the build of the TBS is to balance your motors, balance your props and balance the aircraft. I';ll start off with this aircraft balance technique and I';ll post two others later on about balancing the motors and the balancing the props.

This is a technique my dad showed me many many moons ago! I normally use it to set up the CG of my 3D aerobatic planes, BUT I';ve adapted it for quads and multi rotors. I think it works beautifully as the video I';ll be posting in a few minutes proves.

1) You will need a long piece of string, a bubble level and a plumb weight.
2) Run the string through each of the holes in the feet of your quad.
3) Tie a knot and hang your quad upside down from a nail or similar in the door frame. I used a clamp!
4) From the same point on the nail above the door, hang another piece of string with a plumb weight on it.
5) Now put the bubble level on the quad and level it.
6) Wherever the plumb weight rests is the current CG.
7) Add / Remove / Move things around until the CG is perfectly where it needs to be.

The first photo shows the quad hanging upside down from the clamp on the door frame The second photo shows the CG slightly forward of where it needs to be. The third photo shows the lead weight which I';ve added to the front which will eventually be replaced by the GoPro (this is the same weight). The final picture shows the CG is now perfect.

The End! Enjoy!

[mickyt]

They claim its 15cm forward of the center of thrust mark on the bottom plate 

[bunnygirl80]

Hi Mick, Yeah, Craig mentioned that to me on the forum this morning and I did try it with the 15mm adjustment, but it was definitely more stable and hovered better with the original TBS CG. So, I';ve left it at that!

[mickyt]

Cool so when do we get the video of its maiden flight outside then treacle?

[SimonS]

They claim its 15cm forward of the center of thrust mark on the bottom plate 

Try mm not cm.

Just to reiterate, the mark on the bottom plate of the TBS Discovery is the centre of thrust NOT the centre of gravity.

[bunnygirl80]

Hi Simon,

I hope you';re ready for a laugh! Nice of TBS to change the definition of the CG symbol! LOL. I';m not an aircraft engineer or mathematician, but my understanding of center of thrust is that it involves the angles of the motors and their relationship to each other and the frame of the multirotor. Furthermore, the center or angle of thrust cannot be calculated easily without some serious calculus, but equally can easily be changed by small variations in motor parameters such as speed which is generally induced by the MC. So, this makes me think that it would be pointless to even calculate or mark the bottom plate with a centre of thrust symbol as it would be useless to most men and beast! The purpose of the MC is to keep the thrust line within the parameters of a level aircraft which is why the CG is most important. Personally, the only indicator that we as builders can really work with (unless you have a degree in newtonian physics) is the CG and this has been the CG symbol for many years (check out wikipedia). I have been tinkering with things that can fly since I was a nipper and I';ve always recognised the CG symbol as the equilibrium of the aircraft and the line with arrow eminating from it as the thrust line! Perhaps you have a degree in newtonian physics and can enlighten me further! Ok, that';s my Wednesday morning rant over with! lol. Go, get your coffee! xx

[eyeflying]

Hmmm... maybe this is being over thought a little. The Discovery';s Centre of Gravity does appear to be almost exactly where the CoG mark is located. That';s when its fully built and fitted with a 4 cell 4.5mAH battery - and of course its also fitted with a GoPro and an FPV camera, Tx, etc.

All I ever do before flying is balance the TBS frame between two fingers just in front of the point where the rear ams connect with the frame (this is in line with the CoG mark) - I then move the battery a couple of cm';s or so to get it to balance. It then will always fly perfectly.

Correct me if I';m wrong, but I';ve always assumed that a quad';s CoG should be at the point where two lines between the centre points of diagonally opposing motors intersect? - this would surely be the Centre of Thrust too?

[SimonS]

Hi,

I';m well aware that they have used the wrong symbol.  There is a good diagram somewhere on FBVLAB but the thread is over 300 pages and I';m not going through it all again to find it.  From memory, the mark on the bottom board is at the intersection of the diagonals from the motors 1&3 & 2&4.  The c of g is suggested to be at the mid point  between the lines through 1&2 & 3&4.  if you plot it out this is 15mm forward of the mark. This of course will change according to the weight and distribution of everything else on the craft.  I like your method and will try it in the future.

[Jumpy07]

Far too complicated for my simple brain..     will stick to my "two" finger balancing method and sliding the battery to compensate 

[mickyt]

Why do people have to go all bloody technical 

[Smeagol]

Far too complicated for my simple brain..     will stick to my "two" finger balancing method and sliding the battery to compensate 

Gets my vote, but then no aerobatics for me, i';m still learning to fly straight 

But it looks like a good accurate method to check and set, 

[icedfusion]

Technically, you "do not need" to have the FC at the CoG. If it is off, you can compensate by separately adjusting the roll/pitch PIDs. Makes things more complicated in terms of tuning or if you have a copter that is not symmetrical, however, with so much other stuff to worry about it is easier to get the FC properly placed!

ice.

[bunnygirl80]

Yeah, lots of positive comments here. I do think it is best to have everything mechanical, balanced as best as possible. I balanced the props, motors and frame. The method I use for balancing the frame is very accurate and also works in both x and y planes. My Naza is currently sitting in the recommended area between the two lines, but the quad doesn';t turn well, so I may move this back to see what it does. I';ll keep you posted.