here's a little something to help multirotor beginners and
provide food for thought for the experienced chaps.

*  placement of the flight controller board does not have to be right at the centre

* the multirotor does not have to be perfectly balanced at your perceived centre of gravity ( within reason)

here's why....

1 . most flight boards today use a three axis gyro accelerometer combo as the primary sensor .  Three axis means X , Y ,Z.  .... so at best you can locate the flight controller only at the centre of X ,Y axis .. you will never be able to locate the flight controller at the true center of all X,Y,Z axis.

2.  the chips measure g- force and angular vector about itself  so where ever you place them - the software will treat as centre.

3. the software can easily compensate for any weight imbalance . A good and simple example is the Kk 2 mixer menu. It is possible to have a quad with 3 arms at 25cm and one arm at 50 cm and the quad will still fly normally.

4. An F-16 does not have its gyros and accelerometers placed at its exact centre of gravity ( neither does an A320. )

what is important for multirotors is the following....

1. vibration should be at a minimum .. and isolating the flight controller from vibrations will give you better performance.

2. the frame should not flex.

3. take the time to study the basics of PID loop and tune your multirotor.

APM 2.6 w/ arducopter really cares about COG(stabilize/acro mode) and there is no compensation whatsoever from firmware while thats not the case on a kk 2. So sensitivity to COG is dependent on the platform being used.

That is why kk2 is widely suggested for beginners as you have less things to worry about, it works on default PIDS on most of the average sized frames.

As for PIDS, I may get bashed for this, but understood it better when I started tuning by holding the quad in hand and adjusting pids on the fly(multiwii). Throttle it only till the time you feel it is about to takeoff then roll/pitch slightly and check the reactions. It helps to get better feedback and adjust parameters so that it begins to fly. I wont suggest it to anyone, but if done carefully with adequate precautions, its not so impossible.

@sidewinder...

think 3d not 2d

you are missing the point. the sensors used are 3d . if you have to place at actual COG then it would be
somewhere in the middle of the quad. You would not think twice before using standoffs and place the
FC 1/2 inch above the frame ... this means you have just moved the FC 1/2 inch away from actual CG.

secondly - the PID loop works the same in all cases. This applies to all controllers ... yes.. the defaults
may be different each case but the underlying logic is the same . This is why it is left to the user to adjust
as per his frame. i.e if a PID loop is implemented then the code HAS to compensate.

In vertical (z)axis the copter compensates well even Arducopter for that matter if the CoG is 1/2 inches above or below, but it does effect stability. Its not optimum. Ideally Z axis CoG should be at prop level, if its lower or higher, copter has to compensate while rolling or pitching. Yet its a common belief that placing CoG lower than the FC provides great stability, but it isnt. Its a different thing that the firmware compensates & stabilizes for it(thats why I wont think about it) but naturally its not stable.

More details can be found here
http://diydrones.com/forum/topics/center-of-gravity-question

So ideally, well balanced copter x/y CoG's should be dead center. And CoG(z axis) and FC should be at prop level centered. Again while KK2 handles off balance x/y axis CoG's, but APM 2 doesnt. I can put up a video to show that.

A little off the topic though, I have got the HK FC board with 3 axis gyros and Atnega644 , (clone of KK5.5) , Which is the default Quad orientation in this board ? (+ or X ?) . The front direction of board is one corner, so for X configuration, do I have to keep this corner to the flying direction of quad ?
And if the default firmware is + config , can I get it going in X mode with simply changing the physical orientation of FCB , or do I have to flash it with X mode firmware?

Its hard to predict default orientation, but usually its + on kk 2.
Yes,arrow always shows the front, so it should always be pointing in forward direction with respect to frame.
And you wont be able to just change the orientation of a + to x by changing the position of the board, you have to flash the x  firmware.

My board is not KK2.x It's the Hobby king version of KK5.5 (Three gyros + Atmega 644A) .
I have made a Aluminum frame, Do I have to keep the arms in Right angles ?

Just flash the firmware you want rather than second guessing what is the default firmware, or you can check it the hard way by connecting everything.
+ and x are the same in all aspects, other than the fact that forward orientation changes. So yes all four arms should be at right angle to each other. There can be variations but the position of all motors should be in x or + in a quad.