I have a question on CG of a foamy. Now by calculation/ thumb rule it should be ~ 1/3 from LE of the wing chord if I am correct?

But when I lift my plane (all fixed except battery) from it's spine it "balances" about 8cms behind that. Now If i were to put my battery on the dot here so that the CG remains where it was before the battery what good/bad things can i expect to happen (or is it just "plane" suicide?) vs I were to put the battery where I can get the CG further ahead on the calculated CG point.

PS: If the Q is plain stupid do tell & point me if it's already answered here, could have missed..

@flyingbrick
Is it possible that you have never built a chuck glider?
Maybe never even seen one?

If you had, this doubt would not have arisen.
The existing CG of the model has nothing to do with where the CG should be. If the tail had been a little heavier, the CG would have been a little further back.

The thumb rule is to start with the CG at 25% of chord back from LE, then move it gradually back to improve response.
The surest way to crash a new model is to fly it tail heavy, ie, CG too far back.

Try to build a chuck glider like this. Try flying it without any nose weight (plasticine).
It will teach you all about CG!

Try balancing the airplane on your fingers by placing your one finger tip 1/3 from the LE. Then you'll get to know where the CG lies. If there is any nose up/down try balancing it with battery.

@KK Iyer Sir thanks for an extremely enthusiastic reply but it does not answer my question nonetheless. Why does it have to be 25-30-.. of Wing Chord from LE?

CG being the force downwards and CP being upwards..and these need to cancel out in flight I presume ..CP being more dominated by lift and so Wing area, position w.r.t to the fuselage, Wing AA and the empennage HS VS, also play a role in flight vs a static model etc...but I could be wrong .Thoughts?

Looking for rcpilotacro Sir's old posts, he may have touched upon this somewhere

Ok.
Not sure whether you want to find:
1. A reasonably safe CG position for a test flight. If so put CG between 25-33% of chord from wing LE.
2. An academic understanding. If so, we'll go one level deeper.

The CG position has to relate (NOT to the CP of the wing) but to the effective CP of the wing and (horizontal) tailplane. This is called the Neutral Point, and is the position through which the lift forces effectively pass.
For this discussion, let us take it as a working assumption that the NP is perhaps in the range of 25-50% of the wing chord from its LE. (Obvious exception if free flight power duration models with huge stabs)

If the CG is at the NP, the model is neutrally stable. Usually we need some amount of positive stability. This is provided by keeping the CG say 5-10% of chord ahead of the NP. This is called the Static Margin.

Remember that in models we may rarely know where exactly the NP is. From experience aeromodellers know that keeping the CG at around 25% of chord from LE gives a reasonably safe Static Margin.
Ie, fairly strong stability (hands off ability to fly) at the cost of elevator responsiveness.
Then you gradually move the CG back to suit the sensitivity you need.
Once the CG goes aft of the NP, the model becomes unstable, ie, any divergence from flight path will magnify. It will continuously need control input.

Even though we don't know the exact position of your model's NP, if the CG is behind the NP, you will come to know without doubt in the very first launch/take off. It will be practically unflyable.

For simplicity, effect of decalage on stability, CP movement etc not discussed. Nor is how the static margin contributes to stability. Can be discussed, but may not be of interest to too many model flyers, as these issues are taken care of in RTFs and ARTs.

My humble request. Please do try out CG experiments with a chuck glider. It is amazing.

Why does it have to be 25-30-.. of Wing Chord from LE?

One line answer: Because it has to be ahead of the Neutral Point to provide a static margin of stability.

thanks