ORIGINAL: Silver Flyer
I am confused by some post I have been reading, I hope some one can straighten me out. Several have stated that at the recommended 100MM from the leading edge their model was tail heavy. Then they state that moving the CG to 90 MM from the leading edge corrected the tail heavy problem. In my mind if its tail heavy at 100MM from the LE and you move the CG to 90MM from the LE it is going to be even more tail heavy. What am I missing here? Thanks
Silver Flyer:
You are correct!
What has been called the CG, is not!
What we do is to locate that point along the fuselage where there is no torsion force or torque trying to pitch the fuselage, in any direction.
When the wings are supported at that point, the fuselage remains horizontal, and, if disturbed, it always returns to that position.
This happens because the forces and levers are balanced, in front, and behind that point.
By adding or taking away weight at the nose or the tail, we can make ANY point in between the CG or balance point.
Now, any model will fly at its best if it balances around a "magic point".
The location of that "magic point" has to do with the location of the lifting force produced by the wings and the tail, which only appears while the model is flying.
That is the reason it is referenced to a point of the wing, generally, the leading edge next to the fuselage.
By playing with the distribution of the weights along the fuselage, we make the CG and that "magic point" to coincide.
In your example, that post you have been reading, has stated that at the recommended by the designer "magic point", the model was flying like tail heavy models fly. Then it states that adding enough weight to the nose moved the CG (or balance point) closer to the leading edge, where the "magic point" was verified by experimentation to be.
Two things may have happened there:
1) A design or typo mistake. The "magic point" was assumed, calculated, located or described wrongly.
2) Some deviations from the original drawings were introduced during the construction of the model, enough to cause a 3/8" slide of the "magic point".