Does it make any difference how you balance a plane so that the CG is correct.

I mean assuming the same light model which of these two configurations will be better:
1 two weights one at front one at the back,
2 both weights together just over the wing?

I've just electricized a small sailplane and have the motor and battery close together just over the wing, the CG is correct but will this config be 'twitchy'?

From : Don Stackhouse

Two weights spread far apart have more rotational inertia (in layman's terms "flywheel effect") than two weights closer together.

In general, more inertia is bad. Yes, it can reduce the plane's initial response to a gust or other disturbance (which also makes it more difficult to see the plane's initial reactions to entering thermals), but it also means that the oscillations that follow involve more energy and will be harder for the flying surfaces to damp out, hurting the plane's dynamic stability. The initial response to control inputs will be delayed, and the plane's tendency to overshoot the desired attitude at the end of a control input will be worse, making it harder to control the plane's attitude precisely.

The size of the "sweet spot" for the zone of wing dihedral and vertical tail size that results in both an absence of dutch roll and an absence of spiral instability depends a great deal on the plane's inertia. If you have too much weight in the extremities, you may find that the zones of spiral instability and dutch roll actually overlap, and that there are no combinations of dihedral and tail size that don't result dutch roll, spiral instability or both!

The best aerobatic planes often go to extreme measures to minimize weight in the extremities and thereby minimize inertia. For example, on the Bucker Jungmeister full-scale classic aerobatic plane (considered the finest of its day, and the plane that quite literally "wrote the book" for modern aerobatic competition), its designer Lars Anderson went so far as to use seven different telescoping diameters of tubing for the fuselage longerons, larger near the wing and engine where the loads were higher, and tapering to lighter, more slender diameters in the more lightly-loaded tail.

If you have a choice, barring any other significant factors, it's nearly always best to keep large masses as close to the C/G as possible, to keep rotational inertia as low as possible.

Don Stackhouse
DJ Aerotech