I was wondering that spoilers in most of the aircraft are always placed aft (towards the tail) the CG location. Why aren't they placed ahead of the wings or near the nose of the aircraft? The same is the case with parachutes. For small UAVs, the parachutes are either at the CG location or placed behind it. Is there a really strong design point to this method? Any suggestions/information would be appreciated?
Surfaces sticking out ahead of the Centre of Gravity act in an unstabilising manner: any dissymmetry will want to amplify itself. A dart thrown with the feathers first is in an unstable equilibrium and will very quickly turn around 180º.
Surfaces behind the CoG act as stabilisers, keeping the nose pointing forward. An aeroplane has vertical and horizontal tail surfaces at the back just for this purpose. When spoilers are deployed at low speeds, the aeroplane is in a situation where you don't want any destabilising moments.
Aerodynamic behaviour is stable if a disturbance auto-corrects: a deviation is automatically blown back. Since motion in flight is relative to the Centre of Gravity, it follows that the aerodynamic Centre of Pressure must be behind the CoG.