Weight shift for trim and control is one of the most origional and basic forms of aircraft control. In a hang glider, for example, body english-weight shift is everything.I've flown a variety of aircraft that involved vast changes in payload and consequently CG, throughout the course of the flight. When performing ag, the weight loss and CH change might occur over fifteen minutes as the chemical was sprayed out over the crops. In this case it wasn't a shift so much as a loss, though the CG certainly did change; it required considerable retrimming.
Flying skydivers in Cessna 182's and 206's involved having everyone lean forward during the takeoff. After a thousand feet, everyone would sit back to a more comfortable position, with a significant change in the feel of the airplane, and the trim required. The CG would move aft noticably. The same when people got out of the airplane, or climbed out on the strut or the wing.
When dropping retardant on fires, we would lose 18,000 to 27,000 lbs in varying increments, from 1/8 at a time, to all of it in under two seconds. This created very noticable rapid chagnes in center of gravity. Typically we would drop one aft tank, then a forward tank, then an aft, etc. If this change were done at one second intervals, the result would be a significant porpoising tendency that had to be anticipated to be controlled. Even in large airplanes such as these, someone getting out of their seat and walking to the back of the airplane could be felt in the controls.
In small aircraft such as the Learjet, I've noticed a definite control feel change as someone gets out of the rear bench seat and walks forward to the cockpit. The pitch change can be felt as the weight and CG shift. A degree of attitude change, or more, may be required to compensate for the change. This doesn't sound like much, but it can be felt.
During flight test operations to explore the CG envelope and attendant aircraft performance, very often a sliding weight is used to vary the CG. This weight is usually run on a cable and rod arrangement or with a chain and sprocket, in order to explore a variety of CG applications during one flight. The CG is precalculated, and can be set as required at intervals to conduct the test.
Some aircraft trim systems have used weight shift arrangements using a weight in the floor or in a track somewhere in the aircraft.
Some helicopters, such as the Rotorway Executive (an experimental) must use weights on the skids to compensate for passengers in or out of the aircraft. Some aircraft use a battery case that is mounted on a track, and the battery moves to adjust the CG in flight.
Some major aircraft accidents have occured over the years due to shifting cargo in which the cargo moved aft (or forward) and prevented recovery. Somewhere I used to have photos of a Skyvan that stalled and rolled over into a spin when too many jumpers gathered on the tailgate in preparation for an exit.
Remember that when having a discussion of spins, the aft CG isn't what causes a spin. A spin results following a stall; the aircraft must be stalled first. Many other factors are involved in spin recovery, including aircraft design, power application, control input, etc.
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