There are several different ways elevator trim can be installed. Inside the cabin, control for the trim is either an electric switch on the elevator control, a manual trim wheel, or a combination of the two. Most aircraft have an external trim tab on the right elevator, while our DA20s have an electric motor to control spring pressure on the elevator (no trim tab). The advantage to the DA20 trim system is that, in the event of a trim failure, the elevator can still deflect fully to control the aircraft.
When used correctly, trim does not control the aircraft — the elevator controls the aircraft. Trim simply relieves the pressure on the elevator to make flying easier. Since the amount of air flowing over the wings changes as we speed up and slow down, it makes sense that the required elevator pressure changes with airspeed. Essentially, a trim setting is for a given airspeed. Example: If we are trimmed in cruise at 120 knots and we power back to begin a descent, the trim will try to hold 120 knots during the descent. If we were to add power, the trim would try to make us climb at 120 knots. It should be noted that older aircraft tend to have sloppy/loose trim systems. Trim during a descent at 120 knots may be different than a cruise at 120 knots. In general, the required change in trim is negligible. When I am teaching trim technique I try to allow my customers to experience this principle hands-off. On a calm day, we let go of the elevator controls for a bit and just change the power to see what happens. In the DA20, the airspeed typically remains within 5 knots during climb, cruise, and descent — with no elevator input. Go try it!
Trimming properly can make your landings very consistent. A perfect approach will give us a constant airspeed once established on final approach. In the DA20s, for example, we recommend 60 knots. Since trim changes with airspeed it is important to do your final trimming AFTER establishing 60 knots. As you approach the runway threshold, the trim will assist in keeping a consistent attitude and airspeed (60 knots). During the roundout and flare the airspeed will slow down and back-pressure on the control will increase. Since the trim is adjusted for 60 knots, the amount of back-pressure required for that perfect landing should be the same, every time.