Flaps do several things: they modify the airfoil to increase lift and / or drag to allow the plane to fly at a slower airspeed, and to help slow the plane for landing. Whether they create more lift than drag depends on the angle they are deployed, as well as the particular design of the flap. A simple flap increases the camber of the airfoil, and alters the angle of attack by changing the chord line. Spilt flaps do the same thing, but are "draggier" because you have both the lowered flap and the trailing edge of the wing spoiling the slipstream over the rear portion of the wing. Slotted and Fowler flaps are "cleaner" because they allow a small stream of air through the hinge area, which smooths the airflow over the top of the flap, decreasing the drag. Fowlers usually move rearward as well during deployment, which increases the chord of the wing slightly. There is a point on either type where drag becomes greater than the extra lift, dependent on how far they are deployed. For example, for take-off you only use 10-20 degrees, where for landing you can go full deployment, as much as 60 degrees on some planes, to add braking by increasing the drag.

A model might benefit with as much as 40 degrees or so for landing. Using them may also change the flight characteristic of the model. Some will pitch up and some will pitch down with flap deployment, requiring a compensatory application of elevator to maintain the desired attitude.
Are you in the USAF, or family of one? I used to fly in and out of McChord back in the late '80's. I was in a medevac squadron, we flew in C-141B's, and had some friends there at McChord.