When you decide to fly off water, you’ll need to learn a few aeronautical terms. The reference point for measuring the incidence of the wing is the “datum line” of the fuselage. The datum line runs horizontally through (or is parallel to) the horizontal stabilizer. “Wing incidence” is the angle between the wing chord line and the datum line of the fuselage. With flat-bottom airfoils, the “chord line” is not the lower surface of the wing; it is the straight line from the forwardmost point of the wing’s leading edge to the most rearward point of the trailing edge. In most flat-bottom airfoils, the lower surface of the wing is flat only from the main spar to the trailing edge, but from the spar forward, it sweeps up slightly. The chord line is usually about 2 degrees more than the flat surface aft of the spar. This angle of incidence is set and cannot be changed once the wing has been fitted to the fuselage. It is not to be confused with the wing’s angle of attack, which varies greatly during flight and depends mostly on flying speed.
Add to the above terms two more that apply to floatplanes and flying boats: “beam” and “keel flat.” The “beam” for a single-hull flying boat is the width of the hull at its widest point, usually near the step; with a floatplane, the beam will be double the width of each float. The “keel flat” is considered to be the bottom surface of the hull or floats directly under the center of gravity. It is the area from directly under the wing’s leading edge back to the step. When the model flies at high speed on the water just prior to liftoff, it is what we call “on the step,” and it rides on this small keel-flat section of the floats or hull. If the model is blocked up on the workbench so that the fuselage datum line is parallel to the bench surface, the only part of the floats or hull touching the bench would be the bottom tip of the step. The keel flat should angle upward slightly, at about 2 degrees. Sometimes, however, it can also be set at 0 degrees, with the entire keel flat touching the bench. Getting the keel-flat setting correct for the floats is akin to checking the incidence for the wings. Forward of the leading edge of the wing, the keel gently sweeps up toward the nose.