Wing incidence angle
The incidence angle is the angle between the wing airfoil chord and the fuselage reference axis. This angle determines the wing's angle of attack during straight and level flight. Typical wing incidence values for trainer models range from 1 to 3 degrees (positive, meaning the leading edge is above the trailing edge relative to the fuselage axis).
Too much incidence increases drag and slows the model. Too little means the model must fly faster to generate sufficient lift. A change of a fraction of a degree can noticeably affect flight speed and glide angle.
Decalage: the incidence difference
Decalage is the difference between the wing incidence angle and the horizontal stabilizer incidence angle. Positive decalage means the wing has a larger incidence angle than the stabilizer. This is the standard setup for models with a conventional layout (stabilizer at the rear).
Positive decalage provides longitudinal stability. When the model accelerates (for example in a dive), the stabilizer generates a force that lifts the tail, reducing the wing's angle of attack and arresting the dive. When the model slows down, the stabilizer lets the tail drop, increasing the angle of attack and restoring lift. A typical decalage for trainer models is 1 to 3 degrees.
Washout
Washout is a reduction of the wing incidence angle from root to tip. The wing tip has a smaller angle of attack than the root. This means that during a stall, the root section loses lift first while the tips retain attached airflow longer.
Washout improves model behavior at low speeds and increases safety, especially in turns. A typical washout value is 1 to 3 degrees. In free flight models, washout is often built into the wing structure. In RC models it is achieved by slightly twisting the wing tip during construction.
The opposite of washout is wash-in (increasing incidence toward the tip), which should be avoided because it worsens stall behavior and increases the tendency to enter a spin.
Stabilizer incidence
The horizontal stabilizer in most trainer models is set at an angle of 0 to minus 1 degree relative to the fuselage axis (slightly negative, with the leading edge lower than the trailing edge). This setting, combined with a positive wing incidence, creates positive decalage.
In free flight models the stabilizer angle is critical because there is no way to correct it after launch. A change of 0.5 degrees can turn a stable glide into a dive or a pitch-up. Therefore stabilizer angle adjustments are made delicately, preferably by using shims (wedges) under the leading or trailing edge of the stabilizer.
Wing dihedral and polyhedral
Dihedral is the upward angle of the wings relative to the horizontal. Both wings form a shallow V shape as seen from the front. Dihedral provides lateral (roll) stability. When the model banks to one side, the lower wing sees a greater angle of attack than the upper wing. The difference in lift forces returns the model to level.
Polyhedral is multi-panel dihedral, used mainly in free flight gliders. The wing is divided into several panels, each at a different dihedral angle. A common arrangement is a small dihedral on the inner panel and a larger dihedral on the outer panel. Polyhedral gives a stronger stabilizing effect than simple dihedral with less increase in drag.
In RC models, dihedral is usually smaller (3 to 7 degrees) than in free flight models (10 to 20 degrees) because the pilot can correct banks in real time. Excessive dihedral in an RC model causes a tendency to Dutch roll (lateral oscillation).
Building board and geometry control
Precise wing and stabilizer geometry requires building on a flat, stable surface. A building board is a sheet of cork, MDF or polystyrene on which you pin the plan and build the structure. A good board must be perfectly flat and resistant to glue.
During construction, check incidence angles with a digital or analog inclinometer. It helps to prepare blocks of known thickness (for example 3 mm, 5 mm) to place under the leading or trailing edge to achieve the desired angle.
After construction, verify wing symmetry by measuring the distance from each wing tip to the fuselage centerline. A difference greater than 2 to 3 mm requires correction. Asymmetry causes the model to turn to one side.
Practical trimming
Start by setting the CG according to the plan, then do hand-toss test flights on a field. Observe the flight path. If the model dives, slightly reduce the stabilizer incidence (raise its trailing edge). If the model pitches up, increase the stabilizer incidence (lower its trailing edge) or move the CG forward.
Make changes in small steps. One parameter at a time. Several test tosses after each change. Keeping a notebook with a record of changes and their effects is invaluable, especially in free flight models where there is no way to correct the flight path after launch.