RUDDER-STEERED MODELS

Models which rely for directional control on the rudder only, without ailerons, rely on the dihedral to turn. Insufficient dihedral may cause lack of turning power even with a large rudder. The rudder yaws the aircraft, thus causing the wing on one side to present a larger angle of attack, and the dihedral rolls the aircraft into a banked position. The total lift force of the whole wing is then tilted and this force, not the rudder, turns the aircraft. With well matched vertical tail areas and dihedral angles, the model turns quite efficiently since the brief yaw at the start is promptly countered by the roll. Once the turn is established, there should be little of no slip or skid with a suitable angle of bank. Flat, skidding turns are very inefficient

12.28 AILERONS AND DIHEDRAL

With ailerons, the amount of dihedral required for radio controlled models is quite small, and for aerobatics none at all.

Figure 12.18 shows that too large a dihedral angle reduces the efficiency of a wing. The drag force will be no less, but because of the inclination of the wing, a component of the lift is directed horizontally. The true wing area is represented by the vertical projection in plan, rather than by the length of the wing span as the model is built on the building board. Only the minimum dihedral needed for stability should be used. For free flight models and thermal soaring R. C. gliders, it is advantageous to employ polyhedral. The steeper dihedral of the outer wing panels is more effective, due to their greater leverage arm from the model centre line. Polyhedral is a means of reducing the total dihedral of the wing compared with a straight wing model. To achieve the same effect with a straight dihedral requires a greater average dihedral angle. As with planform, an elliptical dihedral form is theoretically best, but although models have been built in this style, the gain in efficiency is very slight and there is considerable difficulty in laying out such a form on the building board. Extreme forms of dihedral should always be avoided since they promote cross flows on the wing and create vortices, increasing drag. It is also possible to have so much dihedral that turning becomes impossible.