ANGLE OF ATTACK AND TRIM

However large and fast a model may be, its ability to gain lift will depend almost entirely on the form of the wing and its angle of attack relative to the airflow. The angle of attack is measured in degrees from some more or less arbitrary reference line, usually the straight line through the extreme leading and trailing edges of the wing aerofoil section or profile. In some cases, especially for an aerofoil with a flat underside, such as the Clark Y, a line tangential to the undersurface may be used. The angle between the reference line and the airflow at a distance from the wing is the geometric angle of attack. The aerodynamic angle of attack, i. e., the angle at which the air actually meets the wing, is almost always different from this, as will be explained in later pages.

The angle of attack (both geometric and aerodynamic) of the main wing is governed in orthodox models by the relative setting of wing and tailplane. The tailplane is a small wing which may or may not contribute lift to the total, but whose main function is to trim the mainplane to the desired angle of attack and hold it there. The angle of incidence of tail and wing to the fuselage must be distinguished from the angle of attack to the air. The fuselage itself may not be aligned with the airflow. (In this book, the term angle of attack is reserved for the angle of wing or tail airflow, and angle of incidence refers only to the rigging angle of such surfaces relative to some datum line on the drawing board. This convention is not always observed in other works on aerodynamics.)

Tails are sometimes arranged in V form, or even inverted V, when the longitudinal pitching and lateral stabilising and trimming functions are combined in the two surfaces of the V. Many other layouts than the orthodox wing-tailplane-fin style are possible, including tailless, tandem, delta and tail-first or canard aircraft All these and more can be made to fly and sometimes for special purposes may be superior to the standard arrangement.

Strictly, almost all ordinary aeroplanes and gliders are ‘tandems’ in that they have two wing-like surfaces disposed one behind the other and set at different rigging angles relative to one another. The relative areas and spans of these surfaces are matters of the designer’s choice. Whether one wing or the other carries most of the load or all of it is a matter of trim and centre of gravity position. If one of the pair of wings carries no load or very little, it functions only as a stabiliser and control surface and may then be very small relative to the main load-carrying wing.

In certain circumstances, the so-called ‘canard’ layout with a small, load-carrying wing ahead of a larger mainplane has certain advantages over the more usual mainplane/ tailplane arrangement. The first successful aeroplane, the Wright Brothers’ Flier of 1903, was a canard.

The reason why most aircraft have tailplanes rather than foreplanes will appear in more detail in Chapters 12 and 13. Although unorthodox aircraft sometimes appear to offer great advantages, the tailless type because it saves the drag and weight of fuselage and stabiliser, for instance, there are always certain disadvantages too, either in terms of excess drag from other causes, structural complexity, or, more often, problems of control and stability.