Before starting to build any machine, either full size or model, the following points require to be known by the constructor: First, the weight of the complete machine; second, the area of supporting surfaces necessary; third, the amount of power required; fourth, dimen sions; fifth, size and speed of propeller, and lastly, methods of control. Weight forms the basis of designing. It may seem somewhat difficult to assume the weight of an object, before knowing its dimensions, but it is not difficult to build an object of such dimensions that it will not exceed a certain weight.
In flight, the weight of the aeroplane is supported by the reaction between the wings and the relative wind created by their motion through the air. So long as the proper relative motion continues, the wings perform their function of maintaining an upward pres sure, but it depends on a variety of circum stances whether that pressure continues to be applied exactly in the correct way. At this point it will be necessary to explain two technical terms: the centre of gravity (C.G.), and the centre of pressure (C.P.). The centre of gravity is the point where the weight of the machine seems to be concentrated, and the centre of pressure is the point at which the lift of the wings seems to be focused. Any object that is supported exactly at its centre of gravity is always balanced in any position in which it may be set. When it is not supported, it tends to fall into such a position as will bring the point . of support vertically in line with the centre of gravity. Like ships, aeroplanes are potentially liable to pitching, and yawing, and it is essential from the beginning to recognize that some of these acts may at times be essential to the guidance of the machine from one point to another through i space. If, for example, an aeroplane were n capable of being made to swerve at will it could not be steered; on the other hand, a tendency to make erratic changes of direction of its own accord would be described as direc tional instability. It is important to bear in mind that the air does not provide a fixed platform. When an aeroplane is canted, so that one wing is lower than the other, the C.G. of the machine has not necessarily been disturbed. There is a very ingenious side-show, often to be found at large exhibitions, that may assist the imagination in grasping the breadth of the subject. It consists of a flexible track that ceaselessly undulates in supposed pres entation of the waves of the sea. The raft like trolley serves as a very good object-lesson in two forms of stability. The machines in question are stable in the ordinary sense to the degree of absolute security, for they cannot conceivably capsize. On the other hand the instability of their direction makes it almost impossible to guide them three or four yards without running into either side of the barrier.
When an aeroplane is canted, the forces brought into play correspond with those that make the trolley run into the barrier, and their effect is equally to tend to make the aeroplane slip down sideways through the air. In the broader use of the term, what ordinarily is called longitudinal stability has to do with the pre vention and cure of pitching; lateral stability — similarly related to rolling, and directional stability to yawing from the course.
Lateral When an aeroplane is seen advancing from directly in front, the up ward pressure or lift of its wings may always be assumed to be acting in a direction per pendicular to the spars. The downward force of the weight acts always vertically toward the earth. If, therefore, the wings are canted, their pressure is no longer precisely in line with the weight, and there is, necessarily, a sideways component tending to make the machine swerve off its former course. On the assumption that the machine ascends into the air with its wings level, it is necessary to account for the disturbance of the balance by the introduction of some extraneous force. This, however, is readily supplied by supposing that the machine is struck by a gust. A gust, for present purposes, is assumed to be a sud den veering or backing of the relative wind. It is not necessary that the machine should be possessed of vertical fin surfaces, against which the oblique wind may strike, in order to account for the tendency of a gust to cant an aeroplane. The characteristics of wing-forms as ordinarily employed suffice in themselves to explain the occurrence, for if a gust is a sudden veering of the wind, it is equivalent to a sudden spinning of the wings about their vertical axis, as a propeller might spin on a vertical shaft. Under such conditions there is an obvious tendency for one wing to lift more than the other, and so to upset the balance. Lateral balance in the modern machines is secured by various methods. First, warping, which means that the relation of the wing surfaces to the relative wind are changed by the pulling of wires for a temporary period to aid in the control of balance. Secondly, machines a the biplane type are constructed with their upper wing in advance, slightly, of the lower wing, which enables the pilot to manoeuvre the machine very effectively in gusty weather. (Consult Berri man, 'Aviation' ). Thirdly, dihedral forma tion of wings has been introduced with much success. That is, wings which rise from a point at which they meet (exactly above the body of the machine) to their outward surface.