CHAPTER XII.
CONTROL SYSTEMS.
The mechanism by which the aeroplane is controlled in flight forms the connecting link between the pilot and machine, and constitutes a vitally important and somewhat vulnerable item of the complete structure.
Main Principles.
The control of all modern aeroplanes is effected in a lateral direction by small planes hinged to the rear spar of the outer ends of the wings, and known as “ailerons”; in a longitudinal or “fore-and-aft” direction by the elevator planes; and for steering by the rudder. Although these functions are alluded to separately, they are more often than not combined in their actions. The correct proportion of the controlling surfaces is an important factor in determining the ease or otherwise with which a machine can be handled in flight, and faults in this direction are responsible for the terms “heavy” or “stiff” on the controls being applied to a machine. The use of subsidiary flaps or ailerons for lateral control is a comparatively modern innovation. At one time it was usual to warp the entire plane, or in some cases the outer section only, and although the principle is the same—that of forming a negative or positive surface to the line of flight—structural considerations are wholly in favour of ailerons. With warping, the whole plane is subjected to continuous torsional movement, and to obtain this some of the trussing wires have necessarily to be arranged as control wires, the result being that the plane curvature loses its uniformity, and the whole girder system of the planes is less efficient under load than if the wires were permanently fixed; and the latter item is only possible with aileron control. Although it is usual to attach ailerons to both top and bottom planes of a biplane, there are occasions when sufficient control can be obtained with ailerons to the upper plane only, usually when the span of this plane is greater than that of the bottom.
Control by Inherent Stability.
With machines of the inherent stability class the lateral control is effected by additional means, the planes being designed to automatically right the effects of gusts. This element of inherent stability is obtained by suitably grading the camber and incidence of the wings, until at the wing tips the chord of the plane section forms a negative angle to the line of flight. Although this arrangement is undoubtedly of value, especially for the touring machine of moderate power, its chief fault lies in the relatively slow righting movements, which, although of no great consequence at a reasonable altitude, becomes a source of danger when alighting, and certainly entail the use of ailerons, or warp, to counteract it. The type was well exemplified in this country by the Handley-Page monoplane and biplane, while in Germany it achieved great popularity, surviving in some makes until the latter part of 1916. In the matter of control-surface design it is interesting to note the contrast between the preferences of English and German designers. In almost all German machines the ailerons, elevators, and rudder are balanced, i.e. surface is disposed each side of the hinge-axis, this applying to the small Albatross scouts and to the large machines of the Gotha class; while in this country few examples of this practice occur. The reason for the balancing of controls lies in the desire to reduce the manual strain on the pilot to a minimum; and it appears that with large machines balanced surfaces will be imperative. Several automatic controls have been produced, the most notable perhaps being the Sperry gyroscopic, this being a combination of servo-motor and gyroscope. This apparatus has been well tried.
So far as the arrangement of the control surfaces is concerned, little variation occurs, which condition has obtained from the early days of aviation, but the mechanism governing or directing these movements varied at one time considerably, and although in this country one type of control is used, there are still instances of the use of widely different systems. In former days the practice of individual makers fitting different controls resulted in some arrangements being in exact contradistinction to others, which not infrequently meant, to a pilot taking on a new type, the unlearning of a great deal which practice had rendered instinctive.
The Instinctive Principle.
All modern controls are based on the instinctive principle, i.e. the movements of the control lever coincide in direction with the promptings of natural instinct. Thus, to change the course of a machine flying level into an upward one, the column is pulled towards the pilot, and for descent, the reverse, while to correct a bank, the column is moved in a direction opposed to that of the bank. For steering, a foot-bar is employed, so arranged that for a turn to the left the left foot is pushed forward, and the reverse for a right turn. On one well known machine of former days, the foot-bar actuated the lateral control, which is sufficient indication of the great diversity of opinion then existing.
Vertical Column Control.
A typical control of the immensely popular “joy-stick” type is shown by Fig. 100. This consists of a vertical column pivoted through the medium of a fork-joint to a rocking shaft. The elevator wires are taken round pulleys mounted under the seat, and the aileron wires from a form of bell-crank, flanged and welded to the steel tube. A disadvantage with this system, in addition to the complication of the wires, is that lateral movement also affects the elevator, although the extent of this is of no great moment. It is obvious, although somewhat paradoxical, that if the elevator is to be depressed by a forward movement of the column, the control wires will required to be crossed, i.e. the wire running from the base of the tube to the pulleys will be attached to the arm on the top side of the elevator, and vice versâ. On single-seater machines it is sometimes necessary for the pilot to have both hands free of the controls, so that it becomes necessary to install some form of locking device for the elevator control, there being many simple ways of accomplishing this. The locking of the control lever fixes the flight path of the machine, but, of course, lateral equilibrium can be maintained by movements of the lever sideways, and steering by the rudder bar. The German machines of the Fokker and Albatross types are both fitted with the single lever control with a locking arrangement. Another method which achieves the same purpose consists of bracing the lever in a normal flying position, with rubber cable or coil springs anchored to various parts of the fuselage, and although this permits of movement, the control column always tends to return to the normal position.
Wheel Controls.
While the “joy-stick” type of control is greatly in favour, there are various forms of wheel control in use. American machines are almost entirely fitted with wheel controls, and all things considered, it appears that modern practice is evenly divided between the two types. The sequence of movements of the wheel type may be varied in a number of ways, the general arrangement shown by Fig. 101 being typical of an average system. In this case the hand-wheel is mounted on a central column, which in turn is rigidly fixed by some form of Tee joint to a transverse rocking shaft. A sprocket attached to the wheel centre engages with a short length of chain, which connects to the aileron control, while the elevator wires are connected to short tillers, arranged to work on the outer side of the fuselage. With this system the hand-wheel is rotated for the aileron movements, a fore-and-aft rocking motion for the elevation, and the rudder is actuated by an outward movement, with either foot on the rudder bar. A development designated “three in one” embodies all these movements in the wheel column, which in this case is pivoted at its base: a to-and-fro motion in the column for the elevators, sideways for the ailerons, while the rudder control is effected by the rotation of the wheel. This system is fitted to a number of American machines, but it is a moot point whether the rotation of the wheel for warping or steering is quite such an instinctive action, as the sideways movement of the lever combined with the movements of the foot on the rudder-bar; in any case, there is just a suspicion of complication in its working which is undesirable, that is, for machines intended for popular use.
The “Dep” Control.
The type of control used on the Deperdussin monoplanes of 1910 and onwards has survived until the present day, and forms a distinctive arrangement. Its chief attribute is that, compared with other systems, much greater room and freedom is afforded the pilot, which is evident by a consideration of the diagrammatic sketch, Fig. 102. The inverted U-shaped lever is composed of either ash, bent to shape, or steel, or duralumin tube, the general system of its working being the same as the wheel control shown by Fig. 101. Incidentally, passing reference may be made to the fact that the usual close proximity of the compass to the controls precludes the use of steel in any great quantity for the construction of the lever, as the various movements adversely affect the compass readings.
The Wright System.
Another variant of the wheel control is instanced by the Wright system, this consisting of a general lay out similar to that shown by Fig. 101, but no rudder-bar is fitted. The rudder control is provided by a small lever, mounted concentric with the wheel, the latter carrying a rigidly attached sprocket. The hand-lever is also connected to a sprocket, this running free on the wheel shaft, so that by gripping both hand-lever and wheel it is possible to operate the ailerons and rudder simultaneously, this action being a characteristic feature of all the Wright productions. Although there are many types of control in use, those described in the foregoing chapter are illustrative of general practice.