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.
Fig. 100.—Arrangement of 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.
Fig. 101.—Arrangement of wheel control.