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Aviation History
1934
1934 - 1438.PDF
SUPPLEMENT TOPLIGHT •7666 m THE AIRCRAFT ENGINEER JULY 26, 1934 rolling of the vessel. The direct use of a gyroscope to pro- vide stability in an aircraft would, however, be quite im- possible on account of the prohibitive weight. The gyro- scope used for controlling an aircraft provides only an accurate datum from which deviations can be detected, and to which the craft is directed when a disturbance has occurred. Since the gyroscope itself does not provide stability, the aerodynamic characteristics of the aircraft must receive some consideration. It must be remembered that while the degree of stability of an aircraft in pitch and in roll is largely under the control of the designer by suitable dis- position of the wings and tail surfaces in relation to the centre of gravity, it is not possible to design an aeroplane to have inherent directional stability.- In other words, no aeroplane could be designed to maintain its flight upon a predetermined course without some form of control. The first essential feature of any automatic control is, therefore, that it shall be capable of directing the aircraft along a predetermined course, and it is only essential to control the pitching motion of the craft if it is unstable in pitch, or if a rapid damping of a pitch oscillation is re- quired. In practice, however, a single gyroscope is capable of providing control about two axes, and it is usual to con- trol the aircraft both in azimuth (i.e., directionally) and also in pitch. Stability in roll is very largely determined by the dihedral angle, and control of the ailerons is not usually necessary except for certain special purposes. In this paper we are only concerned with the control of rudder and elevator movements. The Importance of Stability Since the gyroscope is only the datum, to which the aircraft is controlled, it is of importance to consider the effect of applying corrections when a deviation is detected. Considering the rudder movements, for example, it would be feasible to apply a correction of fixed amount as soon as an error in course is detected, and to reverse the direc- tion of the correction as soon as a reversal in the sign of the error is indicated. Such a control would, however, cause the rudder to move in a "bang-bang" fashion, and as a result the motion of the aircraft would be continually disturbed. A great improvement is effected by arranging that the amount of rudder angle applied varies directly with the displacement of the aircraft from the datum line. The motion resulting from this form of control will generally consist of a complex oscillation, the damping of which is of considerable importance. Negative damping is, in fact, by no means impossible with such a control. By negative damping we mean that the amplitude of the oscillations would continue to increase instead of dying away. Disaster would obviously be the consequence of -iiv, uninterrupted negatively damped oscillation. While a full explanation of the reasons for the setting up of an oscilla- tion would require mathematical treatment and would be out of place in these pages, a brief explanation is required. It must be remembered that a movement of any one of the control surfaces not only exerts a primary effect about its own axis, but, in addition, it usually has important secondary effects about the other axes. "Aileron drag" is, perhaps, the most familiar example of this secondary effect, when the depression of an aileron so increases the drag on that side of the aircraft that the machine yaws to one side in consequence, in addition to the primary rolling movement. Similarly, a movement of the rudder not only causes a yawing movement, but also introduces roll and sideslip. It is these secondary effects which com- plicate the damping of an oscillation. Fortunately, there are two methods by which satisfac- tory damping of the disturbed motion may be produced. The first method -was originated by the Sperry Gyroscope Company, and consists in applying a movement of the ailerons in proportion to the roll displacement. This, however, involves the use of a second gyroscope, and is therefore a serious complication. The second method, which is due to Mr. F. W. Meredith, consists in applying rudder angle in proportion to the roll displacement. The total application of rudder angle is therefore the sum of two amounts, one being determined by the angle through which the aeroplane has turned off its course, and the other being determined by the angle to which the aeroplane is banked. It was predicted by mathematic investigation, and con- firmed by practical experiment, that the application of rudder angle when the aircraft is banked tends to increase the lateral stability of the craft in much the same way as if the ailerons were used to correct the bank. The application of rudder angle in response to a roll dis- placement is accomplished in a very ingenious but simple manner by tilting the axis of the gyre- rotor upwards in the forward direction. As will be seen later, from the detailed description of the gear, the effect of inclining the forward end of the gyro axis upv/ards is to cause a relative move- ment between the gyro axis and the aircraft when the machine banks, and this relative movement causes the application of rudder angle, even though the aircraft has not rotated about the azimuth axis. By this means, ade- quate damping of the disturbed motion is achieved and lateral stability assured without the added weight and complication of a second gyroscope, which, for all ordinary purposes, is quite unnecessary. • • . . The Automatic Pilot Having thus briefly outlined the essential features of an automatic control and the aerodynamic requirements to be satisfied, we may proceed to a more detailed descrip- tion of the Automatic Pilot, familiarly referred to as " George," which has been developed at the Royal Aircraft Establishment, and the patent rights of which are now controlled by Smith's Aircraft Instruments throughout the world. Unlike some other systems, the Automatic Pilot de- scribed below depends on pneumatic means for its opera- tion. The use of compressed air has so many advantages FIG. 1.—Air Compressors, showing Interior Details. over electrical operation, particularly in that it results in an exact and almost instant response of the -control sur- faces, that it is most improbable that it will be super- seded. So far as the actual gyroscope is concerned, there is little to chopse between electrical or pneumatic opera- tion. It is in the means whereby a relative movement between the aircraft and the gyrosocope is translated into a movement of the control surfaces that the pneumatic system demonstrates its superiority. In an electric system a relative movement between the gyroscope and the air- craft has to complete a circuit which brings some form, of reversing motor into operation. No matter what form this motor may take, it must possess considerable inertia, and the resulting lag of the control surfaces seriously affects the damping of an oscillation. The small amount of power available from an electric motor without reduction gear-
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