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Aviation History
1935
1935 - 0683.PDF
MARCH 28, 1935 17 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT the definition of the vertical is, achieved, it is a simple matter to utilise such a mechanism for con trolling the ailerons of an air craft. It is necessary no.w, how ever, to describe the manner in which the gyroscopic mechan ism described above succeeds in achieving an accurate defini tion of the vertical. In order to visualise the man ner in which the gyroscope maintains a constant definition of the vertical, it is perhaps easiest to consider the sequence of events which occurs if, for any reason, the weight W is dis placed from the vertical Let it be supposed that the aircraft is flying on a straight and steady course, and that, in some manner, the weight W has been displaced from the vertical. Remembering that the axis of the gyroscope is situated athwartship of the aircraft and that the pivots which support the cuter gimbal ring are placed parallel to the fore-and-aft axis of the aircraft, it will be clearly- seen that the displaced weight W will exert a gravitational torque on the outer gimbal ring. The effect of this torque about the longitudinal axis is to initiate a precession of the gyro scope and the inner gimbal ring about the vertical axis of the inner gimbal ring, thus causing the gyroscope and the inner ring to depart from the normal position orthogonal to the outer gimbal ring. However, precession of the inner gimbal through a very small angle from its normal position at right angles to the outer ring brings the detector valve C into operation and permits the admis sion of compressed air to one side or the other of the pre cessing cylinder D, according to the direction in which the weight W is displaced. The admission of compressed air to the precessing cylinder causes a torque reaction to be applied about the axis of the outer gimbal ring in such a sense as to oppose the gravitational torque due to the original displacement of the weight W. In addition to the operation of the detector valve which results from the precession of the inner gimbal ring from its neutral position, the spring mechanism E is brought into operation by the relative displacement of the inner and outer gimbal rings, and exerts a small torque about the vertical axis of the inner ring in such a sense as to tend to restore the inner rings to its orthogonal position. The effect of this small restoring torque is not, as might at first sight be supposed, to return the inner ring to its normal position at right angles to the outer ring, but to cause a small precession of the outer gimbal ring about its horizontal axis in such a direction as to return the weight W towards the vertical. It will thus be seen that if the weight W is displaced from the vertical, it will gradually subside to the vertical m the manner described, and it therefore follows that the weight will not of itself tend to depart from the vertical. U7 • WaS stated m tne above description that if the weight w is displaced from the vertical, the gravitational torque Fig. 2. The complete aileron unit, showing gyroscope assembly which it exerts on the outer gimbal ring causes an azimuth precession of the inner gimbal ring, thus causing the inner gimbal ring to depart from its normal position orthogonal to the outer ring. As was explained, the departure of the inner ring from the orthogonal position brings the detector valve, the precessing cylinder and the spring E into opera tion, but it is not self-evident how the inner ring is itself returned to the orthogonal position as and when the weight W again lies in the vertical. This may be briefly explained as follows: The gravitational torque on the outer ring caused by a displacement of the weight W is not a constant torque, but varies with the sine of the angle of displacement. The counteracting torque of the precessing cylinder, however, is independent of the displacement angle, and at small angular displacements the torque of the precessing cylinder considerably exceeds the gravitational torque due to the weight. The excess torque about the horizontal axis of the outer gimbal ring results in a precession of the inner gimbal ring about its vertical axis, and the inner ring is thus gradually returned to its normal orthogonal position. When the two rings are again orthogonal the detector valve is closed and the precessing cylinder no longer operates. Thus we see that in straight flight, at any rate, the gyroscopic system as represented by the gyroscope, the gimbal rings and the erecting gear illustrated in Fig. 1 does, in fact, always tend to erect towards the vertical. It remains to consider the behaviour of the system when ifi-
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