122   THE GYROSCOPIC PENDULUM

together with the periods of the oscillations. It is a pendulous gyroscope* mounted so as to be capable of rotation about two perpendicular intersecting axes that normally are horizontal. The degree of pendulousness is such that the instrument has high dynamic stability together with a period of vibration that is many times greater than the period of either the roll or the pitch of any ship. Hence the direction of the spin-axle is practically unaffected by the oscillations of the ship. The vibrations of the instrument

are damped by the motion of a mass of liquid contained in an annular passage concentric with the spin-axis. The passage is constricted so that the flow of liquid back and forth will be sufficiently out of phase with the oscillations of the gyro to produce the desired damping effect.

Figure 96 shows the instrument as mounted on a ship with the outer gimbal axis athwartship and the inner one in the fore-andaft direction. The pen nearest the divided scale records the angle of roll on a strip of paper that is moved parallel to the keel of the ship at a constant rate by means of a clock mounted on the bed

* U. S. Patent. Snerrv. No. 1399032. 102

GYRO-HORIZONTALS AND GYRO-VERTICALS   123

plate. The pen to the left records angles of pitch and the pen to the right indicates time intervals.

The roll-recording pen is held by a horizontal arm fastened to a vertical counterbalanced circular loop capable of rotation about pivots in line with the fore-and-aft gimbal axis of the instrument. The concave side of the upper part of this loop is deeply grooved. Into this groove projects a rod fastened to the top of the gyro-case and in line with the spin-axis. When the ship rolls, the spin-axle and the loop remain in the vertical plane while the paper carrier moves athwartship under the pen. If at the same time the ship pitches, the groove in the loop permits motion of the loop relative to the paper carrier without affecting the movement of the paper relative to the roll 'recording pen.

The pitch-recording pen is supported by a parallel motion device that permits motion of the pen only in the athwartship direction. It carries two horizontal guides that for part of their length are inclined at about 45 degrees to the fore-and-aft axis of the ship. In the space between the two guides is the upper end of a vertical rod, the lower end of which is fastened to the outer gimbal ring. When the ship pitches, the parallel motion device moves relative to the gyro spin-axle along the fore-and-aft axis of the ship. In so doing, the guides are pushed in the athwartship direction by the rod extending upward from the outer gimbal ring. Thus the attached pen traces a curve that indicates the angle of pitch with respect to time.

78. The Sperry Automatic Airplane Pilot. - This apparatus maintains an airplane on any predetermined straight course as long as desired. It is especially useful in " blind flying " through opaque clouds. The aviator may leave the cockpit for any purpose, may even walk out to the end of a wing, with confidence that the machine will continue in its course on a horizontal keel. If he were to faint for a brief period the automatic pilot would keep the machine in a straight course on a horizontal keel. He can disconnect the automatic steering device instantly whenever he decides to take control.

The automatic pilot comprises two universally mounted gyros, one with spin-axle vertical and the other with spin-axle horizontal. The first keeps the airplane horizontal by controlling the position of horizontal rudders. The second maintains the course of the airplane in azimuth by controlling the position of vertical rudders. The power required to spin the gyros is supplied by the