The Gyro-Horizon Indicator affords the nearest approach to the natural horizon an artificial horizon within the cockpit of the airplane. By means of a miniature airplane and a gyro-actuated horizon bar, it shows the pilot what he would see if he had good visibility outside the airplane, that is, whether the airplane is banking, climbing, gliding or flying level. Gyro- Horizons and Directional Gyros are used in the transport planes of all commercial airlines and are standard equipment in military and naval aircraft. This article covers operation and maintenance aspects of the Gyro-Horizon for instrument technicians. A similar article on the Directional Gyro will appear in June Air Tech and the July issue will carry a comprehensive article on the Sperry Automatic Pilot which comprises both these instruments.
Probably no single illusion has caused airmen as much woe as the idea that they can "feel" their position in the air relative to the horizon. In conditions of normal visibility, the pilot can line his ship up with the horizon. But when haze, fog or darkness obscures this natural reference level, the pilot may be forced to depend on his physical sensations to determine the attitude of the airplane relative to the earth.
These are drawn from his inner ear which, in practice, acts as a minute form of liquid level and from his "deep muscle sense" or the feel of his own weight. Centrifugal force, shifts in posture and turbulence, make this method of "flying by the seat of one's pants" unreliable from the safety point of view.
An accurate method for determining the ship's attitude has been devised in the form of the Sperry Artificial Horizon, a gyroscopic instrument which gives the pilot his airplane's attitude in reference to the earth's surface with an accuracy exceeding the pilot's natural senses. The axiom for instrument pilots, drawn from years of experience, is that the artificial horizon has neither sense nor feeling and therefore makes no mistakes. If there is a choice between trusting the instrument or one's senses, a wise flier trusts the gyro.
The modifying condition, of course, is that the instrument be in proper operating condition. The faith that has been built up by pilots in this instrument in over a decade of operation on airlines as well as in the military field, puts a heavier-than-normal responsibility on the maintenance crew for the proper operating condition of the artificial horizon.
The motivating element in the artificial horizon, the directional gyro, the turn and bank indicator and the automatic pilot is the gyroscope. Almost everyone is familiar with the gyroscopic top, a spinning wheel so mounted or suspended that it is free to rotate on any axis.
Once set in motion, the gyroscope resists any force tending to alter the plane in which it revolves. This resistance, known as rigidity, depends on the gyro's speed and weight. If a force exerted on a gyro is great enough to overcome the gyro's resistance, the gyro will turn at right angles to the outside force until the direction of rotation coincides with the direction of the outside force. This action is called precession, and is the principal motivating force in many of the gyroscopic instruments.
The gyro-horizon is one of the few three-dimensional instruments on the modern airplane panel. By means of a fixed miniature airplane and a gyro-actuated horizon bar it shows the pilot what he would see with respect to the horizon if he had good visibility outside; whether his plane is banking, climbing, gliding or flying level. The structure and operation of the instrument is best explained by the use of the cut-away and sectional views of the instrument.
The instrument is operated by a vacuum of four inches of mercury, supplied by means of a venturi tube or by an engine-driven vacuum pump. The gyro rotor or spinning element is mounted in the gyro housing (see cutaway at top of color page), and spins at about 12,000 rpm around the axis Z in the direction indicated by the arrow on the rotor housing. Obeying the gyroscopic principle of rigidity, the gyro maintains its spinning axis upright irrespective of the movement of the airplane, and thus establishes a means of obtaining a horizontal flight reference. Any movement about the Y axis (bank) or the X axis (climb or glide) is shown on the face of the instrument by the horizon bar (2). This is carried on an arm pivoted at the rear of the gimbal ring and controlled by the gyro through the guide pin (3), (see sectional view at bottom of color page) which protrudes from the gyro housing through the gimbal ring (4).
By means of a caging knob located at the lower right hand side of the instrument face, the gyro may be caged and secured in its normal operating position during maneuvers or aerobatics which would exceed its operating limits of 70° climb or glide and 100° right or left bank.
Any tendency of the gyro to depart from its upright position, which might be caused by bearing friction or other disturbance, is corrected by the pendulum body assembly shown in the sketches below. Four pendulum plates, one of which is shown at A, are suspended from the four sides of the pendulum body. Each one of these plates partially covers one of four air ports that exhausts air from the gyro housing.
The four plates are balanced in diametrically opposite pairs by their balancing nuts so that the gyro attains its normal operating position when the plates bisect the air ports. If the gyro housing departs from its upright position as shown at left, gravity holds the plates vertical and one plate closes its port while the plate diametrically opposite opens its port. The reaction of the air from this open port moves the gyro in the direction C back to its normal position, left. This corrective movement, which is at right angles to the air force, is a manifestation of precession, the second basic characteristic of the active gyroscope
During a turn, centrifugal force acts upon the pendulum plates, tending to displace the gyro in such a way that the horizon bar would tilt slightly and go down. This tendency is over come by erecting the gyro so that its spinning axis, Z, is normally inclined from the vertical axis 2½° the amount necessary to compensate for the error which would result from a standard 180° per minute turn.
The gyro-horizon, like most other gyro instruments, if properly installed should require little attention between 300 to 400 hour overhaul periods other than an occasional cleaning out of air screens or replacement of filters Even a cursory examination of the instrument's cutaway views will reveal its complexity. The general direction for all field repairs send it back and let the experts work on it applies to gyro instruments probably more than any other instrument or the panel.
However, certain troubles whose correction does not require disassembling the instrument, can be handled by the average competent mechanic, If the horizon bar fails to respond, check the caging knob and make sure the instrument is uncaged. Next, check to see if the air filter is clean, Clean it or replace it if necessary. Check the vacuum lines with a suction gage. If the indication is below 3.5 in of mercury, go over the following items: See that the suction regulator is properly adjusted, check or adjust gauge calibration; see if the pump or venturi is in operating order and finally, check the vacuum line for kinks or similar obstructions. If none of these work, it's a job for the depot.
Insufficient vacuum may also keep the bar from settling, as may excessive vibration. For the latter, test the installation with a vibrometer. If the amplitude is more than .004 in, examine shock mountings and note whether connections are pulling on the instrument. If neither remedy works send it back to the depot!
If the bar oscillates or shimmies, the gyro may not be completely uncaged. Check the caging knob. If that is not the trouble, check for excessive vacuum. If the lines show a vacuum in excess of 5 in Hg, check the filter for cleanliness and the suction regulator for proper adjustment. Also, as a last resort before sending the instrument in for defective mechanism repairs, check it for excessive vibration.
Replacement of air filters or the cleaning of an air screen is not considered a major disassembly of the instruments, and may usually be accomplished without removing the instrument. Frequency of replacement depends somewhat on the service conditions. Fifty hours is the average service replacement period for air filters.
Air filter replacements are simple. They are replaced by removing the four fillister-head machine screws, and lifting off the filter body cover (8). Remove the snap-ring which holds the filter in place and replace the filter with a new one. Replace the snap-ring and filter body cover. Make certain that when you make this change, the instrument is protected from dirt or dust entering the opening while the filter body is off.
The shafts, pivots and bearings of the instrument are lubricated before assembly in the case and no further lubrication should be necessary between overhaul periods. It should be borne in mind, however, that if the instrument is used in hot climates continuously, the oil will evaporate at a faster than normal rate, and periods between overhaul may be shortened.
This article was originally published in the May, 1943, issue of Air Tech magazine, vol 2, no 5, pp 54-58.
The original article includes, besides the diagrams shown above, 7 photos, a cutaway drawing and a schematic diagram.
Photos and diagrams are not credited.
An interim PDF of this article[ PDF, 9.3 MiB ] is available.