THE automatic pilot has taken its place as an important factor in both civil and military aircraft operation. Its value is based upon its recognized ability to relieve the human pilot of the physical effort of continuous flying and to hold the airplane on a much steadier and truer course. The automatic pilot, or gyropilot, as it is often called, provides complete automatic control for lateral, longitudinal and directional motions of the aircraft. Compact, light in weight, an integral part of the instrument panel, it provides the human pilot with a continuous, running picture of all the airplane's movements, whether it is automatically or manually controlled. The gyropilot is designed and built to give dependable service over a period of years, but as is the case with any fine mechanism, the best results are dependent upon understanding, intelligent use, and proper maintenance of the equipment. "Elmer the Gyropilot," or "George," as it is known in the RAF, does not require too much attention.
The autopilot is essentially composed of three integral units: the frame or mount, the turn gyro control and the bank-climb gyro control. Visual indication of the attitude of the airplane as a reference for manual or automatic flight control is provided by the gyroscopic flight instruments which form an integral part of the automatic pilot. Other instruments in the flight group, such as the compass, bank-and-turn, rate of-climb and airspeed indicators on the instrument panel give the pilot supplementary information in this respect. However, the gyropilot can be provided with accessory means for automatic and manual remote control which will govern the course and/or attitude of the plane.
The turn gyro control unit provides the directional reference for both manual and automatic flight control. A directional gyro with its card and lubber line identical to that contained in the flight instrument of that name is incorporated in the control unit. The caging mechanism provides a means of locking the gyro so as to prevent damage to the instrument during maneuvers that might exceed its operating limits. This control unit has two circular cards graduated in 360° of azimuth; the lower, or gyro card may be set to any desired heading by pushing in and turning the caging knob; the upper, or rudder follow-up card, which may be set by turning the rudder knob, is attached directly to the air pick-offs which are in a neutral position when the readings of the two cards coincide. It is necessary that they do coincide before the gyropilot is engaged in order to have automatic control to a selected heading.
The bank-climb gyro control unit contains the gyro which supplies the lateral and longitudinal reference for both manual and automatic control of ailerons and elevators. This unit is also equipped with a caging device for locking the gyro. The artificial or gyro-horizon dial is attached to the gimbal ring of the gyro and, due to the gyroscopic characteristic of rigidity, provides a horizontal reference as the airplane banks. The bank scale indicates the degree of bank with reference to the bank index above it. The horizon bar in front of the dial is actuated through linkage by a pin in the side of the gyro case, so that the bar rises as the plane noses down and descends as it noses up, remaining horizontal as it banks. The position of the miniature plane in relation to the horizon bar thus affords the pilot with a visual indication of flight attitude. In order to compensate for load conditions, the miniature plane can be raised or lowered with respect to the horizon bar by adjusting the knob beneath the dial.
The elevator and aileron follow-up indices provide a visual indication of the air pick-off setting for these controls. By means of the aileron knob, the pick-offs for lateral control may be set for the desired lateral attitude. The same holds true for longitudinal control and attitude by the use of the elevator knob. The controls are neutralized by matching up the aileron and elevator indices with the bank index and elevator alignment index respectively. For level flight, or preparatory to engaging the automatic pilot, the indices for each control must be matched.
The gyro control frame or mount assembly consists of a shock-mounted frame to which air relays, balanced oil valves and follow-up pulleys are attached. It is also the support for the control units which slide into the mount on tracks. All mechanical and air connections in the rear of these control units are established at the time they are bolted into place. The follow-up pulleys, to which cables are attached, are provided with clutch discs which carry their motion to the control unit. Inside these pulleys are springs which keep the cable taut during the follow-up action that returns the air pick-offs to a neutral position. The pressure and drain manifold on the bottom of the mount is piped to the three balanced oil valves. This serves to distribute pressure oil to, and collect drained oil from the valves. The air intake connections for the two control units are connected to the suction manifold, permitting the entire system to be connected to an air filter. A dial giving readings in inches of mercury indicates the amount of suction in the system.
Two types of mount assemblies are now in service. The most recent and probably most efficient type incorporates a variable air relay bleed operated by thumb discs which eliminate the need for speed control valves. These valves served to regulate the rate of oil flow from each hydraulic surface control and thereby the rate of response of the mechanism in general to various settings. The use of the variable bleed dispensed with a fourth unit called the servo speed unit, which contained the speed control valves. The hydraulic surface control mentioned above consists of pistons and cylinders to operate each of the main controls of the airplane by an amount determined by the automatic pilot system. They are connected to the balanced oil valves on the mount assembly by tubing and by cable to the follow-up pulleys. An oil pressure regulator, oil filter, a return manifold and sometimes a drain trap are included in the hydraulic system. An air filter is also required on the intake line to insure a clean airflow to the gyros and air pick-off system.
After the engines have been started and the ship is balanced oil valves from the servo unit. This is done warming up, check the vacuum; it should be anywhere by use of a variable bleed disc which is turned in the from 3½ to 5 inches Hg. Then uncage gyros after setting the turn gyro, and center controls. Set rudder, aileron and elevator follow-up indices so they coincide and then open sensitivity controls to a setting of 3. Engage the automatic pilot and check operation by turning each control knob. However, maximum sensitivity setting of the variable air relay bleeds in the most recent type unit may result in rapid oscillation of a control. If this occurs, reduce sensitivity until oscillation ceases. Next, check the oil pressure (ten pounds recommended) and overpower valves. The latter is done by operating controls manually against the gyropilot. If when overpowering, the controls are moved too far from automatic position, they will not return automatically when released. Manual movement back toward the position from which they were overpowered will bring them within range of automatic control, which will then bring them back to position. After taking off, trim the ship hands off. Set rudder, aileron and elevator follow-ups to match turn gyro card, bank index and alignment index respectively. Do not align elevator follow-up index with horizon bar as relative movement between alignment index and horizon bar is in opposite directions. Turn on the automatic pilot by slowly moving the lever into that position. Adjust sensitivity dials for desired control response and set your course by turning the control knobs as desired.
Some operating cautions are well worth repeating at this point. First, turning any sensitivity control (variable bleed disc) to 0 turns off the automatic control of that surface and locks the hydraulic surface control in whatever position it happens to be. Very low sensitivities are not recommended for flight in rough air since control response may be too slow for proper recovery from disturbances. Second, banked turns may be made by setting in bank with the aileron knob, and then by continuous rotation of the rudder knob, applying enough rudder to keep the ball-bank indicator centered. However, a convenient alternate method of making turns is to cage the turn gyro, and immediately add the amount of bank necessary for the turn. In using this method no indication of the amount of turn is given. To resume straight and level flight, level the ship by rotating the aileron knob and then uncage the turn gyro. This method may also be used in making spirals, both ascending and descending, the ship being nosed up or down by the use of the elevator knob.
When it is desired to resume manual control, move the engaging lever to the OFF position and take over the controls. As an added safety measure, hydraulic surface control relief valves are provided so the human pilot can overpower the automatic pilot by applying about twice the normal force on the controls. One fact that is of utmost importance, however, is that instruments should be uncaged at all times except during aerobatics or when the engines are not running.
To the mechanic charged with the responsibility of maintenance and overhaul of the automatic pilot, the breakdown will appear somewhat different. In his eyes, the gyropilot is composed of three basic and closely related systems: the vacuum system, which operates the gyro control units, in turn regulating the hydraulic pressure system by a series of valves; the servo units, which change the pressure into mechanical energy and transmit this energy to the control cables; the cable system, which restrains the servo units, preventing over-control. Except for the way in which servo unit control speed is adjusted, both types of mount units are basically identical. In one type the speed control valves are in the hydraulic system and regulate the rate at which oil returns from the servo units. The more recent type is controlled by regulating the airflow in the vacuum system through the diaphragm portion of the air relay valve, which in turn is connected to the balanced oil valves from the servo unit. This is done by use of a variable bleed disc which is turned in the opposite direction of the speed control valve for time control. In the past, the procedure used for bleeding both types of servo units has been the same: experiments and tests, however, have proven that the following method, which differs slightly, gives better results when bleeding a gyropilot equipped with variable bleed discs.
With the engines running at approximately 1000 rpm, turn on the oil pressure valve of the auto-pilot. Check the vacuum gage on the bank and climb control unit it should read 4" Hg. plus or minus ¼". Center all controls and align follow-up indices, then check hydraulic pressure gage for proper oil pressure. After placing all sensitivity controls at a minimum setting of 3 on the dials, turn servo controls to ON position. Using control knobs, move each control to an extreme position. With the controls in hand-over position, continue turning the trim control knob until indices part approximately 10° for thirty seconds. Repeat this procedure in the opposite hand-over positions. This should completely bleed the auto-pilot system of air. To complete the test for either or both types, engage the gyro-pilot and kill the engines. The controls should feel as if they were locked, and not resilient, springy or spongy.
The hydraulic surface control is generally constructed in such a manner that it is seldom that repairs other than replacement of the packing or gaskets are necessary. However, each part of the unit should be inspected carefully for corrosion, mishandling or excessive wear. After such inspection, the unit should be thoroughly tested for freedom of operation, overpower and leaks. The procedures involved are simple and very easily followed. During operational inspection, each piston must be free to move throughout its full stroke with the cylinders full of oil, with a maximum push or pull of fifty pounds. The by-pass valve must rotate its full throw in both directions with a maximum torque of five inch pounds. In the overpower valve test, one end should be open to the atmosphere and the opposite end under oil pressures over a range from 50 to 150 pounds per square inch in both directions. Final setting of the valve must be at 110 pounds per square inch within a plus or minus five pounds. The leak test is made while subjecting the unit to 200 pounds per square inch pressure. A total of not more than four drops per minute is permissible from the six piston rod glands. No one rod-end gland may leak more than one drop per minute. Internal leakage around the piston must not exceed twenty-five cc per minute.
The first gyroscope ever used in an airplane was made and installed in 1909. In 1914 a gyro-stabilizer won the Grand Prix at Paris for safety in flight. During World War I, research and development gave way to accelerated production, and it was not until 1920 that any further step in design took place. In succeeding years, a considerable reduction in size and weight took place and in 1926 a special type of automatic control was developed for application in the first Ford trimotor transport planes. The weight of this equipment was amazingly light for those days, being only 160 pounds. By 1930 a further weight reduction had taken place and the equipment was condensed to 120 pounds. The greatest advance took place, however, during the period between 1930 and 1933. The directional gyro and the artificial horizon were incorporated into the unit and the present automatic pilot was born. Such a set of equipment, the predecessor of the modern gyro control, was installed in the Winnie Mae used by the late Wiley Post on his solo flight around the world. Development since that date has merely been a refinement of the type fitted to that plane. By constant refinements and improvements in design through the years, the weight of the complete equipment has been reduced until it is now only twenty-four pounds.
A. Gyro element
B. Suction Pump
F. Air relay
J. Balanced oil valve
L. Servo unit
M. Piston rod
N. Pressure regulator
P. Overpower valve
Q. Follow-up cable
T. Suction regulator
U. Vacuum gage
W. Oil pressure gage
X. Drain trap
Y. Variable bleed disc
Z. By-pass valve
This article was originally published in the June, 1944, issue of Air Tech magazine, vol 4, no 6, pp 58-61.
Photos and diagrams credit Jack & Heinz, Co.