Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 30. Automatic Pilot, Type A-2

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XXX - AUTOMATIC PILOT, TYPE A-2

141. Purpose and use.-a. The purpose of the automatic pilot is to provide a mechanical means for automatically controlling the flight attitude of an airplane. By its use the human pilot can be relieved of strain and resultant fatigue when manually flying an airplane, particularly on flights of long duration. It also allows the attention of the human pilot to be devoted to navigation and tactical problems, engine operation, or other important flight factors.

b. In addition to maintaining mechanical control, the automatic pilot furnishes visual indications of the attitude of the airplane in yaw, pitch, and bank similar to the indications of the flight indicator and the turn indicator.

e. While the automatic pilot is used normally to maintain straight and level flight, it may also be used for all of the normal maneuvers of flight such as climb, descent, flat turns, spirals, etc., with greater precision than is possible by a human pilot.

142. Description.-a. The automatic pilot consists of a combination of gyroscopes, pneumatic and hydraulic actuated mechanisms which, when their functions are coordinated, simulate the operation of the human brain, nerves, and muscles. The mounting unit consists of a frame to which the air relays, balanced oil valves, follow-up pulleys, exhaust and drain manifolds, vacuum lines, and electrical contacts are attached as shown in figure 86. The two control boxes, directional gyro unit, and bank and climb gyro units (fig. 87) are carried by the mounting unit. The control units slide into place on tracks and are easily secured by inserting four screws. They are interchangeable with other like units, but their construction makes it impossible to place them on the wrong track or to interchange them with each other. Twelve-volt lamps are used for illuminating the dials of the directional and bank and climb gyro controls. Access to these lamps may be had through a small door on the front of each control unit. Two spare bulbs are carried behind the door on the bank and climb gyro control for use as replacements in case one of the lamps burns out. When the control units are placed on the tracks and secured, all the necessary connections are made automat ically by contact. at the rear of each unit. and the front of the mounting unit.

FIGURE 86.- Automatic pilot mounting unit and attachments.

b. The directional gyro control (rudder) (figs. 87 and 88) contains the directional gyro mechanism, rudder knob, follow-up card, directional gyro card, ball bank indicator, and caging and setting knob. This unit slides into its respective track on the left side of the mounting unit and is secured by two screws. The bank and climb gyro control unit (figs. 87 and 89) contains one rotor which maintains the lateral and longitudinal attitude indication and control. The unit also contains the air pick-offs, follow-up systems, caging knob, control knobs, a vacuum gage, and level flight control. It slides into its position on the right-hand side of the mounting unit and is also secured by two screws.

FIGURE 87.- Automatic pilot gyro control units.

FIGURE 88.-Cross section of a directional control unit.

FIGURE 89.-Cross section of a bank and climb unit.

c. Figure 90 shows the component parts of the complete automatic pilot, and their relation to each other and to the control surfaces of the airplane. A brief description of each unit is as follows:

(1) The on-off valve Z located in the hydraulic surface control is operated by the engaging lever, which is located on the control column in the pilot's cockpit. To engage or disengage the automatic pilot from the control surfaces of the airplane, the lever is set to the on or off position, respectively.

(2) The oil sump 0 is an oil reservoir for the hydraulic system and has a capacity of about 1 1/2 gallons.

(3) The engine-driven oil pump J provides the necessary pressure and flow of oil for operating the automatic pilot. Pump bases are provided to fit various types of standard engines. One-half horse-power is required for the pump. Its capacity is 3 gallons of oil per minute.

(4) The oil pressure regulator N is a spring-loaded ball type regulating valve connected on the pressure side of the pump in a position to permit easy access to its adjusting screw and lock nut.. When properly set after ground tests, further adjustments will seldom be needed. On equipment in which the hydraulic system is interconnected with other hydraulic applications, such as landing gear and flaps, no sump is supplied. In this case a special pressure relief valve is used.

(5) The oil pressure gage W indicates the pressure at which the oil is being supplied to the automatic pilot. This is a conventional Bourdon tube pressure gage with a range of 0 to 300 pounds per square inch.

(6) The bypass valve VV allows the output of the hydraulic pump to be applied to the main system or merely to circulate around the pump.

(7) Speed control valves Y control the speed of the action of the hydraulic surface controls by regulating the rate of flow of the oil which actuates them. There is one speed control valve for each surface control.

(8) The oil relays H are five port valves each provided with a four-step piston. One passage of the valve provides a channel for the passage of oil under pressure from the speed control valve. Two of the ports lead to the servo unit. One of the ports provides a means for drainage and the other for exhaust. The piston is balanced by means of a spring located at one end. Means are provided for a very sensitive adjustment of the spring tension.

(9) The hydraulic surface controls K, which make up the servo unit, are mounted permanently and connected in or to the airplane control system. Each unit consists of three hydraulic cylinders, one for each surface control. The speed at which the pistons move may be regulated by the rate of flow of oil from the oil relay to the servo cylinder. Each control is made up of a piston within a cylinder. The piston is attached to the. center of a long piston rod which extends outside of each end of the cylinder and to each end of which is attached the cable controlling the movement of a control surface using turnbuckles L and L'. Oil pressure acting on one side or the other of the piston under the direction of the balanced oil valves in the mount assembly controls the movement of the surface control cables. Integral with each cylinder is a relief valve which is normally set for a pressure of about 25 pounds per square inch more than the operating pressure. In an emergency, if for any reason it is impossible to disengage the automatic pilot from the airplane controls, the pilot can, by exerting excessive pressure on the stick or rudder, overpower the automatic pilot and take over control of the airplane manually.

(10) The oil drain trap X is connected between the drain manifold on the mount assembly and the suction line to the oil pump. On an alternate installation which omits the drain manifold, the oil drain trap is connected between the oil drain outlets on the balanced oil valves of the mount assembly and the suction line to the oil pump.

(11) The main oil filter V is located in the reservoir.

(12) The auxiliary oil filter ZZ is connected in the main oil pressure line between the pump and the speed control valves. The filter element can be withdrawn for cleaning without the necessity of disconnecting any piping or fittings.

(13) The drain manifold I is connected between the drain outlets of the balanced oil valves on the mount assembly and the oil drain trap. It is secured to the base of the mount assembly with two screws. On an alternate installation, the drain manifold may be omitted. In this case, the drain outlets are connected directly to the oil drain trap.

(14) The exhaust manifold XX is connected between the exhaust outlets of the balanced oil valves on the mount assembly on the oil pump side. It is secured to the base of the mount assembly with two screws.

(15) The vacuum pump B is engine-driven. It furnishes the source of suction which operates vacuum-actuated units of the control system.

(16) The suction or vacuum gage U is an integral part of the bank and climb gyro control unit and shows the amount of vacuum in the control system. It has a graduated range of 0 to 10 inches Hg.

(17) The suction regulator T is connected in the air line between the vacuum pump and the mounting unit.

(18) Each air pick-off system A and A' consists of the air nozzle and the air nozzle plate. The nozzle plate is secured to and is part of the vertical ring. Inasmuch as the vertical ring is held in position due to the property of the gyroscope's rigidity, then likewise the air nozzle plate will remain rigidly in the same plane unless the entire mechanism is turned manually. The air nozzle is pivoted above the vertical ring but is not part of it. It is free to rotate about the same axis of the airplane in relation to the control surface associated with it. The air nozzle has a small slot approximately 1/2 inch long and 1/64 inch wide and is supported above the air nozzle plate, the distance between the two surfaces being 0.000017 inch.

With such a minute opening, the air passing between the air nozzle and the air nozzle plate will be extremely limited.

(19) The air relay C consists of two convex disks separated by a leather reinforced metallic diaphragm. The two convex portions are held together with screws allowing the leather diaphragm to be suspended between them. On the top of the air relay are two air screens and, at the bottom, the suction nipples which lead into the evacuated area of the control boxes. Figure 90 shows the three air relays and their location in the mounting frame. The leather diaphragm is very sensitive and a vacuum of approximately 0.4 inch of mercury will be sufficient, to cause the diaphragm to deflect from an equilibrium position to an extended position, moving with it the balanced oil valve.

FIGURE 90.-Complete automatic pilot in neutral position.

(20) The follow-up cables Q are connecting units for coordination between the air pick-off system and the hydraulic system. There is one cable for each control. The follow-up clutch R is a cork-lined clutch automatically making contact with the clutch plate when the control units are in the mount assembly and the follow-up clutch spring S is used to keep the cable tight and to take up slack when moved away from the winding.

143. Operation. a. The hydraulic oil and vacuum pumps, being engine-driven, start the operation of the automatic pilot immediately after the engine is started. Oil pressure is built up in the system and a partial vacuum is created in the areas as shown in figure 91. This figure shows the method of lateral control only, that is, of the ailerons.

b. The operation of the automatic pilot is dependent upon the fundamental principle of the gyroscope, rigidity. Because of this property, it is possible to establish a fixed reference in space. The gyro mechanism as used in the automatic pilot consists essentially of small air-driven rotors so mounted as to have 3° of freedom of rotation. The spinning axis of the rotor in the directional gyro control unit is always in a horizontal plane and in the bank and climb gyro control unit in a vertical plane. The rotor is pivoted in a gimbal ring, this inner ring is likewise pivoted in a vertical ring, and it in turn is pivoted in the case proper. Due to this type of rotor suspension, the rotor sets up a force which is transferred to the vertical ring, causing the entire assembly to remain rigidly in position regardless of any outside torque imposed upon it. The gyro then maintains a fixed position and allows all associated units to move about it.

c. When the air nozzle and air nozzle plate edge are exactly in balance (fig. 92 (1)) , each air port or opening in the air nozzle is open and cutting the edge of the air nozzle plate by the same amount. While this condition exists, the same amount of air will escape from each air nozzle port, A and A'. Since the air nozzle plate is part of the vertical ring and is immovable, then any change of attitude in flight, such as a cross wind in the case of the rudder control, would cause the airplane to leave its course. The result of the action would affect the air pick-off system and cause it to become unbalanced and appear as in figure 92 (2). This occurs because the spinning rotor retains its position in space holding the nozzle plates rigidly and allowing the air nozzle to rotate about it. This demonstrates that the pressure in lines 3 and 4 are not equal because the pressure in line 3 is restricted as the air nozzle A has assumed a position over the nozzle plate. Any difference of pressure at the nozzles immediately causes deflection of the diaphragm in the air relay. This movement of the air relay causes movement of the piston in the balanced oil valve as can be seen in figure 91. Any movement of the oil valve unbalances the pressure on the two ports leading to the servo cylinder. Unequaled pressure in opposite ends of the servo cylinder will cause the piston to move. The piston being a continuous part of the control cable will cause the control surface on the airplane to change attitude. This change in position of the airplane control surface will cause the airplane to resume straight and level flight.

FIGURE 91.-Diagram of automatic pilot showing lateral control only

FIGURE 92. Operation diagrams of air relay and air nozzle.

d. In controlling an airplane, it is not only necessary to apply control to bring the craft back to level flight when it has been disturbed, but also to begin to remove the applied control as the craft is returned to level so that the control surface will be back to neutral when the change of attitude has been fully corrected. Over control and the return to neutral is accomplished by the follow-up system. One end of the follow-up cable is attached to the servo piston and the other is wound around the follow-up pulley. Figure 93 shows the follow-up differential gear box, pulley clutch and a differential gear box and shows that any movement of the follow-up clutch will be taken up in the differential gear box and cause the air nozzle to turn above the air nozzle plate. The air pick-off system is then in an unbalanced condition and as the pressure changes in the control box it also changes in the air nozzle ports. However, in A the air flow is restricted. The restriction of the air is immediately taken up by the deflection of the diaphragm in the air relay and the air relay causes the balanced oil valve to move. This displacement causes application of pressure in the line leading to the servo piston. The servo piston and follow-up cable, being integral, move simultaneously. The follow-up cable winds on the pulley, which in turn turns the shaft in the differential gear box and rotates the air nozzle. When the air nozzle becomes neutral with the plate, the entire system is balanced and all control correction ceases.

FIGURE 93.-Diagram of air pick-off arrangement, follow-up pulley-clutch, and differential gear box

144. Installation.-All automatic pilot control units are tested before leaving the factory or the Air Corps repair depots and are ready for service when received. Initial installations of the automatic pilot are made at the airplane factories and repair depots. Removal or replacement of major units should be accomplished in the service by qualified instrument personnel only.

145. Maintenance.-a. The general points on maintenance given in section II are applicable to this instrument and its component units.

b. The following method of procedure and order of execution will be followed on each preflight inspection by the airplane crew chief :

(1) Check oil level in sump (should be from one-half to three-fourths full).

(2) Start engines.

(3) Check emergency oil bypass for "open." Normally this valve will always be open, but should always be checked and set to the full open position.

(4) Uncage gyros.

(5) Check level flight knob for "off."

(6) Check suction. This should not be less than 3 inches Hg. or more than 5 inches Hg., 4 inches being desired.

(7) Close speed control valves. Turn knobs on these valves clock-wise until the scale reads zero.

(8) Check oil pressure. As the pressure varies on different air-planes, the normal reading for this measurement must be obtained from the handbook on the airplane.

(9) Open speed control valves. Match cards on directional gyro and aline follow-up indices on bank and climb control.

(10) Engage pilot.

(11) Check for air in the hydraulic system. Press on each control in each direction. If instant resistance can be felt, that part of the hydraulic system is free of air. If the control moves easily and the resistance builds up as the control is pressed and if there is a sudden kick-back when the control is released, it is a positive indication that there is air in the system which must be removed before the airplane takes off, inasmuch as it is not possible to accomplish the removal of air after the airplane is in flight. To insure successful operation of the automatic pilot it is absolutely essential that the air be worked out of all three of the control systems in the following manner:

(a) Open speed control valves wide. Each numeral represents one turn of the knob or displacement of the valve seat 0.025 inch.

(b) Turn on oil and vacuum.

(c) Centralize all controls (approximately).

(d) Line up indices, rudder, aileron, and elevator.

(e) Engage pilot with engaging lever.

(f) Turn all control knobs hard over to extreme limits until indices remain separated.

(g) Disengage pilot and hold controls in this hard-over position for 15 seconds.

(h) Reengage pilot and repeat operations (f) and (g) in the opposite direction.

(i) Recheck for air in the hydraulic system.

(12) After it is certain that all air has been removed from the system, reengage the pilot and operate each control knob in a slight amount. In each case, check the movement of the rudder, the ailerons, and the elevator for the proper movement as their respective control knobs are turned.

c. All screens are checked for excessive accumulation of dirt and removed and cleaned at the required intervals.

d. Oil is added to the sump as required and whenever this operation is performed only "oil, lubricating, aircraft, hydraulic mechanism, specification 3580-for Sperry Automatic Pilot" is used.