Aircraft technical Basics: TM 1-406, Aircraft Electrical Systems, 1940: 4. Starting Systems

TM 1-406, TECHNICAL MANUAL,  AIRCRAFT ELECTRICAL SYSTEMS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON, October 18, 1940.

SECTION IV - STARTING SYSTEMS

30. General.-The majority of aircraft engines are equipped with starter systems to provide a means for starting the engines without resorting to hand cranking. The common types are –

Hand inertia starter.

Combination hand and electric inertia starter.

Direct cranking starter.

Cartridge starter.

Hand turning gear starter.

Air injection starter.

31. Inertia type starters.-a. The starting ability of an inertia starter depends upon the energy stored up in a rotating flywheel. There are two types of inertia starters: the hand type (fig. 68) in which the flywheel is accelerated by a hand crank; and the combination hand and electric type (fig. 69) in which the flywheel may be accelerated either by an electric motor or a hand crank.

FIGURE 68.-Typical hand inertia starter

FIGURE 69.-Typical combination hand and electric inertia starter.

b. A sectional view of a hand inertia starter is shown in figure 70, and its operation is described as follows :

When the hand crank is rotated, all moving parts within the starter are set into motion from the starter jaw to the flywheel. The small bevel gear on the end of the hand crankshaft drives its mating bevel gear, held in a bearing in the barrel extension. The barrel rotates on two cone bearings, and rotating with the barrel are the clutch assembly, screw shaft, jaw advance spring, and starter jaw. Attached to the barrel assembly are three planetary pinions which revolve with the barrel. These planetary pinions are in constant mesh with a fixed internal gear held stationary in the starter housing, and also with a sun pinion. Every revolution of the hand crankshaft turns the barrel assembly at an increased speed through the medium of the two bevel gears. The rotation of the barrel with the planetary gears around it provides a definite step-up ratio. Further increases in speed are accomplished through the sun internal gear meshing with the spur pinion of the spur bell gear. The final step-up in ratio is accomplished from the spur bell gear to the flywheel pinion on the flywheel shaft. The train of gears described steps up the speed of rotation from one revolution of the hand crank driveshaft to 100 revolutions of the flywheel. The energy thus stored in the flywheel is transmitted from the flywheel to the starter jaw in a reverse direction through a torque overload release, which consists of a multiple disk clutch under spring pressure. The torque is transmitted from the clutch to an internally threaded nut member, which operates a longitudinally movable screw shaft splined to the starter driving jaw. Most of the energy used in cranking the starter is stored in the rapidly revolving flywheel, which if left to rotate would continue to spin for several minutes. When the starter jaw is engaged to the engine jaw by means of a meshing rod, the stored energy of the rotating flywheel is sufficient to rotate the engine crankshaft approximately four revolutions under ordinary conditions. The multiple disk clutch previously mentioned prevents damage to the starter in case of engine kick-back or excessive overloads.

FIGURE 70.-Sectional view of hand inertia starter.

c. The combination hand and electric inertia starter has the same gear arrangement as the hand inertia starter, except that the flywheel may be accelerated to its required speed by either the hand crank or an electric motor. The electric motor used in connection with the combination inertia starters is series wound and expressly designed for high speed and high torque requirements. The electric motor is a separate unit, the housing of which is bolted to the flywheel end of the starter proper and is connected to the flywheel only during the period of flywheel acceleration, its engagement and disengagement being provided for by an automatic mechanism. When the starter is operated by hand, the electric motor unit is inoperative.

(1) The electric motor used in conjunction with these starters is similar in construction to the electric generator, except that it converts electrical energy into mechanical power, whereas, the generator converts mechanical power into electrical energy. The armature, fields, commutator, and brushes are practically the same as in the generator construction, the difference being in the size and number of conductors in the armature and the type of field winding employed. Direct cur-rent motors are classified either as series wound, shunt wound, or compound wound; however, those commonly used with aircraft engine starters are of the direct current, series wound type. Figure 71 illustrates the typical circuits of series wound motors of both the multiple and bipolar types.

FIGURE 71.-Typical series mound motor circuits.

(2) The operating principles of an armature in a four-pole electric motor may be explained by considering the magnetic action of the poles of the field magnets and the poles formed on the armature core by the magnetomotive force of the armature windings. When the current is conducted through the armature windings at brush (A), figure 72, and leaves at brush (B), the direction of current flow in groups of conductors (C), (D), (E), and (F) is as indicated. The flux around the individual conductors combines to form poles on the armature core midway between the tips of two adjacent pole pieces, as indicated at N' and S'. Since unlike magnetic poles attract each other, and as the field poles and armature poles are unlike, a clock-wise rotation of the armature is established. The individual armature conductors change from one group to the adjacent group as the armature rotates, but as the direction of current in the armature conductors changes during this action, owing to commutation, the polarity and location of the poles formed on the armature core are unchanged; therefore, the direction of rotation of the armature will be the same.

FIGURE 72.-Operating principles of an electric motor.

(3) When an armature is forced by motor action to cut across the flux of the field magnets, an electromotive force is generated in the armature conductors, called counterelectromotive force. The armature of a motor has a very low resistance. and if held so that it could not rotate, the full voltage from the source of power would be impressed across the motor terminals, resulting in damage to the windings. When the armature of the motor is free to rotate, the counter-electromotive force established in the active conductors acts in direct opposition to the impressed voltage and thus limits the current through the armature conductors. As the speed of the armature increases, the counterelectromotive force also increases, and the armature finally reaches a speed when the opposing action of the counter-electromotive force, plus that due to the ohmic resistance of the windings, is such that just sufficient current is taken by the motor to develop the required torque. The voltage that is actually effective in forcing current through the motor is the difference between the impressed voltage and the counterelectromotive force. This difference is usually only a few volts, because the resistance of the armature is so low that only a low effective voltage is required to force the current through the motor windings.

(4) The turning torque of an electric motor varies directly as the strength of the armature current and is expressed in foot-pounds. The measurement of the torque of a motor is accomplished by the use of a dynamometer or prony brake (fig. 73).

FIGURE 73.-Prony brake set-up.

(5) The series motor (fig. 71) is so named because the same current passes through both the field coils and the armature. The field current of a series motor increases in proportion to its exciting current, hence, at starting, the torque of a series motor varies practically as the square of the current. These motors are therefore especially valuable where a high torque is required, such as in starting aircraft engines. The speed of a series motor varies inversely with the load, and for each particular load there is a corresponding speed. A very objectional feature of a series motor is its tendency to "race" with diminished loads.

d. Various types of control units are employed for accelerating and engaging inertia type starters to the engine.

(1) The hand inertia starter employs a pull rod or cable to mesh the, starter jaw with the engine jaw, after the reqnired r. p. m. is attained by hand cranking. When a battery operated booster system is installed to assist in starting, a suitable switch is used to close the booster circuit during the cranking period of the engine. A typical control system used with a hand inertia starter system is shown in figure 74.

FIGURE 74.-Typical hand inertia starter engaging control.

(2) Electric inertia starters may be equipped with a remote control, solenoid type starting switch, operated from a control switch, and desirable where the starter is located at some distance from the control compartment. The starter solenoid consists of a coil with suitable connections and contacts and is usually mounted on the starter flange. Figure 75 illustrates the operation of a starter solenoid switch. When the control switch (A) is closed, the low amperage current energizes the solenoid, causing the iron plunger (B) to advance against spring pressure. This action closes the switch contacts (C) and (D) and completes the battery circuit through the starter motor. When the required flywheel speed of the starter is attained and the switch circuit opened, the coil loses its magnetic effect, and as the iron plunger is under spring tension it returns to its original position, this opening the battery circuit to the starter motor. To engage the starter after its flywheel has been accelerated to proper speed, the plunger in unit (A) is pulled out and held in its extreme position until the engine starts or the starter exhausts its energy. When the the plunger is released it returns to a neutral position.

FIGURE 75.-Remote solenoid starter control.

(3) A single heavy duty starting switch is also suitable for use in conjunction with an electric inertia starter. A booster switch is incorporated in the assembly for use when desirable. With this type of switch a push-pull rod is used to provide a positive action in both directions. The booster system operation is automatic on engagement of the starter to the engine. By referring to figure 76, when the plunger shaft (A) is pushed inward, the motor circuit is completed through contact (B) with the booster switch contact (C) resting on the insulating sleeve (D), thereby keeping the booster coil circuit open while the starter flywheel is being accelerated. To mesh the starter jaw with the engine jaw and to close the booster circuit, the plunger shaft (A) is pulled outward. This action also opens the starting motor circuit. When the plunger (A) is released, a spring returns it to a neutral position.

FIGURE 76.-Heavy duty starting switch control.

(4) Where complete electrical control of the starter system is desired, electric inertia starters are equipped with a control system consisting of a double contact push and pull switch, or a single pole double-throw toggle switch, used in conjunction with a starting solenoid switch and a solenoid meshing device. This control system eliminates the need for long push-pull rods or cables when the starter is located at some distance from the control compartment, as is the case of outboard multiple engine installations. The control system (fig. 77) operates as follows: When the switch is pushed in to the "start" position, the battery circuit is completed through the coil in the solenoid starting switch, which causes the iron plunger to advance against spring pressure. This action closes the switch contacts and completes the battery circuit through the motor to energize the starter flywheel. When the required flywheel r. p. m. is attained, the switch is manually pulled to the outward position marked "engage." This operation opens the battery and motor circuit and makes electrical contact through the coil of the electrical meshing device. The meshing device is similar in construction to the starting solenoid switch with the exception of the iron plunger which is connected to the starter meshing rod for automatically engaging and disengaging the starter to the engine. A booster system may also be used with this type of control switch if desired.

FIGURE 77.-Complete electrical starter control.

32. Direct cranking type starters.-a. This type of starter (fig. 78) is a combination unit for hand or electrical operation, and provides instantaneous and continuous cranking of the engine. It consists basically of an electric motor, a gear reduction, and an automatic engaging and disengaging mechanism which operates through an adjustable torque overload release. As the engine is cranked directly by the starter, there is no preliminary storing of energy required as in the case of the inertia type starter.

b. For electrical operation, the starting switch is closed and the battery energy is applied across the motor terminals. The torque of the motor is transmitted through a gear reduction to the adjustable torque overload release mechanism. This, in turn, actuates a splined screw shaft which moves the starter jaw axially outward to engage with the engine cranking jaw before it begins to rotate. Thus, complete engagement is accomplished before cranking begins. The torque overload release mechanism consists of a multiple disk clutch, adjusted to a predetermined value to deliver a sufficient yet not excessive cranking torque to the engine. In case of engine kick-back, the clutch slips, preventing damage to the engine or starter. When the engine starts, the starter automatically disengages from the engine.

FIGRE 78.-Direct cranking type starter

33. Cartridge type starters.-The cartridge type starter may be used in aircraft as a satisfactory substitute for the hand inertia starter. The system includes a starter, breech, intake tube, exhaust tube, toggle switch, and cartridge (fig. 79). The operation of the starter is illustrated in figure 80. A cartridge is placed in the closed breech barrel and ignited electrically. The initial pressure developed within the cartridge after ignition propels the ignited fuel through the intake tube to the fuel combustion chamber where its combustion is completed. This combustion of the fuel in the cartridge is accompanied by a subsequent increase in gaseous pressure within the system. The power derived from this pressure moves a piston which engages the clutch jaw of the starter with the engine jaw. Immediately after engagement of the two jaws, the continuing forward movement of the piston is converted into rotary movement by means of a series of helically splined shafts, thus cranking the engine. At the end of the expansion stroke, the exhaust valve within the fuel combustion chamber automatically opens, releasing the pressure in the cylinder. The special coil spring, located below the piston, which was compressed during the forward stroke, returns the piston to its normal position, expelling the burned gases through the exhaust tube and disengaging the starter clutch jaw from the engine. At the completion of its return stroke, the piston automatically closes the exhaust valve. This completes one cycle of operation. The starter is protected by a safety disk located in the fuel combustion chamber. In case the pressure in the starter exceeds the normal working pressure, the safety disk is ruptured and the gas pressure is exhausted through the exhaust tube. New safety disks can be readily installed.

FIGURE 79.-Typical cartridge type starter installation.

34. Hand turning gear type starter.-The hand turning gear type starter is used on low horsepower engines and on installation where an electrical source of power is not available. This starter consists of a gear reduction unit which operates an automatic engaging and disengaging mechanism through an adjustable torque overload release. Mechanical features are incorporated to safeguard both the operator and starter from injury in case of engine kick-back. In the event of kick-back, the torque overload release automatically disconnects the starter from the engine, thus preventing damage to the starter mechanism. As a further protection to the operator, a ratchet is provided on the hand crankshaft to prevent the possible transmission of any reverse motion to the crank handle.

FIGURE 80.-Sectional view of cartridge type starter.

35. Air injection type starter.-This type of starter operates from engine generated air pressure and is particularly adaptable to application wherein an electrical installation is not suitable and hand cranking is undesirable. The system includes a compact engine-driven air compressor ; a timed rotating distributed valve, integral with the compressor; a storage tank; an automatic pressure-regulating valve; a pressure gage, and a fuel primer. The starter is operated by the air pressure in the storage tank. The release of this pressure is controlled from the cockpit and transmitted by the distributor valve to the cylinders in proper firing order. The air pressure dissipated in starting is rapidly replenished by the engine-driven compressor. Provisions are made to introduce a priming charge into the distributor as the starter is being operated.

36. Portable field starting system.-a. The purpose of the portable field starting unit is to conserve manual effort required in operating hand starters, and in case of the electric starter to relieve the battery in the aircraft from the excessive current discharge required, particularly in starting the engine in cold weather. The two types of portable starting systems in common use are the portable battery cart and portable energizer.

(1) The portable battery cart consists of a pull cart in which several standard storage batteries, connected in parallel, are installed. With this system the portable batteries are wheeled to the airplane and connected to the load circuit of the electrical installation through a suitable master battery switch arrangement in the airplane. This switch opens the aircraft battery circuit when the portable battery circuit is closed. The cart is suitable for use in operating electric inertia and electric direct cranking starters.

(2) The portable energizer (fig. 81) consists of a universal electric motor designed to operate from an alternating or direct current 110-volt circuit; however, where a permanent electrical source is not available a separate portable electric power plant is used to furnish the current. The electric motor is attached to a gear reduction driving unit which incorporates a spring adjusted multiple disk clutch to protect it from excessive overloads. The starter driveshaft protrudes from the driving unit for engagement into the engine starter crank arm.

(a) To operate the energizer, a trigger switch assembly is installed at the electric motor end where it can be readily manipulated. The direction of rotation of the electric motor and driving unit can be reversed by changing the position of the, reverse snap switch installed adjacent to the trigger switch assembly.

(b) To use the energizer with an inertia starter the driveshaft is placed in the opening of the starter crank extension shaft protruding from the side of the fuselage or engine nacelle, and the trigger switch closed. The driveshaft pin fits itself into the spiral slot of the starter extension and accelerates the starter flywheel. As soon as the desired cranking speed is attained, the trigger switch is released and the energizer removed from the starter extension allowing the flywheel to run free. The operator may then engage the starter with the engine at will. The energizer should never be used to continue to accelerate the flywheel when the starter is engaged with the engine, as undue stresses may cause serious damage to the mechanism or injury to the operator.

FIGURE 81.-Portable field starting energizer

b. The portable electric power plant. (fig. 82) consists of a generator driven by a gasoline engine and is used for various ground equipment requiring 110-volt direct current power. However, its main purpose is to furnish the power for the operation of the portable starter energizer. When the plant engine is started and its speed increased to required r. p. m., the generator, attached to the engine, reaches a maximum output of 110 volts. The engine speed is controlled by a governor; however, if it becomes necessary to increase or decrease the engine speed, this may be accomplished by varying the tension of the governor spring. The voltage of the generator is controlled by a fixed field resistor located in the terminal box to which the energizer motor attachment plug and cord are also connected.

FIGURE 82.-Portable power plant.

37. Maintenance.-The following instructions pertain to the maintenance of the various units of engine starting systems and are confined to minor repairs, tests, and adjustments which can be accomplished in the field.

a. Inertia starter.-(1) Inspect the starter for cracked housing and mounting flanges, security of mounting, tightness of housing bolts, and safetying of all attaching or connecting bolts. Replace the starter if cracks appear in the housing or flanges. (The maintenance of the electric inertia starter motor described in (2) below also applies to energizer motors.)

(2) Check all connections and terminals for condition and tightness. Remove the motor brush strap and check for worn or binding brushes and improper brush spring tension. Worn brushes must be replaced and the new brushes properly seated by inserting a strip of No. 000 sandpaper between the brush and commutator, with sanded side next to the brush, and pulling the paper in the direction of commutator rotation. Binding brushes and brush holders are wiped clean with a gasoline moistened cloth. Weak brush springs must be replaced. If the commutator is rough or dirty, smooth it with No. 000 sandpaper while it is revolving; however, if it is badly scored or worn replace the motor assembly.

(3) Before installing a new starter on the engine, remove the cover over the starter jaw, which is provided for shipping and storage purposes only, and examine the end of the engine crankshaft to note if the engine jaw and starter jaw are of the same type and of correct rotation.

(4) Check the solenoid switch and meshing device for security of mounting and tightness of leads and terminals. If operating troubles are experienced with either of these devices, use the proper test instruments to determine the cause; however, if the solenoid is found to be inoperative it must be replaced.

(5) To check the operation of inertia starters proceed as follows:

(a) To crank the starter by hand, insert the hand crank in the starter extension arm and rotate it to a speed of approximately 80 r. p. m. and remove the crank handle. Engage the starter to the engine by use of the manual meshing control. When the engine starts, the starter jaw should automatically disengage from the engine. If the engine fails to start and the starter jaw remains in mesh with the engine jaw, it will be necessary to turn the propeller by hand about 1/3 or 1/2 of a revolution in either direction of rotation to release the starter jaw.

(b) To turn the starter electrically, the procedure is determined by the type of control device employed. If the manual push-pull type control is used, push in the control for approximately 10 seconds until the starter flywheel has reached its normal r. p. m., and mesh the starter jaw with the engine jaw by pulling out the switch or control rod handle as quickly as possible. As soon as the engine starts, the control handle or push-pull switch is released and the starter jaw should automatically disengage. If the engine fails to start on the first trial, make sure the starter jaw is disengaged and that the fly-wheel is not revolving before operating the starter again. To operate starters equipped with a double contact switch, starting solenoid, and meshing device, close the control to the "start" position for approximately 10 seconds, then move the switch to the "mesh" position. When the engine starts, release the switch to permit it to return to its neutral position. Do not operate an inertia starter manually or electrically while the starter is engaged with the engine.

b. Cartridge-type starter.-(1) Check the security of the starter and all attachments. When required, remove the breech, intake and exhaust tubes, and the combustion chamber, and check for general cleanliness. If necessary, disassemble the breech and clean it with a cotton swab on a cleaning rod saturated with penetrating oil. The unit should be lubricated before reassembling. After this has been done, test the electrical circuit for proper functioning. When the combustion chamber is removed, check the starter exhaust valve and make sure that the locking nut is holding the assembly securely on the exhaust valve opening bolt. In the event that the lock nut is loose, adjust the exhaust valve. Clean the residue from the perforated disk by removing it from its seat, but do not remove it from the exhaust valve bolt. Use water, gasoline, or kerosene. Check the piston and cylinder head and if hard particles of residue are found which impede the movement of the piston, replace the starter.

(2) To check the operation of a cartridge-type starter, open the breech lever 90° in an upward direction and place a cartridge in breech barrel. Close the breech and operate the electric contact switch to ignite the cartridge, which in turn operates the starter. When the engine has started, remove the empty cartridge case by opening the breech lever 45° upward, holding it in this position until all of the back pressure in the system has escaped. When the starter imparts sufficient torque to the engine for normal starting, but the engine fails to start after discharging three or four cartridges, investigate for probable engine difficulties. In case of failure of a cartridge to fire after a first attempt, close the contact switch two or three times more, and if the cartridge still fails to fire, after waiting for approximately 5 minutes, remove the cartridge. If, after inserting a new cartridge, it fails to fire, check the firing pin and contact block in the breech assembly for proper functioning. When-ever a loud report and a slight movement of the propeller are noted, it is probable that the safety disk has blown out. Replace the safety disk mounted in the safety disk holder, and if they continue to burst recheck the starter as previously described.