Aircraft Engine Starting Basics

5-1 Hand crank adapter Hard steel insert Centrifugal clutch FlywheelTorque overload release clutch Starter driving jaw

Introduction

Most aircraft engines, reciprocating or turbine, require help during the starting process. Hence, this device is termed the starter. A starter is an electromechanical mechanism capable of developing large amounts of mechanical energy that can be applied to an engine, causing it to rotate. Reciprocating engines need only to be turned through at a relatively slow speed until the engine starts and turns on its own. Once the reciprocating engine has fired and started, the starter is disengaged and has no further function until the next start. In the case of a turbine engine, the starter must turn the engine up to a speed that provides enough airflow through the engine for fuel to be ignited. Then, the starter must continue to help the engine accelerate to a self-sustaining speed. Turbine engine starters have a critical role in starting of the engine.

If the starter turns the turbine engine up to a self-sustaining speed, the engine start process will be successful. There are only a few types or methods used to turn the engine. Almost all reciprocating engines use a form of electric motor geared to the engine. Modern turbine engines use electric motors, starter/generators (electric motor and a generator in the same housing), and air turbine starters. Air turbine starters are driven by compressed air through a turbine wheel that is mechanically connected through reduction gears to one of the engine’s compressors, generally the highest pressure compressor.

Reciprocating Engine Starting Systems

In the early stages of aircraft development, relatively low powered reciprocating engines were started by pulling the Engine Starting Systems propeller through a part of a revolution by hand. Difficulty was often experienced in cold weather starting when lubricating oil temperatures were near the congealing point. In addition, the magneto systems delivered a weak starting spark at the very low cranking speeds. This was often compensated for by providing a hot spark using such ignition system devices as the booster coil, induction vibrator, or impulse coupling. Some small, low-powered aircraft which use hand-cranking of the propeller, or propping, for starting are still being operated. For general instructions on starting this type of aircraft, refer to the Aviation Maintenance Technician— General Handbook, Chapter 1, Safety, Ground Operations, and Servicing. Throughout the development of the aircraft reciprocating engine from the earliest use of starting systems to the present, a number of different starter systems have been used. Most reciprocating engine starters are the direct cranking electric type. A few older model aircraft are still equipped with inertia starters. Thus, only a brief description of these starting systems is included in this section.

Inertia Starters There are three general types of inertia starters: hand, electric, and combination hand and electric. The operation of all types of inertia starters depends on the kinetic energy stored in a rapidly rotating flywheel for cranking ability. Kinetic energy is energy possessed by a body by virtue of its state of motion, which may be movement along a line or spinning action. In the inertia starter, energy is stored slowly during an energizing process by a manual hand crank or electrically Chapter 5 5-2 Starter inertia + Bus Ener. Mesh Battery relay Battery control switch Starter control switch Starting solenoid Engaging solenoid Crank socket Engaging level Flywheel Barrel Mounting flange Springs Starter driving jaw Disks Starter + Bus To auxiliary igniter device Starter solenoid Starter switch Battery switch Battery solenoid with a small motor. The flywheel and movable gears of a combination hand electric inertia starter are shown in starter is shown in Figure 5-2. During the energizing of the starter, all movable parts within it, including the flywheel, are set in motion. After the starter has been fully energized, it is engaged to the crankshaft of the engine by a cable pulled manually or by a meshing solenoid that is energized electrically. When the starter is engaged, or meshed, flywheel energy is transferred to the engine through sets of reduction gears and a torque overload release clutch. [Figure 5-3] Direct Cranking Electric Starter The most widely used starting system on all types of reciprocating engines utilizes the direct cranking electric starter. This type of starter provides instant and continual cranking when energized. The direct cranking electric starter consists basically of an electric motor, reduction gears, and an automatic engaging and disengaging mechanism that is operated through an adjustable torque overload release clutch.

A typical circuit for a direct cranking electric starter is shown in Figure 5-4. The engine is cranked directly when the starter solenoid is closed. As shown in Figure 5-4, the main cables leading from the starter to the battery are heavy duty to carry the high current flow, which may be in a range from as high as 350 amperes to 100 amperes (amps), depending on the starting torque required. The use of solenoids and heavy wiring with a remote control switch reduces overall cable weight and total circuit voltage drop. 5-3 + 60- 60 -30 +300 A M P Heavy current to starter 24 Volts DC input 24 Volts DC input Starter vibrator Ground through switch actuates battery solenoid Auxiliary voltage input Heavy current to starter Solenoid actuating voltage External power receptacle Left magnetos Circuit breaker Starter switch left Left engine starter Left starter solenoid Battery solenoid Battery switch Right engine starter Right starter solenoid Ammeter shunt BUS Starter switch right Right magnetos The typical starter motor is a 12- or 24-volt, series-wound motor that develops high starting torque. The torque of the motor is transmitted through reduction gears to the overload release clutch. Typically, this action actuates a helically-splined shaft moving the starter jaw outward to engage the engine cranking jaw before the starter jaw begins to rotate. After the engine reaches a predetermined speed, the starter automatically disengages. The schematic in Figure 5-5 provides a pictorial arrangement of an entire starting system for a light twin-engine aircraft.

Direct Cranking Electric Starting System for Large

Reciprocating Engines

In a typical high horsepower reciprocating engine starting system, the direct cranking electric starter consists of two basic components: a motor assembly and a gear section. The gear section is bolted to the drive end of the motor to form a complete unit. The motor assembly consists of the armature and motor pinion assembly, the end bell assembly, and the motor housing assembly. The motor housing also acts as the magnetic yoke for the field structure. The starter motor is a nonreversible, series interpole motor. Its speed varies directly with the applied voltage and inversely with the load. The starter gear section consists of an external housing with an integral mounting flange, planetary gear reduction, a sun and integral gear assembly, a torque-limiting clutch, and a jaw and cone assembly. [Figure 5-6] When the starter circuit is closed, the torque developed in the starter motor is transmitted to the starter jaw through the reduction gear train and clutch. The starter gear train converts the high speed low torque of the motor to the low speed high torque required to crank the engine. In the gear section, the motor pinion engages the gear on the intermediate countershaft.

[Figure 5-6] The pinion of the countershaft engages the internal gear. The internal gear is an integral part of the sun gear assembly and is rigidly attached to the sun gear shaft. The sun gear drives three planet gears that are part of the 5-4 Jaw spring Bronze clutch plates Planetary gear Planetary carrying arm Jaw engaging spline Sun gear shaft extension and jaw stop retainer nut Return spring Starter jaw Conical clutch suface Traveling nut Sun gear Steel clutchClutch spring retainer Internal gear Motor pinion Motor shaft Intermediate countershaft Countershaft pinion planetary gear assembly. The individual planet gear shafts are supported by the planetary carrying arm, a barrel-like part shown in Figure 5-6.

The carrying arm transmits torque from the planet gears to the starter jaw as follows: 1. The cylindrical portion of the carrying arm is splined longitudinally around the inner surface. 2. Mating splines are cut on the exterior surface of the cylindrical part of the starter jaw. 3. The jaw slides fore and aft inside the carrying arm to engage and disengage with the engine. The three planet gears also engage the surrounding internal teeth on the six steel clutch plates. [Figure 5-6] These plates are interleaved with externally splined bronze clutch plates that engage the sides of the housing, preventing them from turning. The proper pressure is maintained upon the clutch pack by a clutch spring retainer assembly. A cylindrical traveling nut inside the starter jaw extends and retracts the jaw. Spiral jaw-engaging splines around the inner wall of the nut mate with similar splines cut on an extension of the sun gear shaft. [Figure 5-6] Being splined in this fashion, rotation of the shaft forces the nut out and the nut carries the jaw with it. A jaw spring around the traveling nut carries the jaw with the nut and tends to keep a conical clutch surface around the inner wall of the jaw head seated against a similar surface around the underside of the nut head. A return spring is installed on the sun gear shaft extension between a shoulder, formed by the splines around the inner wall of the traveling nut, and a jaw stop retaining nut on the end of the shaft. Because the conical clutch surfaces of the traveling nut and the starter jaw are engaged by jaw spring pressure, the two parts tend to rotate at the same speed. However, the sun gear shaft extension turns six times faster than the jaw. The spiral splines on it are cut left hand, and the sun gear shaft extension, turning to the right in relation to the jaw, forces the traveling nut and the jaw out from the starter its full travel (about 5⁄16 inches) in approximately 12° of rotation of the jaw.

5-5 Off 1 2 3 4 Bus Starter bus Starter bus cable Master bus Ignition boost switch Primer switch To induction vibrator To primer Starter relay To prop deicing relay To feathering pump relay Firewall junction box Engine selector switch Start switch safely switch Starter primer and induction vibrator Starter motor (engine No. 1) The jaw moves out until it is stopped either by engagement with the engine or by the jaw stop retaining nut. The travel nut continues to move slightly beyond the limit of jaw travel, just enough to relieve some of the spring pressure on the conical clutch surfaces. As long as the starter continues to rotate, there is just enough pressure on the conical clutch surfaces to provide torque on the spiral splines that balance most of the pressure of the jaw spring. If the engine fails to start, the starter jaw does not retract since the starter mechanism provides no retracting force. However, when the engine fires and the engine jaw overruns the starter jaw, the sloping ramps of the jaw teeth force the starter jaw into the starter against the jaw spring pressure. This disengages the conical clutch surfaces entirely, and the jaw spring pressure forces the traveling nut to slide in along the spiral splines until the conical clutch surfaces are again in contact.

When the starter and engine are both running, there is an engaging force keeping the jaws in contact that continue until the starter is de-energized. However, the rapidly moving engine jaw teeth, striking the slowly moving starter jaw teeth, hold the starter jaw disengaged. As soon as the starter comes to rest, the engaging force is removed, and the small return spring throws the starter jaw into its fully retracted position where it remains until the next start. When the starter jaw first engages the engine jaw, the motor armature has had time to reach considerable speed because of its high starting torque.

The sudden engagement of the moving starter jaw with the stationary engine jaw would develop forces sufficiently high enough to severely damage the engine or the starter were it not for the plates in the clutch pack that slip when the engine torque exceeds the clutch-slipping torque. In normal direct cranking action, the internal steel gear clutch plates are held stationary by the friction of the bronze plates with which they are interleaved. When the torque imposed by the engine exceeds the clutch setting, however, the internal gear clutch plates rotate against the clutch friction, allowing the planet gears to rotate while the planetary carrying arm and the jaw remain stationary. When the engine reaches the speed that the starter is trying to achieve, the torque drops off to a value less than the clutch setting, the internal gear clutch plates are again held stationary, and the jaw rotates at the speed that the motor is attempting to drive it. The starter control switches are shown schematically in Figure 5-7.