In the early stages of aircraft development, relatively low powered reciprocating engines were started by pulling the 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. 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.
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 with a small motor. The flywheel and movable gears of a combination hand electric inertia starter are shown in Figure 5-1. The electrical circuit for an electric inertia 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.
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.