The magneto is a completely self- contained ignition device. Magnetos have been around since the early 1900s and have been refined for the last 100 years to give us the reliability we have come to expect from aircraft magnetos. As maintenance professionals, it is important that we understand what makes a magneto tick. In this article, we will discuss magneto theory of operation as well as items necessary to prepare a magneto for installation.
Theory of Operation
Changes in magnetic fields produce electricity. The faster the magnetic field changes the more electricity is produced. The spinning motion of the magnetized rotor induces a small voltage of less than 100 volts in the primary windings of the coil. This voltage makes the iron core in the center of the coil become an electromagnet, just like wrapping a nail with copper wire and attaching it to a battery like we did as children.
The rotor only spins fast enough to produce about 100 volts in the primary windings of the coil, not nearly enough to fire a spark plug. In order to boost that voltage to above 14,000 volts, we use two integral parts of the magneto.
Remember we mentioned that how fast we change the magnetic field is directly related to voltage output. If we can instantly turn off the voltage flowing through the primary coil windings there will be a much higher voltage output. The way we “unplug” that voltage in the primary windings of the coil is by opening the breaker points in the magneto. This instantly collapses the magnetic field that we have built up in the iron core of the coil. This is the first method to boost the voltage.
The second way we boost the voltage is by means of a secondary coil. Wrapped around the primary coil windings are about 100 times as many secondary coil windings. The instantaneous collapse of the magnetic field by opening the points creates a very high voltage in the secondary windings of the coil. This is the voltage that fires the spark plugs.
It takes about 8,000 to 12,000 volts to jump a spark plug gap. However, if two pieces of metal are touching and then moved apart, like the magneto contact points, it takes less than 20 volts to jump the resulting gap.
If voltage was allowed to jump the points gap, our magnetic field would collapse much more slowly and greatly reduce our output voltage as well as quickly erode the faces of the points. So, the capacitor acts like a sponge soaking up the 100 or so volts from the primary coil, until the points are far enough apart to prevent the current from jumping across the point gap. If the capacitor has failed, it will result in a high magneto drop, and severe point erosion. Accurate capacitor testing equipment is complex and expensive.
Preparing Magneto for Installation
Let’s get back to the basics. Most IAs have probably nstalled a number of magnetos. However if you deal primarily with turbine or jet engines this process may not be familiar.
More than twice as many magnetos are returned to the factory due to incorrect installation than all of the other warranty claims combined. When troubleshooting an ignition problem, verify at least twice that the magetos are installed correctly. A second set of eyes is always helpful in these situations. In hopes of eliminating some of these installation issues, let’s review the basics of installing a Slick magneto.
First, let’s install the magneto driver or drive gear from the old mags onto the new mags and torque the nut to 120 to 320 inch pounds. Use the supplied cotter pin to secure the castellated nut with the long leg up toward the end of the shaft. On impulse coupling magnetos, there should be .010 to .020 inch end play on the gear. This is normal and not a reason for concern.
Next, prepare the magneto for installation by setting the mag to fire the No. 1 cylinder. All Slick magnetos that were manufactured in the last 25 years use a small pin that is inserted through the distributor block and through a corresponding hole in the distributor gear. When the pin is installed correctly, the magneto is ready to be installed on the aircraft.
The techniques and opinions on how to start aircraft engines are as varied as the airplanes, engines, and operators that encompass this topic.
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