Just so there is no confusion, the RPM sensors we refer to, are the ones used for measuring the RPM of the Bendix Magneto or the Slick Magnetos in aircraft. Some aircraft use Dual Magnetos but Bendix Magnetos and the Slick Magnetos are more widely used.
One of the main reasons the Bendix Magnetos are popular is because they are easily overhauled and for any aircraft; this is a winning feature as it saves time and money. Amongst the most prominent companies that manufacturer highly accurate aircraft RPM sensors, is J.P. Instrument – they manufacture sensors for Bendix Magnetos as well as Slick Magnetos.
Just in case you are unaware, spark plugs in any aircraft engine are powered by the magnetos – these act like mini power generators and even have a built-in transformer, breaker switch and a distributor – all used to provide high voltage to the spark plugs.
In a standard magneto (or dynamo), there are rotating magnets and electrical coils inside the housing. As the magnets rotate, the electrical coils generate power. The rotation has to be kept within pre-prescribed limits as the output voltage has to be constant and uniform. Any variations will lead to instant problems as the spark plugs will either not fire at all or go out of sync and the engines will misfire. The pilots therefore need to be aware the Bendix magneto is working properly and this is enabled via the RPM Sensor - a small-ish device that is attached to the magneto and provides the RPM readout in the cockpit.
Installation or replacing your RPM Sensor for Bendix Magneto is easy
Once the housing is open, just remove the vent plug from the port on the magneto (the vent plug covers the portion of the magneto that contains the rotating magnet). Remove the existing Aircraft Engine Sensors if any and insert the new RPM sensor (Dual, Bendix and Slick Magnetos have different RPM sensors).
Gently route the wiring bundle towards the firewall – while doing so, make sure you do not attach the bundle directly to the ignition, harness or magneto p-leads. Maintain a bit of slack in wiring so nothing gets unplugged during flight.
Technical data to remember while installing your new RPM sensor:
• Align the sensor with the correct vent plug of your Bendix magneto - when the position is correct, you should be able to see the rotating magnet through the vent – not the ‘gear’.
• The RPM for a 4-cylinder engine would be 3600 rpm, 2400 for a 6-cylinder engine, 1800 for 8-cylinder and 1600 RPM for a 9-cylinder engine. If using a dual mag, a 4-cylinder engine should have rpm of 1800 while that of a 6-cylinder engine should read 2400.
• The red wire of the Aircraft Sensors should be connected to the ‘+ve’ supply rated at 5 volts.
When replacing your old RPM Sensor, make sure you buy a good quality RPM standard sensor – you really don’t want false or inaccurate RPM reading.
More information on JPI manufactured RPM standard sensors here: https://www.jpinstruments.com/shop/rpm-sensor-for-bendix-mag/
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