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COPA U CORNER
A Slick magneto
with impulse coupling attached.
April 21, 1942, for W. Ochsenbein. No computers were around to analyze the data or produce 3D images for testing, just brain power and great imagination. I’m very impressed.
The impulse coupling mounts between the magneto and the engine, allowing it to control when the magneto rotates. The coupling has a couple of spring-loaded flyweights that catch on a post built into the case of the magneto. At very low speeds the flyweights catch on the post and halt the magneto’s rota- tion for those 25 to 30 degrees (lag angle) I mentioned earlier. At the same time, it winds up the coupling’s internal spring. When the rotation gets to the end of the lag angle, a flange on the coupling knocks the flyweights off the posts, and the spring suddenly releases its energy and spins the magneto. At their core, magnetos are just generators and need adequate rotation speed to generate a usable spark. The heavy-duty spring creates this sudden high rotation speed in the magneto.
This is all great for intentional engine starting. The problem is that the impulse coupling engages any time the engine turns at an appropriately slow speed, even if the intent is not to start the engine. Anything that rotates the engine in its running direction will engage the impulse coupling. The only thing that inhibits a spark is the P lead which electrically grounds the magneto through the ignition switch. This grounding is like on those old lawnmowers that you would shut off using your foot to push a metal tab to the top of the spark plug.
The very real danger is that the P lead might be broken, have a loose connection, or have any number of other faulty situations that remove the operation-inhibiting ground from the magneto.
“Anything that rotates the engine in its running direction will engage the impulse coupling.”
Pulling the propeller, a small amount to give clearance for the tow bar is enough to trigger the impulse coupling. You’ve probably heard that “snap” sound from your engine at some point when you turned a prop. The motivation for this article is to help you understand why you should never do anything to hear that snap again.
As a 17-year-old working in Dad’s shop, I was assigned to measure the run out of an engine’s crankshaft flange. The engine was from a plane we had recovered for an insurance company about a month before. The engine was sitting on the shop floor with the top spark plugs removed so I could more easily rotate the crankshaft. Since the engine wasn’t on the aircraft, the magneto P leads were not attached to ground the mags, so they were operational. As I rotated the engine, I heard the snap of the impulse coupling. At that same time, I saw and felt the long jet of flame shoot out of the #1 cylinder’s empty spark plug hole, catching my shirt on fire. Even at that young age, I figured out the effectiveness and dangers of the impulse coupling.
If spark plugs are installed, the magneto is not grounded and the impulse coupling snaps, resulting in a sudden and violent rotation of the prop. It may fire just the one cylinder but possibly two or three. This very dangerous event happens many times per year, and the injuries are significant. Consider possible damage to the plane, hangar, other property or personal injury. A good day is just a broken wrist – a not-so-good day results in nine staples in the back of one’s head and a terrible headache. We don’t need to imagine what a bad day looks like.
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JULY 2023 COPA Pilot