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Secondary Ignition Current

I measured this on my workbench with a magnet attached to a drill spinning next to a spare ignition trigger. I measured the coil as 3.2 ohms and set my power supply to 14v. I used a shunt resistor at the output of the power supply to measure the current draw of a single coil/trigger combo. I expected to measure a peak current of about 14v/3.2ohms = 4.4 amps. It turns out the trigger applies only 11.4v across the coil, explaining why I only see a peak of ~3.5 amps.

The ignition spark is generated when the current drops sharply to 0. There’s a period of almost zero current lasting 2.5ms while the magnet is still close to the trigger. This period is longer at slower RPMs and I believe it gets shorter at higher RPMs. I wasn’t able to test higher RPMs with my drill setup, but I did move the magnet far enough from the trigger that it was barely triggering and saw periods of ~1ms at the same RPM.

Using this data, I simulated the expected waveform at a range of RPMs to show how the average current draw decreases as RPMs increase. It’s only about a 13% decrease in current and probably not enough to be readily noticeable.

While investigating this, I noticed the triggers de-energize the coils after about 2 seconds without a trigger. This greatly reduces their drain on the battery while the engine isn’t running.

Dynamic Propeller Balancer

I’ve been curious about the balance of my propeller/engine combination.  Rather than find someone with access to a dynamic balancer, I decided to design and build my own.

Balancer setup on bench

Moving to the airport

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Cockpit mostly complete

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Throttle, electrical, and fuel

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Attaching engine mount

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Riveting fuselage former

Archie came over again and helped me rivet the last two fuselage formers in place.

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This week I completely disassembled the forward fuselage, deburred them, and added a hole for the rear spar on the left side.  I then reattached the sides, bolted, and riveted the skins.  I started riveting the spar tunnel in place with flush rivets.

Forward fuselage deburring, dimpling, and countersinking

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