Aerovee mag switch

[gr8jab]

Hello all,

First post ever on this forum! I'm going to be asking lots of questions. My EE skills are rusty, and I haven't read a schematic in years. Please excuse me if my questions are rudimentary.

I was reviewing other's schematics and came across a few things that need clarification. Here is the first...

Is this a shielded 18 AWG wire? The shield is grounded to the engine near the magnatron? The switch connects the conductor to the ground shield to disable the spark? Is this to prevent noise from hitting the main grounding block? Is this the normal method for grounding mags? I thought I've seen other schematics with the mag switch wired to the grounding block near the panel.

[Rynoth]

Hello, and welcome!

In the simplest terms, what the schematic is saying is that the magnetrons and the switches need to share a common grounding path that reaches the engine.

In even simpler terms.... connecting ground to the mag lead turns the mag off. This assumes of course that the mag body has a path to the same ground.

IMO the shielding is unnecessary, but simplifies the diagram. In practical terms, here is an example of how my grounding path goes for this particular circuit:

1) Magneto leads go to switches that connect to a (firewall mounted) grounding block.
2) Mags are secured to the (conductive) accessory plate, which is connected by a wire (mine is on the starter bolt) to the battery - /grounding block.
3) Engine case case is connected by wire to the grounding block.

This setup satisfies the required grounding paths (and unifies the engine, accessory case and airframe to the grounding block for all other electrical needs.) Mags are dead when the circuit is closed, active when the circuit is open.

[Bryan Cotton]

Mag grounding wires are usually shielded because they generate radio noise. The schematic says the shield should be grounded forward of the firewall and floating in the cockpit.

[Rynoth]

Bryan is correct.

To clarify, I was basing my assumption on wiring the Aerovee magnetrons, which follow a similar but different schematic. The schematic posted above is as Bryan said, and I can see the reasoning behind shielded wires on magnetrons for which the circuit is closed during operation. In the Aerovee setup, the circuit is open during operation and I don't see how shielding would make any difference.

[wlarson861]

my understanding:
Magnatrons in the AeroVee and Magnetos work the same way. When the mag sparks the electric pulse looks for the shortest path to ground. if the p-lead switch is closed the spark goes directly to ground and no spark is produced by the spark plug. If that switch is open then the spark is forced to seek ground by jumping the gap in the spark plug. All this time the P-lead is energized up to the open switch which radiates RF interference, the P-lead shield captures the RF and caries it to its ground point on the engine.

[Bryan Cotton]

Magnetrons are magnetos, but the Briggs and Stratton flavor. I've played with the B&S mags on the hovercraft a bit.

One thing I totally missed is the switch is supposed to short the center conductor to the shield, rather than a ground behind the firewall. I'll have to fix that on my airplane.

[sonex1374]

All this time the P-lead is energized up to the open switch which radiates RF interference, the P-lead shield captures the RF and caries it to its ground point on the engine.

The wire attached to the magnatron P-Lead acts as an antenna and radiates energy every time the spark plug fires. Most of the energy created by the magnatron goes into firing the spark plug, but a portion will be radiated out the P-lead "antenna". Of course this is just a burst of static, but it bleeds over into our comm and data systems and bothers us. Surrounding the P-lead antenna with a shield intercepts that burst of RF energy and drains it off to ground, where it can't affect our radios.

Pilots can test this effect for themselves by simply connecting a length of unshielded wire to the P-lead and routing it near the instrument panel. Running the engine will then cause this wire to radiate and you can evaluate the need for a shield. You won't damage anything doing this, and sometimes it helps to conduct a test to better understand what's happening with your plane.

Jeff

[gr8jab]

Thanks for the great replies. You all confirmed my suspicions and interpretation of the schematic.

I wasn't thinking about the RF noise as much as the direct route avoiding the grounding block when the switch is closed.

I did (re)learn something!

[Rynoth]

I learned something too!

The schematic in the Aerovee manual doesn't mention the shielded wire.



I just used single-conductor 20AWG for the p-leads (no shielding.) routed to the instrument panel and back to grounding block. I haven't had any issues with RF interference/radio noise, perhaps I was lucky (or the Aerovee mags don't put out much RF.) But everything said above makes sense and if I did it over again maybe I would use the shielded method.

[Onex107]

It's easy to tell if the magnatrons are making the noise on your radio. I had ignition noise only when receiving, Of course if I was broadcasting it I wouldn't know. But when I turned off the mags the noise stopped. Shielded "P" leads for me at 300 hours. I don't remember it being a problem in the early hours.