enginehistory.org

[jsteele]

Hello all. I'm in the process of setting up an engine shop and starting to restore several P & W engines for our local museum (Glenn L Martin Avaition Museum) here outside Baltimore.

A fellow museum member made a statement that confused me regarding the ignintion systems of the Double Wasps. He mentioned that in every a/c he has seen with R-2800's there is a switch marked IGNITION that has 3 positions, BOTH, 1 and 2. He mentions that during the startup sequence the pilot has to do a mag check by switching from BOTH to 1 and then 2, and that if the RPM drops off by more than 200 rpm there is a problem with the ignition system. He states that this means there are 2 magnetos on the powerplant.

I've confirmed his cockpit instrument panel info in the F4U-1 Pilots Manual, also the validity of the test sequence.

However, as I've started to tear down the R-2800 we are starting with, we note that there are 2 distributors, 1 for the front row and 1 for the back row of cylinders, but only 1 magneto.

So my question is, what does the mag check test and how does that work? Is the one magneto divided into two generating elements?

Thanks for any info, John

[kmccutcheon]

The R-2800 is equipped with a dual magneto, which consists of a single rotating magnet, but two magnetic and electric circuits (coils, point sets, condensers). A lead connects the magneto secondary output to each distributor, which then routes it to the correct spark plug in the engine firing order.

A primary lead from each magneto electrical circuit is connected to each of the #1 and #2 positions of the cockpit magneto switch. The switch has four positions: OFF, 1, 2, and BOTH. When in the OFF position, the switch grounds both primary leads; when the switch is in the 1 position, it grounds the 2 circuit; when the switch is in the 2 position, it grounds the 1 circuit, and when it is in the BOTH position, it grounds neither circuit. This DF18LN-1 schematic shows how this works for a high-tension magneto. Low-tension magnetos function similarly, but move the high-tension coils out of the magneto and very near the spark plugs. This is typically done for high-altitude engines where flash over within the distributor becomes a problem.

One day, I hope to write an article about "modern" dual magnetos such as those used on the R-2800 and R-3350, and add it to the Magnetos section of the AEHS web site.
_________________
Kimble D. McCutcheon

[jsteele]

Kim,
Thanks for the concise reply-I now understand.

This leads me to an update: we've gotten into our first project engine, a -77, and it has no magneto. I'm beating about looking for mags for that and the runner (combo of 2 -43's), and beginning to feel pretty stupid again.

I have trouble finding anything that is near the correct original Bendix Scintilla DF18LN for either engine. However, I now see something called Slick magnetos, which seem a modern equivalent. Is there an equivalency chart between the two kinds?

Please let me know if anyone has a non-working or shell to spare, and any points of reference on modern equivalencies great fully appreciated.

Best, John