WIX Archives

Re: Mag "Drop"

Posted by bdk on Tue Jan 29, 2002 11:41:05 AM

In reply top On UK Telly Last Night posted by Paul McMillan on Tue Jan 29, 2002 04:03:35 AM

The "mag drop" comes into play when testing an engine prior to takeoff (for an airplane, obviously). You are checking for 1) The fact that both magnetos are working, and 2) That you don't have a fouled spark plug.

To do the magneto check, you increase the RPM to the specified amount, then switch from "both" to each individual magneto.

If one magneto is "dead," the engine would quit making the problem rather obvious.

If one spark plug is fouled, by switching to one magneto, you cause the cylinder with the fouled plug to not fire, which creates vibration and a reduction in engine RPM (the "drop").

There is a specified drop from the "both" position to each magneto which can assure you of proper engine operation.

Magnetos typically work just fine if they are well maintained. In a 40 year old tank it is easy to have sufficient pitting on the breaker points to reduce the magneto's efficiency to virtually nil. If the engine and accessories were correctly overhauled, they should give years of service. Often, spares come from cannibalization where the history of the part is unknown. Sometimes overhauled engines are installed and the old accessories are reused without overhaul. Thus, you could end up with a 300 hour since overhaul engine with a 3500 since overhaul magneto!

The homebuilt aircraft industry has had great success with electronic ignitions for years, where there is a market with all the new aircraft and engines being built. With a particular overhaul facility maybe overhauling 5 Merlins a year, I'm not sure that it would be worthwhile to engineer a system. It is easier just to overhaul the magnetos which typically give excellent service and are redundant anyhow.

: a: Why didn't the army do this while it was in service?, a
: ll those old war stories you hear (especially about aircra
: ft) whereby they had a 'Mag Drop' and the engine did not w
: ork. Surely a conversion like this would make much sence.
: Also does any WW2 aircraft fly with a similar conversion..
: I know that some restorers want straight out of the facto
: ry rstorations.. But for safetly.. Wouldn't you want somet
: hing other than a Magneto?

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