A question about magnetos

[Gamma]

My first CFI told me that magnetos are designed to only produce a charge when turned in one direction. Therefore, when moving a prop by hand (not hand propping) always turn it opposite its normal direction of rotation to help prevent an accidental start. While "teaching" systems to my CFI instructor this came up and he had not heard of it. I realize that I've never seen any mention of this anywhere since and want to make sure I don't go around spreading bad information. I am hoping one of you experts can tell me if this is true or not?

 

[enigma]

I'm a decade or so removed from studying mags, but my memory says that a mag could possibly make a spark even if turned backward. However, since mags are self-generating, it is virtually impossible to make a spark at any RPM that could be attained by moving a prop by hand. Most Mags include an impulse coupling device that provides the necessary speed to make a spark at starting RPMs.

The impulse coupler IS one way only. It is the impulse coupling that makes that clicking noise when the engine is rotated by hand. Simply put, the impulse coupling connects the mag to the mag drive and it is spring loaded to allow the mag to "get behind" just before the spark is needed, and then it "snaps" the mag at the time when the mag needs velocity in order to produce the spark. It has a bob weight in it that "locks it up" at normal run speeds, because normal RPM provides enough mag speed to make a spark without the help. Because of it's design it can only snap in one direction. (Hopefully, someone like Avbug will give you a proper description)

I would imagine that your first instructor was speaking of this device.

Always treat the prop as if the mags were hot, regardless of the direction you are turning the prop.

regards

 

[Timebuilder]

Magnetos are the reason it was so hard to start the early Sportsters from HD.

Voltage generated by a magneto is directly proportional to armature speed, or "how fast it spins". Fuel dragsters use two magnetos, and at crank speeds of over 10,000 rpm, the voltage is high enough to cause a spark in those super-high pressure combustion chambers.

This speed isn't very much as you turn the prop by hand, but you have to remember that the position the prop is in MAY be the position where the impulse coupling is poised to release its stored energy, spinning the mag a portion of a turn to create a spark. The danger is in the possibility that there is a small bit of fuel and air left in the cylinder in question, and the turning of the prop can be enough to compress and then ignite that small mixture. This can cause the prop to move enough to cause a serious, if not fatal injury.

I taught my students to always treat the prop like a loaded weapon.

 

[bobbysamd]

Magnetos

Very good posts, Enigma and Timebuilder. Excellent mag review.

 

[DIRT]

I did not think you were supposed to turn a prop backwards ever because it is not good for the vanes in the vacuum pump. Maybe it's just an old wives tell....

 

[Timebuilder]

I don't turn any engines backward, whether a 172 or a Conquest. I turn the mag engine slowly, and stand clear as I do so.

You are correct. Vanes aren't designed to be turned backward.

 

[avbug]

The risk of damage to vacum pumps from turning a prop backward is insignificant. No vacum pump manufacturer cautions against it. I recently spoke to the folks at Airborne about their pumps and discussed this particular wives tale. They advised that they have never published data advising people against turning the prop backward, and that if sufficient contamination exists in the pump to cause damage when turning backward, it will likely cause the same damage turning in the normal direction of rotation.

A magneto may provide spark when being turned in either direction. At low speeds of rotation, less voltage is delivered to the plug to fire. One magneto on some airplanes (not all) is often fitted with an impulse coupling, which works off a cam and a spring to increase the rotational velocity in the mag, and produce a "hotter" spark when cranking the engine. This impulse coupling will not be activated when turning the propeller against it's normal direction of rotation, or backward.

By turning the prop backward, you are avoiding the impulse coupling, but the engine can still fire. The previous advice given is good advice.

If you're flying behind a geared engine, don't turn the propeller backward at all. If you do turn the propeller backward on any engine, good practice is moving it forward again a short distance to take up backlash before attempting to start it.

 

[Timebuilder]

I think it was a design engineer that told me not to turn a vane pump backward, due to some sort of stresses placed on the vane that were not considered in the design.

He could be wrong, though.

 

[LR25]

I have seen a few airplanes that sputter backward for a few turns when shutdown, no vacuum pump damage on those.

The biggest thing for vacuum pumps is contamination. I used to work on Navajo's, the vacuum pump is in a tight area and it is real easy when installing them to get some dirt in there if your not careful. If you do it always fails in about 5 hours of installation. They aint cheap! Boss man gets upset too.