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  #21  
Old 08-27-2015, 09:26 AM
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ijustwannafly ijustwannafly is offline
 
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Location: Colorado
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Quote:
Originally Posted by N941WR View Post
My neighbor has had three mage failures in less than 150 hours. They have been attributed to the Sensenich ground adjustable prop. He now spins a Catto and the problems have gone away.
What does Sensenich say about this?
How were you able to trace this back to the prop with 100% certainty out of curiosity.
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  #22  
Old 08-27-2015, 09:36 AM
edbooth edbooth is offline
 
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Location: Trenton, SC
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Default Too many hours, tool little service

Quote:
Originally Posted by videobobk View Post
Mags do fail for other reasons. 30 years ago, rented 172, landed at an airport with the same FBO. Plane wouldn't answer mag check, and I was told to, "Fly it home." I refused, took a 182 back, and they tried to fly the 172 the 20 miles, only to have the other mag fail on the way. No damage, thankfully. I had a single failure on my 9A. BTW, no failures so far on my E-Mags.
My guess on those failures was too many hours, too little service.

Bob
Not necessarily. Had Slick mag fail on my RV-7 with TMX-0-360 at about 940 hours. Ordered new mag, ran very rough. Was in a hurry to get plane going so ordered another mag. This one really ran rough. Called Slick factory and got permission to open up first mag. Reset E-gap, reinstalled, ran just fine. Sent second mag back on warranty and was reimbursed. I think they may have some quality issues. Anyway so far so good.
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  #23  
Old 08-27-2015, 10:45 AM
sblack sblack is offline
 
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Just thinking out loud, but could the common cause for this type of failure be a nasty kickback from a flooded engine? I know that is ripped started castings apart (skytech). It wouldn't surprise me if it could strip a plastic gear. But I don't have direct experience with that, and hope not acquire any.

Nice to hear that it ended uneventfully. Our engines are so reliable these days that most of us never get a forced landing so most of us don't have any experience with them. We just hope that our training was good enough and we are current enough and we are lucky enough that if it does happen it will be a situation with some options.
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  #24  
Old 08-27-2015, 10:54 AM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by ijustwannafly View Post
What does Sensenich say about this?
How were you able to trace this back to the prop with 100% certainty out of curiosity.
They blamed the engine and the engine manufacturer blamed Sensenich. He gave up and bought a Catto and the problem went away.

What no one could answer is if the prop caused internal damage to his engine and no one wanted to pay for it to be disassembled, inspected, and put back together.
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Last edited by N941WR : 08-27-2015 at 10:58 AM.
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  #25  
Old 08-27-2015, 11:40 AM
JDanno JDanno is offline
 
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Location: Tennessee
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Makes a good case for eliminating the mags and installing electronic ignition!
I went with EFII ignition only and couldn't be happier. I know that's not an option for certified birds......too bad!
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  #26  
Old 08-31-2015, 05:27 AM
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pierre smith pierre smith is offline
 
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Default Update

After a two hour flight with the new mags, the engine became rough and upon inspection, several teeth had been lost on the mags!

A mechanic in Florida found what he thought to be the problem. The airplane has air conditioning and the drive pulley has two rubber cushions to soften the engagement torque of the a/c compressor, when it's engaged. the rubbers are 7 years old and not soft anymore. The jarring of the accessory gears as the compressor is engaged, causes the backlash that damages the plastic mag gears!

So they went out and flew several flights without engaging the a/c compressor and examined the mag gears...no damage.

They flew a few more times and engaged/disengaged the compressor and came back and found damage to the gears in the mags. The cushioning effect of the rubbers in the drive had hardened with age and seem to be the culprit!

Even though the compressor is belt driven, the rubbers still play a big role.

Whodathunk?

Best,
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  #27  
Old 08-31-2015, 06:47 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by pierre smith View Post
A mechanic in Florida found what he thought to be the problem. The airplane has air conditioning and the drive pulley has two rubber cushions to soften the engagement torque of the a/c compressor, when it's engaged. the rubbers are 7 years old and not soft anymore. The jarring of the accessory gears as the compressor is engaged, causes the backlash that damages the plastic mag gears!

So they went out and flew several flights without engaging the a/c compressor and examined the mag gears...no damage.

They flew a few more times and engaged/disengaged the compressor and came back and found damage to the gears in the mags. The cushioning effect of the rubbers in the drive had hardened with age and seem to be the culprit!

Even though the compressor is belt driven, the rubbers still play a big role.

Whodathunk?

Best,
The drive pulley is at the prop end of the crank, or is it an accessory drive item?

POSTSCRIPT: Found it:

http://cirrusengineering.blogspot.co...-bushings.html

This photo from 2010; apparently they started out with no soft elements in the drive pulley.

http://1.bp.blogspot.com/_6NMZbtOd12...0/IMG_7354.JPG

Don't know if this is the latest:

http://cirrusengineering.blogspot.co...g-washers.html

Given that the problem has been going on four or five years, I question that this particular mag failure was caused by an AC drive bushing issue.
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Last edited by DanH : 08-31-2015 at 07:02 AM.
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  #28  
Old 08-31-2015, 07:01 AM
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Walt Walt is offline
 
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Quote:
Originally Posted by DanH View Post
The drive pulley is at the prop end of the crank, or is it an accessory drive item?
Continentals drive everything directly from the accessory case.
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  #29  
Old 08-31-2015, 08:37 AM
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Neal@F14 Neal@F14 is offline
 
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Quote:
Originally Posted by ijustwannafly View Post
What does Sensenich say about this?
How were you able to trace this back to the prop with 100% certainty out of curiosity.
Somebody determined that the carbon fiber prop on the little 4 cyl Continental engines was suffering from 6th order harmonics being transmitted all the way down the crankshaft ending up causing chattering of the mag drive gears.
The little 4 cyl Continentals have a design flaw... they have no flywheel!!!
The carbon fiber prop on Lycs are not experiencing this issue because Lycs have flywheels which dampen out these high order harmonics.
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  #30  
Old 08-31-2015, 08:58 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by Neal@F14 View Post
Somebody determined that the carbon fiber prop on the little 4 cyl Continental engines was suffering from 6th order harmonics being transmitted all the way down the crankshaft ending up causing chattering of the mag drive gears.
The little 4 cyl Continentals have a design flaw... they have no flywheel!!!
The carbon fiber prop on Lycs are not experiencing this issue because Lycs have flywheels which dampen out these high order harmonics.
The prop is the "flywheel". It has a mass moment of inertia far higher than any other rotating component.

In general, the very large prop MMI is the anchor against which the other components oscillate, so yes, the amplitude of torsional vibration tends to be worst at the accessory end of the system.

A prop is typically modeled as a spring-mass system, with the blade roots being the spring and the blades themselves being the mass. Very stiff blade roots would tend to increase oscillation amplitude at the accessory end.

The Lycoming flywheel has little or no effect on torsional vibration in the first vibratory mode, because it is located near the node, the point in the system with no oscillation. Its effect at higher orders would require analysis, but again, it is a relatively small inertia located in proximity to a huge inertia with no significant connecting spring between them, other than the blade roots. I would predict its effect to be pretty near zip.

IIRC, pendulum absorbers on Lycoming cranks are tuned to the 6th and 8th orders. I don't know about Continental cranks.
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