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  #31  
Old 12-14-2018, 07:56 AM
DanH's Avatar
DanH DanH is offline
 
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Quote:
Originally Posted by RV7A Flyer View Post
I used Lord mounts with the inner isolator part, and have experienced no problems...but I'd really like to know for sure what the root cause of the failures of those gel-filled things is.
When the inner spacers pinch the gel bag against the inside of the isolator rings, the ends of the bag will be peeled back if the engine moves in the mounts. If I had to pick (caution, opinion), I'd say they sagged enough to pinch the bags due to a combination of four factors.

The first two are pretty common. #1 is calendar age, about ten years since hung, and eight flying. That's probably beyond the manufacturer's suggested replacement interval. Shame on me.

Second is supporting an IO-390, heaviest of the 4 cyl models, with a metal Hartzell. There is a very large difference between this combination and an 0-320 with a Catto, yet it's the same isolator part number. Seems logical that big motors will need new isolators at shorter intervals.

Loading is a little less predictable. This one sees 3.5G on an irregular basis. Some RVs never pull G, some acro on every flight.

The unusual factor is heat. I operate with lower cowl temperatures which are somewhat higher than average.
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  #32  
Old 12-14-2018, 10:08 AM
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Just one other data point (therefore anecdotal). I just pulled an engine - Superior XP400 - from an RV14A for the Superior recall. One of my 4 mounts had failed with the other 3 in excellent condition. Time in service is 21 months and 380 hours - mostly cruising but some mild acro. The failure was eccentricity of the stiff discs with a ruptured gel donut. While it looked like it could have been an installation error where one of the interior tabs on the disc snagged/pinched the membrane on the donut, I now believe because of the eccentricity and this discussion that it is a similar failure mode to what Dan has described. These 390/400 engines can move a lot, and can shake pretty violently at start/shutdown.
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  #33  
Old 12-14-2018, 11:42 AM
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Example of the eccentricity due to sag. You can see how there is very little room remaining for the center spacer and gel bag, and the raised ribs can't help:



Thickness of this bottom compression element is way down too, about 0.86" rather than the original 1.115"~1.085" spec range.

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Last edited by DanH : 12-14-2018 at 11:45 AM.
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  #34  
Old 02-18-2019, 08:24 AM
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The rest of the story. Previously I wrote:

Quote:
Originally Posted by DanH View Post
Ok, the 50016-08 acro mounts are now installed, and I got an opportunity to get in three hours today over to Baton Rouge and back.

Initial impression: Vibration transmission is acceptable, but different. Previously it was smoothest around 2400~2450. Now it's glass smooth at 2700, acceptable at 2450, and shakes the glareshield at 2350 and below.

There are two possible explanations, maybe more. It was dynamically balanced recently, but the Lord mounts were sagging enough to have internal contact, so that may have skewed the balance process. It feels like it needs balancing, but I know that human perception is a very poor gauge. The other possibility is perhaps more likely. Short version is that I've raised the system's natural frequency, so it's now closer to the disturbing frequency, increasing transmissibility. Isolation gets better as the ratio of Fd/Fn is increased, and I've decreased the ratio.

I'll get another balance and we'll see. More later.
The answer seems to be ....both.

With the VIP acro mounts, the airframe shake below 2400 felt like an out of balance prop. Vic had done a dynamic balance not long ago, before I discovered dripping silicone, and realized how far gone my Lord mounts really were. With new VIP acro mounts installed, we were both curious about how failed Lord mounts might affect the dynamic balance process. If one or more gel bags and center spacers were against the inner steel ring, would it skew the dynamic balance results?

Answer...apparently yes. I flew over to Fresh Fried Chicken where Vic and I repeated the balance done just a few months ago. Getting down to 0.01 required the removal of a #10 screw, washer, and compact nut installed when we balanced with bad mounts. IPS immediately dropped below 0.04 (or maybe it was 0.03). A washer or two in the right place dropped it to 0.01. The lesson, now learned, is obvious; check your mounts before balance.

However, when flying home, there was no significant difference in cockpit feel. Pulling the prop to down to around 2300 would still make my sunglasses dance. Ok, fine. If it ain't balance, it must be resonance.

As previously noted, the VIP acro mounts are simply two thin isolators and a slightly different internal bushing length. I called VIP, and they were willing to sell me the thick isolators and the correct bushings, since I already had plenty of thin isolators. The resulting thick-thin set is exactly what you would get from Vans or Aircraft Spruce. Blocked out the day Saturday and got 'em in there.

Drum roll please; a short test flight yesterday confirmed the return of good vibes. It's been a long time since I had new Lord mounts, so a direct comparison is hard to judge. There is no objectionable behavior with the VIP mounts; I can pull RPM down with fairly high MP and the previous large amplitude shake is gone. I did notice one difference between the VIP and Lord mounts. With the Lords, I recall pulling 3G would cause a noticeable change in engine sound and feel. There does not seem to be any change with the VIP mounts. I assume G would push the Lord gel bags into contact with the rings. I do not know how long ago I noticed the sound change with G; could be after they had sagged a little.

I'll fly the new isolators a while and eyeball them for sag. Given this is a return to the "one size fits all" isolator set, I'm interested to see if the large engine actually does shorten isolator life. Seems logical that a 320 would not load the isolators as much as a 390.

For those who wish to know more, there is a nice tutorial regarding isolation and transmissibility in the Lord isolator catalog:

https://www.lord.com/sites/default/f...torCatalog.pdf
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Last edited by DanH : 07-22-2020 at 06:37 AM.
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  #35  
Old 02-18-2019, 03:21 PM
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Default great info

Thanks Dan - this is great info. I learned a lot, and I suspect many others did too.
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  #36  
Old 02-18-2019, 03:31 PM
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https://www.youtube.com/watch?v=KvyUK10y0Cc&t=182s

Start watching at 2:05. Interesting how soft the mounts are.
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  #37  
Old 07-22-2020, 12:33 AM
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Quote:
Originally Posted by rocketbob View Post
https://www.youtube.com/watch?v=KvyUK10y0Cc&t=182s

Start watching at 2:05. Interesting how soft the mounts are.
FYI: I recommend you use this link to go right to the applicable part:

https://youtu.be/KvyUK10y0Cc?t=86
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