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  #21  
Old 01-27-2019, 10:31 AM
SledDog SledDog is offline
 
Join Date: Nov 2010
Location: Lac du Flambeau, Wisconsin
Posts: 16
Default Similar Issue

Mark,

I found a very similar issue with the Sensenich ground adjustable prop on my Sport Cub. After working with Sensenich support, we determined it was cosmetic. The crack I have is in the paint where an internal metal strip ends and carbon fiber begins. Both blades had the paint crack in the same position with the same shape. Definitely work with Sensenich support.

Here's a picture:

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  #22  
Old 01-27-2019, 11:03 AM
Marc Bourget Marc Bourget is offline
 
Join Date: Apr 2009
Location: Stockton, California
Posts: 195
Default

Can you provide a fairly accurate measurement of the position of the crack in relation to the overall length of the blade (and in respect to the length outside of the hub) ?

Wonder if the crack correlates to a vibration node? (my term)
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  #23  
Old 01-27-2019, 11:36 AM
SledDog SledDog is offline
 
Join Date: Nov 2010
Location: Lac du Flambeau, Wisconsin
Posts: 16
Default Similar Issue

Mark,

Here is the email I received from Sensenich about my issue:

it's not uncommon to have slight stress cracks beside the metal erosion shield on a composite (or wooden) propeller.
the difference in metal and composite material properties can sometimes result in slight paint cracks where they overlap.

the primary concern here is not the crack, but whether there has been any loosening of the metal from the composite structure beneath.
small areas of loosening are allowable as long as they do not exceed airworthy limits.

a tap test with a quarter is done to detect any hollow or "dead" spots, which indicates a local debond.
a high pitch sound or tone indicates good structure or bond with no loosening.
a hollow or "dead" spot indicates loosening or an air pocket.
note: the tap tone is proportional to material density. it will change when going from all-composite to metal bonded over composite.
tap along the metal erosion shield to become familiar that that tone, and likewise tap the
inboard leading edge (without metal) to hear what all-composite should sound like.
if any hollow or dead spots are evident, then tap in a grid pattern to determine the overall debond dimensions.


airworthy limits:

total debond area under 0.39 in^2; this equates to a 5/8 x 5/8" square or a 1 x 3/8" rectangle.
no debonds over 1"

mark limits of paint crack with permanent marker and check during each preflight for growth.
if any growth is detected, then perform tap test and compare the airworthy limits above.


--------------------------------------


Here is the spanwise measurement of the crack. It's about 12 1/8" from the hub:



--------------------------------------


This may be related or not. The continued airworthiness for Sensenich GA propellers requires it be disassembled and inspected annually. It is very easy to get a mismatch in the blade pitch during reassembly. This sets up a very noticeable vibration. I found this out on a post condition inspection flight. The vibration actually got worse at lower RPM.
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VAF donation through November 2019
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  #24  
Old 01-27-2019, 11:37 AM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
Join Date: Apr 2012
Location: Collierville, TN (KFYE)
Posts: 1,071
Default

Absolutely...I've sent an email to Don Rowell at Sensenich and hope to hear from him tomorrow in re next steps. I have the blades in the car and can measure anything he asks for
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Titan IOX-370, Dual P-Mags, AFP FM200A FI, Sensenich GA Prop
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  #25  
Old 01-27-2019, 11:39 AM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
Join Date: Apr 2012
Location: Collierville, TN (KFYE)
Posts: 1,071
Default

Quote:
Originally Posted by SledDog View Post
Mark,

Here is the email I received from Sensenich about my issue:

it's not uncommon to have slight stress cracks beside the metal erosion shield on a composite (or wooden) propeller.
the difference in metal and composite material properties can sometimes result in slight paint cracks where they overlap.

the primary concern here is not the crack, but whether there has been any loosening of the metal from the composite structure beneath.
small areas of loosening are allowable as long as they do not exceed airworthy limits.

a tap test with a quarter is done to detect any hollow or "dead" spots, which indicates a local debond.
a high pitch sound or tone indicates good structure or bond with no loosening.
a hollow or "dead" spot indicates loosening or an air pocket.
note: the tap tone is proportional to material density. it will change when going from all-composite to metal bonded over composite.
tap along the metal erosion shield to become familiar that that tone, and likewise tap the
inboard leading edge (without metal) to hear what all-composite should sound like.
if any hollow or dead spots are evident, then tap in a grid pattern to determine the overall debond dimensions.


airworthy limits:

total debond area under 0.39 in^2; this equates to a 5/8 x 5/8" square or a 1 x 3/8" rectangle.
no debonds over 1"

mark limits of paint crack with permanent marker and check during each preflight for growth.
if any growth is detected, then perform tap test and compare the airworthy limits above.


--------------------------------------


Here is the spanwise measurement of the crack. It's about 12 1/8" from the hub:



--------------------------------------


This may be related or not. The continued airworthiness for Sensenich GA propellers requires it be disassembled and inspected annually. It is very easy to get a mismatch in the blade pitch during reassembly. This sets up a very noticeable vibration. I found this out on a post condition inspection flight. The vibration actually got worse at lower RPM.
The prop has been very smooth and I've been (up tp this point of course) very pleased with it.. I did pull the prop in accordance with their instructions last year and had no issues, so this is something that's happened in 2018. I haven't flown it yet this year.
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Titan IOX-370, Dual P-Mags, AFP FM200A FI, Sensenich GA Prop
First Flight 11/20/2016
www.marksrv8.com
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  #26  
Old 01-27-2019, 12:03 PM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
Join Date: Apr 2012
Location: Collierville, TN (KFYE)
Posts: 1,071
Default

The crack is 8-7/8" from the hub end of the prop.

Never having conducted a coin test, I thought I'd video doing this and put it up on youtube for anyone to watch and listen to. It's at https://youtu.be/_fzn2H0zJGg. I can't hear any difference between the area that's got the crack and other areas, but perhaps someone with a more experienced ear will hear a difference. Maybe it is just paint? I'll let Sensenich have a final word on that.
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Titan IOX-370, Dual P-Mags, AFP FM200A FI, Sensenich GA Prop
First Flight 11/20/2016
www.marksrv8.com
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  #27  
Old 01-27-2019, 01:13 PM
Gusmax Gusmax is offline
 
Join Date: Jun 2014
Location: Sherwood Park, Alberta
Posts: 32
Default

Did you ever put a dynamic balancer on to see if you had any strange vibrations prior to this issue?
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  #28  
Old 01-27-2019, 02:31 PM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
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Location: Collierville, TN (KFYE)
Posts: 1,071
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Quote:
Originally Posted by Gusmax View Post
Did you ever put a dynamic balancer on to see if you had any strange vibrations prior to this issue?
No, but it's been on the list for a while. Once I get this squared away, I plan to make it a priority
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Titan IOX-370, Dual P-Mags, AFP FM200A FI, Sensenich GA Prop
First Flight 11/20/2016
www.marksrv8.com
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  #29  
Old 01-27-2019, 03:18 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,009
Default

Quote:
Originally Posted by Mark Dickens View Post
Good grief...all I wanted to do was let others know....
I'm sorry -- I really was not trying to be critical, just cautionary to others. It was a good catch to find it. The prop is one of the things that is easy to look at every time you fly, and can have a big impact on the outcome of your flight.

And I like Walt's idea of the post-flight. I don't do that very often, but will start.

From my composites experience, it looks to me like a bending failure of the thin composite wrap over a wood propeller. That wrap may or may not even play a significant role in the structural strength and stiffness of the prop. But it does get highly stressed if/when the prop bends. Another post observed the high-stressed fibers at the outer edge of the cross section.

It will be interesting to hear what Sensenich says.
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  #30  
Old 01-27-2019, 05:42 PM
Mark Dickens's Avatar
Mark Dickens Mark Dickens is offline
 
Join Date: Apr 2012
Location: Collierville, TN (KFYE)
Posts: 1,071
Default

Quote:
Originally Posted by scsmith View Post
I'm sorry -- I really was not trying to be critical, just cautionary to others. It was a good catch to find it. The prop is one of the things that is easy to look at every time you fly, and can have a big impact on the outcome of your flight.

And I like Walt's idea of the post-flight. I don't do that very often, but will start.

From my composites experience, it looks to me like a bending failure of the thin composite wrap over a wood propeller. That wrap may or may not even play a significant role in the structural strength and stiffness of the prop. But it does get highly stressed if/when the prop bends. Another post observed the high-stressed fibers at the outer edge of the cross section.

It will be interesting to hear what Sensenich says.
No apology necessary. I just didn't want the thread to veer off in a different direction. Truth be told, I should have found this on a preflight. It just shows you that even after flying for 37+ years and somehow managing not to bust my you know what, there's always something to learn. I've always run my hand along the leading edge of the prop. A holdover from what I was taught in 1981...long before composite props. An old dog has to learn new tricks!
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RV-8 #81077 Super Slow Build
Titan IOX-370, Dual P-Mags, AFP FM200A FI, Sensenich GA Prop
First Flight 11/20/2016
www.marksrv8.com
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