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