Spinner backplate rivets

[Breezy]

Too All,

During a condition inspection for a RV-8, O-360, Blended Foil prop, approx 800 hrs on engine/prop; missing rivet heads (Manufactured head) were on the spinner backplate. They were both in the same quadrant of the plate, looked like they were simply sheared off. Backplate and spinner show no signs of vibration or cracking. No indication of unusual vibration anywhere on the airframe.

Anyone observe this?

 

[Ralph Inkster]

Had 2 planes come in exhibiting this. Replaced the broken rivet and installed another one about 1/2" away, plus one on opposite side of the backing plate for balance. One of those planes came back the next year with no further problems. The second plane came back with a crack in the center mounting bolt hole and the whole backing plate was replaced.
These instances were a couple years before I bought the prop balancer.

 

[Scott Hersha]

I am helping a friend with a condition inspection on an RV8 with a BA Hartzell, and I found the same thing on the backplate. Two rivet heads popped off, both adjacent to one of the blades. No cracking noted anywhere. I replaced both rivets. The prop hasn?t been dynamically balanced.

 

[BillL]

I have two missing rivet heads at 320 tach hours. No special vibration, most cruise at 2450-2500 RPM M1B Factory Lyc with standard mags and Cr. This is the Hartzell composite with a 2.5" spacer for the James Cowl, and Vans spinner. Balanced 200 hrs ago, w/weights added to the flywheel. 22 ft-lb torque confirmed on through bolts. Note the streamer from one rivet and to the other. No crack yet found after cleaning with naphtha. That could change if cracks are found.

Should the rivet size be increased? They had to be drilled in the center with #40 bit to punch them out, but is it bad form to just replace?

Odd, why just one blade? and the one with factory balance weights attached.

I will contact Hartzell to gather their comment. If no cracks, the rivets will be replaced and checked in the future as usual.

Any thoughts/comments invited. Thanks

edit: 7/28/25 No cracks found on close examination. Replacement stock size rivets (4-5) installed.

I think the cause is simply the stress field resulting from RPM and the 4 bolts attaching the spinner to the hub is "containing" the expansion of the discs. The rivets are failing in single shear fashion with a twisting effect on the stiffer backer plate. This is resulting in the head failure mode. An FEMA could define the stress/deformation field and yield a better sizing and location for this particular rivet. Estimated 400 cycles would indicate (very) low cycle fatigue. The mass of the glass spinner is additional on the perimeter of the disc and could be a bit off balance. Also, since the stress is high, having installed the rivets on top of paint may be contributing to failure in this loading case.

My take off is at 2700 rpm, but reduced to 2500 rpm after TO. Most cruise is 2400-2500 rpm. Phase I testing included speed runs at 2700 and other tests with speed changes. IFR practice approaches increase the 2500-2700 cycles.

I sent a note to Vans suggesting and FMEA on this, but not expecting any change. Not much interest in this is expected in light of low severity, low occurrence, production and making money. No judgement, no worries, just curiosity.