Any Ideas why this Broke Off

[lr172]

On my pre-flight yesterday, I noticed that part of the aluminum cover plate on my spinner was cracked and bent. The crack went 90% through to the end and was bent 90*. There are no visible signs that the cover plate was touching the prop and I am at a loss as to how it happened. I know there is a lot of vibration out there, but it seems like an odd part to have fatigue damage.

I wonder if anyone has seen this or may have an idea what caused it.




Larry

[Vlad]

Larry I think it's a perfect example of a stress riser from a burr around rivet hole. It propagated during the many of vibration and finally the tab disintegrated. I also have a micro crack but it stopped propagating long ago.

[Steve Melton]

like Vlad said, a rivet hole stress riser in an area of plate bending because of the centrifugal load starts the crack. once the tab weakens and the fwd edge rises into the flow stream adding some flutter and it doesn't last long. cee ya!

[fixnflyguy]

If it makes you feel any better, I have seen this several times on certificated aircraft as well. Bummer part is taking the prop off and doing re-work/paint !

[AlexPeterson]

It looks like a contributor might be edge distance of the forward most rivet. That rivet almost appears to have the shop head on the outside? Same for the next rivet. Something about them appears strange.

This area sees about 1000 g's acceleration (w^2 * r / 32) at 2400 rpm, assuming that the radius from the crank center is about 6". There would be tension originating from this region of the plate and spinner flexing outward due to this acceleration, which would put a shear load on that rivet. Probably not vibration related, but certainly could be.

[lr172]

Originally Posted by AlexPeterson

That rivet almost appears to have the shop head on the outside? Same for the next rivet. Something about them appears strange.

That part of the spinner got a thicker coat of paint than it should. The surface tension of the wet paint keeps it on the rivet head and it can't flow off into the valley around the rivet. What you are seeing is a somewhat non-uniform build up of paint on the rivet head.

[sblack]

There is probably more vibration there than anywhere else on the airplane. Always good to give the spinner a good look at the walk around. I found cracks on mine once and had to replace it. I know of at least one fatality caused by a spinner failure on takeoff. Not sure why it caused a crash - perhaps it was distraction at the wrong moment but it resulted in a stall spin . The airplane (One Design) was in a steep climbout and the pilot did not get the nose down in time.

Like everything else, if something happens on takeoff you have to force yourself to fly the airplane first, then take care of whatever it is. The good news is that these cracks are usually quite visible and you can find them before they become a problem if you are looking. Yours is an easy fix.

[az_gila]

Another contributing factor may be that the support tab under the blanking plate is smaller than that shown in the plans.

The plans show it taller and with it's forward edge following the spinner cutout.

Less support at the forward tip could allow more vibration.

[lr172]

Originally Posted by az_gila

Another contributing factor may be that the support tab under the blanking plate is smaller than that shown in the plans.

The plans show it taller and with it's forward edge following the spinner cutout.

Less support at the forward tip could allow more vibration.

This makes sense. I didn't realize the type of G force that Alex mentioned on this part. Having the support piece run all the way to the prop likely would have prevented the stress of that end piece trying to be pulled away by 1000 G's. I think I will also throw in another rivet near the end point.

Looks like I have a new project.

Larry

[rightrudder]

It would help to epoxy the aluminum to the fiberglass before riveting, to spread the load out a bit. Use G-Loc epoxy that remains a little flexible when it sets up.