C175 Spinner Cracks

[Keith 160]

I am AN A&P IA in Clarksville TN had a C175 land here Friday with a mag problem noticed The spinner had a wobble to it when they shut it down found that two of the mount bolt flanges on the aft spinner plate were cracked plane has less than 100 hours since last annual. Anyone have any Idea's as to what to look for as a cause?

 

[RotorAndWing]

I am AN A&P IA in Clarksville TN had a C175 land here Friday with a mag problem noticed The spinner had a wobble to it when they shut it down found that two of the mount bolt flanges on the aft spinner plate were cracked plane has less than 100 hours since last annual. Anyone have any Idea's as to what to look for as a cause?

Typically an out of balance will create this, either the spinner is out of balance or the prop.

Over torque, under torque will also effect balance and back plate cracking.

 

[Tom-D]

I am AN A&P IA in Clarksville TN had a C175 land here Friday with a mag problem noticed The spinner had a wobble to it when they shut it down found that two of the mount bolt flanges on the aft spinner plate were cracked plane has less than 100 hours since last annual. Anyone have any Idea's as to what to look for as a cause?

Poor design. It happens way too often.

 

[mondtster]

How do you know it wasn't cracked at the last annual?

If it were me I'd be looking for a vibration. It can be surprising how much stuff will break or crack when you have an out of balance engine or prop.

 

[Tom-D]

How do you know it wasn't cracked at the last annual?

Probably not, but when you remove them for inspection of the prop, and don't get them on straight they will vibrate and crack in very short order.

 

[TEFLONSEAN]

Probably not, but when you remove them for inspection of the prop, and don't get them on straight they will vibrate and crack in very short order.

+1

Less than 100 since annual....most likely they botched reinstall.

There can be other reasons...no doubt.

 

[mondtster]

Probably not, but when you remove them for inspection of the prop, and don't get them on straight they will vibrate and crack in very short order.

Understood, but my point was that I've seen a lot of mechanics take a lot of shortcuts. I wouldn't be surprised if the OP found that nothing had been done with the prop or spinner at the last inspection or two.

 

[bluerooster]

What engine? GO300? or something else? Just curious.

 

[Tom-D]

Understood, but my point was that I've seen a lot of mechanics take a lot of shortcuts. I wouldn't be surprised if the OP found that nothing had been done with the prop or spinner at the last inspection or two.

I doubt very much the backing plate was removed at the last annual, the only time that is done is when the prop must come off, that is not required at annual, but the spinner probably was removed, to inspect the crank & prop mounting bolts.

Very good chance that is was reinstalled 180 degrees out. Unless you mark it prior to removal, how can you tell?

 

[awquick]

This exact thing has just happened to me on my Mooney M20J with a Hartzell 2-blade Q-tip prop...66hrs since annual cracked aft backplate, fwd bulkhead and spinner dome....I can only put it down to the reinstall action after annual...although I didn't feel any vibration issues...the backplate actually failed catastrophically in the end and sent bits of aluminum flying into the engine bay and ground the lower cowling a bit...quite a shocking sight when I pulled the cowling! Now the challenge is getting it all replaced as this was a rare setup for Mooney...


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[Norman]

A trick I learned some years ago is to start all the screws loosely, push the spinner back on the support and then begin in the center between blades and snug the screws toward each blade. Haven't had a cracked spinner in years.

 

[Let'sgoflying!]

I read about how some prop balancer people would install the weights on the backplate and it was not designed for such a load....not to mention a new hole in the metal.
No idea if this was truly the case or not.

 

[Geico266]

I read about how some prop balancer people would install the weights on the backplate and it was not designed for such a load....not to mention a new hole in the metal.
No idea if this was truly the case or not.

It depends on the application. If you have access to the flywheel you use the existing holes. Engines like a Rotax don't have an external flywheel so you have to mount the weight on the bulkhead. An extra hole there with a properly installed screw & washers is not going to affect strength. IMHO.

 

[awquick]

In my case weights were installed in the form of additional washers on the bolts between the back plate and the flywheel/starter ring gear....I doubt this had any deleterious effect...

I think the failure mechanism went something like this:
- The spinner was put back on at annual with a slight out of balance condition
- over the next 66 hours the wobble went undetected by me but caused the backplate to crack...getting worse and worse
- to the point where a large piece of backplate separated and peeled back causing the interference with the lower cowl (which was a closer fit than the top due to the slight engine droop)
- the now large forces on the spinner caused the fwd bulkhead and the spinner dome and the ear support bracket on one side to crack


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[nrpetersen]

I am AN A&P IA in Clarksville TN had a C175 land here Friday with a mag problem noticed The spinner had a wobble to it when they shut it down found that two of the mount bolt flanges on the aft spinner plate were cracked plane has less than 100 hours since last annual. Anyone have any Idea's as to what to look for as a cause?

Let me guess this is with a 180 hp Lycoming conversion 175.

Most spinner cracking is caused by transverse vibration or torsional vibration, and not by simple out-of balance operation.

All engines have torsional vibration due to firing pulses. But only six cylinder engines are mechanically balanced for transverse vibration,

However, a much higher level of transverse and torsional vibration is present in all 4 cyl opposed engines, and it is at twice the crankshaft RPM - IOW about 80 Hz.

Some of this (the squirm) is because the left and right cylinders are not directly opposite each other. Some is because the crank throws are 180 degrees apart. Even a well balanced 4 cyl engine will want to squirm (about a vertical axis) about the engine CG, on its mounts at ~80 Hz (~2400 rpm). This vibration component is caused by the finite length of the connecting rods and is also the source of much alternator bracket failure on 4 cyl Lycoming engines.

In addition, the 80 Hz torsional vibration due to simple internal mechanics is much much greater than the 120 Hz component for a 6 cyl engine.

A spinner is a large highly stressed lightweight structure exposed to high frequency vibration. That's why it wants to crack. The centrifugal acceleration at 2400 rpm and a 6 inch radius is about 1000 g's so any modification or balance weight attachment isn't a trivial affair.

Otherwise I contend the spinner balance itself isn't really a major factor since it is only a 1/start cycle input.