Spinner causing imbalance ?

[Darren S]

Hi All,

Statistics: RV-7, Superior IO-360, Hartzell C/S, dual Slick mags, 490 hours on all.

Recently had mags rebuilt, plugs changed, hub greased. The plane has an aluminum spinner. I didn't build the plane so I'm not sure where the builder got it from.

I have previously had dynamic prop balancing and at cruise setting (22 MP, 2350 prop) all is wonderful. All flight parameters are smooth but between 800 - 900 RPM there is a pretty significant shake.

At idle all is smooth, above 1000 RPM all is smooth but when on the ground and I pull the power back to that 850 RPM zone, shake, shake, shake.

The spinner is the original one from 2006, there was no shake prior to the engine work. When I remove the spinner and ground run the engine, no shake, just like it use to be.

My mechanic says to change the spinner but I would like to try something before I scrap it. Some say live with it as it is only in that small range that the problem occurs. I bugs me though as the engine was never like this. Usually upon start up the engine would run like a sewing machine!!

How do I dynamically balance the spinner ? Probably can't ?
Will doing another dynamic prop balance help at this low RPM ? Or is that primarily for cruise settings ?
Is it a result of the recent engine work and some harmonics at this RPM ?
Has anyone else with an aluminum spinner had this issue ? I've inspected the spinner and hub for cracks and there aren't any. All screws are tight too.

I need some help and some ideas if anyone has any.

Thanks,

Darren

[Darren S]

Is this worth trying ? Do you think I'm dealing with a spinner mounting issue or a spinner imbalance issue ?

Check this link from the RC model world. I'm willing to give it a try

http://www.rcgroups.com/forums/showthread.php?t=587540

[Flybuddy2]

I do the dynamic prop balancing at our airpark for the local group. The prop is balanced with the spinner on so it will take any imbalance into account. It's possible that the spinner was not placed back on the exact same way which would shift weight. Another possibility is mag timing which was just changed is causing a rough idle. Since dynamic balancing is relatively cheap I'd take it back and have done again, ask them to check at 2000 rpm or higher as you had done previously but also at idle. I've checked at various RPMs and they generally run the same. The balancing weights on a lycoming simply go on the flywheel. You can test this yourself by removing the weights (mark the position they came off from) and see how she runs without them.

[TS Flightlines]

Walt Aronow of ExpAircraft-----
Tom

[Mel]

Have you checked the back-plate VERY closely for cracks?

[n700jl]

Darren,
I have seen this before. You need to regrease the prop. Start with a new tube everytime. make sure there is no air coming out of the gun. remove your zert fittings from the one side on both blades and pump the grease across the prop. My guess is that one hub got some air pumped into it and this is causing your imbalance. We balance these props with only about 30 grams of grease. and it does not take to many pumps of air to mess things up. try it and let me know what happened.

[Walt]

Sometimes you just have to start eliminating things one by one, but if was good before the work you did than you can be pretty certain it was something you did that caused it.

If you take care to apply an equal amount of grease to both blades then any balance change will be minimal, if you fill one side of the hub and not the other though you will certainly create an imbalance (make sure when greasing you apply grease at the leading edge of the blade and remove the zirk at the trailing edge). Personally if the temperture is warm, I like to purge the old grease when greasing, this has the added benefit of insuring both sides have the same amount in them.

Make sure the spinner is installed correctly and is not wobbling, then have the balance rechecked. Spinners don't go bad and don't need balancing on their own, the dynamic balancing takes any spinner inbalance into account.

If none of that works then put the old plugs back in and check it and then onto the mags if you still have the problem.

[Darren S]

Ok, thanks for the suggestions. I'll double check the spinner backplate for cracks but I don't think my problem lies there. I checked it close today.

I'm not sure about messing with the flywheel weights especially when everything was fine to begin with and this issue only happened since the recent engine work.

I had thought about the prop grease. I even called the mechanic that did the job. He said that they count the number of pumps that go into each hub so that they are equal. Possibly air got in there??

Another fella told me that he fills the hub till grease comes out the other side. That sounds better to me. More equal.

I didn't know if the weight of grease would be enough to throw things off balance but from what you are saying Jon it can be!!

I will take the plane back and get the prop grease double checked because the spinner can't go out of balance just like that.

Thanks again for the help. I will report back.

Darren

[Mike S]

Originally Posted by Darren S

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Recently had mags rebuilt, plugs changed, hub greased.

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I need some help and some ideas if anyone has any.

Thanks,

Darren

Any chance that when your mech greased the hub, he might have installed the spinner 180* out???

Would be an easy thing to change it, and then do a test flight.

Let us know what it turns out to be, might help someone else in the future.

[Walt]

Originally Posted by Mike S

Any chance that when your mech greased the hub, he might have installed the spinner 180* out???

Would be an easy thing to change it, and then do a test flight.

Let us know what it turns out to be, might help someone else in the future.

Good point, I always "assume" RV spinners will only go on one way but a pre-drilled aluminum one may go on either way (I've never met anyone good enough to make a hand drilled spinner go on both ways!).