Prop vibration/ balancing

My RV-7A came with a Catto three blade prop on an AeroSports Power 0-375.
I've flown it almost 100 hours and have always been impressed with the smoothness and lack of vibration with this prop.
A few months ago I decided to have a refurbishment done at Catto Props.
They fixed nicks and painted the prop with new colors. They also performed a static balance procedure. I installed the prop yesterday and flew the plane for about 1.5 hours. At startup and taxi I noticed a significant amount of vibration that was previously not there. However, after taking off and accelerating to cruise RPM of about 2600rpm, the vibration was gone. Cruised for a while at 9500 ft, at 2500- 2650 RPM and felt very little vibration, ( like I was used to).
When I returned to land and reduced speed to land, the vibration was back!

My hangar mate has a Dynamic Balancing kit and we are planning on performing this procedure tomorrow. Note: There are a couple of weights installed on the starter gear from a previous dynamic balance done by the original builder.....
We are planning on removing these weights from the starter gear and then checking it with the instrument and adding weights as required. Hoping that this will solve the excessive vibration issue.

Would be interested in anyone's thoughts/ experience/ advice on this matter. I just find it curious that IF the prop is out of balance, wouldn't I see vibration throughout the RPM range? Again, the vibration is quite excessive at low rpm, but smoother than ever at 2600rpm......???

Thanks

The old balance weights are almost certainly throwing it out of balance. Remove those and go fly first to establish a baseline.

RPM sensitivity ?

I have been puzzled by this as sell. Max stAtic RPM is about 2000. I would need a hole in my cowl to do balance At cruise RPM. Not an attractive options, any suggestions ??

The vibration we feel is stronger at the lower rpms. It's a normal effect.

Dave

Rvator97 said:

... At startup and taxi I noticed a significant amount of vibration that was previously not there. However, after taking off and accelerating...
When I returned to land and reduced speed to land, the vibration was back!

Note: There are a couple of weights installed on the starter gear from a previous dynamic balance done by the original builder.....
We are planning on removing these weights...

Thanks

Click to expand...

Yep, you found your low rpm shimmy. An imbalance close to the hub will cause a shake at low rpm. Remove the weights before proceeding!

I once ran into a RV7 prop that shook like crazy at low rpm but was smooth at cruise speeds, turned it was the chromed prop spinner (puddle of chrome collected in one spot, chroming shop suspended the spinner in such a way that it didn't drain properly during the process). Changed the spinner & problem disappeared.

Did a dynamic balance yesterday. Got the readings down from .55 to .07. Test flight was great…. Extremely low vibration at all rpms.

I recently balanced my Sensenich prop. It was done about 5 years ago but had recently noticed significant vibs during cruise. Borrowed the local FBO's balancer. My max static is close to 2250 but I cruise around 2350-2450. I did the balance at 2200 RPM's. Of course removed the old weights for the base line... readings were around .57 ips. After several runs managed to get the ips to .04. In flight huge difference at cruise. But.... when power is reduced there is a range somewhere around the 2100-2200 rpm that I do feel the vibrations again. I just try to avoid those R's.

Couple questions. When balancing I initially was using the holes that are already in the starter ring gear but they are IIRC spaced at 30 degrees. Was unable to get anywhere close to desired ips numbers. Started over, I used the spinner back plate. One hole that was already there followed by a small one drilled by me. Only took about three runs to dial it in to the .04 ips. Is this acceptable to use the spinner back plate?
Also... after balancing, is the prop still considered balanced if it needs to be removed? (Of course putting it back in the exact same place). Just wonder if it changes.

Paul 5r4 said:

Couple questions. When balancing I initially was using the holes that are already in the starter ring gear but they are IIRC spaced at 30 degrees. Was unable to get anywhere close to desired ips numbers. Started over, I used the spinner back plate. One hole that was already there followed by a small one drilled by me. Only took about three runs to dial it in to the .04 ips. Is this acceptable to use the spinner back plate?
Also... after balancing, is the prop still considered balanced if it needs to be removed? (Of course putting it back in the exact same place). Just wonder if it changes.

Click to expand...

Short answers - yes, a single weight drilled through the backplate is acceptable, and a very elegant solution. And generally speaking, if you pout a balanced prop on with the same clocking, your balance should stay the same, assuming you haven’t taken the prop apart and put it back together. I have been doing a lot of prop changing the past year doing test runs, and have had excellent repeatability without rebalancing each time. In fact, I asked Jim rust at Whirlwind this a couple weeks ago, and he said the balance should stay the same….and he was right!

Paul

Thank you Paul! Appreciate the information.

Here's a couple of suggestions:

1. Prior to removing a prop that has been dynamically balanced, put a reference mark on the spinner and the prop to the crankshaft flange so that you can install the assembly back in the same position as removed.
   
2. If the prop is being removed for overhaul or repair be sure to remove any weights installed during a previous dynamic balance. When reinstalling the repaired/overhauled prop perform a new dynamic balance. Leaving the previous weights in place will guarantee an out of balance assy.
   
3. Do a Google Search for "Chadwick-Helmuth, The Smooth Propeller" written by Jim Fackler. This booklet will give you a lot of great information about installing weights both on the spinner backing plate or the starter ring gear, sensor locations, max allowed per hole etc.