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  #1  
Old 10-25-2017, 07:10 AM
Bill.Peyton's Avatar
Bill.Peyton Bill.Peyton is offline
 
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Location: St. Louis, MO
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Default Engine/Prop vibration

Over the past 20 hours or so I have noticed that for the first 45 min to 1st hour of flight I have a slight vibration at most any RPM, which previously was not noticeable. Over a period of 45 min to hour of cruise flight, the vibration continuously gets less and less until it is not longer noticeable. This is not a lot of vibration, but enough that my wife noticed it. It's enough to notice in the glare shield, and on the stick. It is consistently repeatable and can be duplicated by letting the aircraft sit for several hours. Flying again for 45 minutes fixes the vibration. I can shut off each ignition in flight and not make any change to the vibration, so it's not an ignition issue.

Engine: IO540 D4A5 690 engine hours

Propeller: Standard RV-10 Hartzell Two blade blended airfoil HC-C2YR clocked in the standard position

I just concluded my annual inspection and did the following, with no change in the symptoms:
  • Reamed all 4 exhaust guides per Lycoming Service Instruction 1425
  • Removed left mag inspected, replaced points and cam, set e-gap etc. and re-timed
  • Verified timing of Electroaire electronic ignition
  • Serviced, gapped and measured resistance of all plugs
  • Measured resistance of all plug wires
When I review my Saavy data online it looks absolutely perfect, I can tell the engine is developing full HP in that my cruise speeds have not changed. My fuel flows and leaning have not changed. My CHT spreads are the same as prior spread a year ago as are the EGT spreads. I bore scoped the cylinders.

The only remaining idea that I have is that the prop grease seal has been compromised and grease is getting into the center of the hub and settling when the plane is sitting idle. When the prop starts spinning and the hub gets filled with warm oil, the grease redistributes itself and the vibration disappears.

So as I write this I am about to leave to drive Hartzell this morning to have them tear down the prop. and hoping that is the issue. Any other ideas from the VAF brain trust are appreciated.
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  #2  
Old 10-25-2017, 07:33 AM
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airguy airguy is offline
 
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Are you certain that the vibration is RPM-dependent, meaning coming from the prop or engine?

I had an interesting vibration chase on my 9A where I finally pinned it down to one of my gear fairings with a broken rivet that would fishtail a bit at cruise speed and cause the gear leg to resonate and transmit the vibration to the airframe. My "Ah-ha!" moment came when I started playing with the RPM and noticed the vibration did not change at all, in intensity or frequency, as I changed my engine RPM from 2700 down to 2000 as long as I maintained a decent airspeed.

Your 45 minutes to an hour timeframe could be caused by something similar, and only happens when the wings tanks are full of fuel and you get close to a resonant frequency. Once you burn off some fuel the resonant frequency gets away from the excitation frequency and it will stop resonating.

Just a thought...
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Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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  #3  
Old 10-25-2017, 03:09 PM
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Bill.Peyton Bill.Peyton is offline
 
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Good Ideas. When I get back from Hartsell assuming the prop is OK thatís the next place Iíll start
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  #4  
Old 10-26-2017, 07:57 AM
rich rich is offline
 
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Try applying your brakes. Might be a out of balance wheel.
Had a nose wheel that would start spinning if I exceeded 180mph. It was out of balance and caused a vibration. Asked my wingman to take a look. He saw the nose wheel spinning. Tightened up the axel bolt and balanced.Vibration gone.
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  #5  
Old 10-26-2017, 11:13 AM
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dwrichey dwrichey is offline
 
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I'm fishing here. Rubber engine isolation mounts heat up and stop transmitting?
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  #6  
Old 10-26-2017, 11:56 AM
WA85 WA85 is offline
 
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Location: Madison, AL
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Default Vibration Frequencies / Dampening

Before you spend the time and money on the prop disassembly, here are few ideas to possibly consider -

1. The (hot) oil in the typical Hartzell passes through a small tube in the center of the hub and feeds the oil to the prop dome, which imparts pressure on the piston inside it to vary pitch. The (hot) oil and grease are typically are not in direct thermal contact, so the grease doesn't typically "liquefy" too much and move around. Prop grease is designed to remain semi-solid through a wide temp range, not sure that linking prop vibes to grease propagation due to the effects of hot oil is probable. (but anything is possible)
2. The grease in the typical Hartzell prop is around 3" from the center of prop rotation and has fairly low mass, so its not likely that the mass of the grease at 3" can impart large mass moments / imbalance vibrations, provided the prop is greased properly. Typically, the standard Hartzell prop grease task is to pump equal amounts of grease into each blade grip. Once the engine is run, centrifugal force will compact the grease into the hub and blade bearings and it pretty much stays there. The only grease seal I have seen in the typical Hartzell prop is around the blade shank and it keeps the grease in the hub around the blade shank. Not sure if there are any grease seals internally that prevent grease propagation and becoming a cause of mass imbalance.
3. I have troubleshot many CS props for "inflight vibrations". A typical dynamic balance should take the "in-plane" dynamic balance down to 0.2 to 0.05 IPS. Note that dynamic CS prop balance can only measure the balance at the max RPM tested and whatever blade angle that provides. Balance can only be improved by making mass changes "in plane" to the prop rotation. The out of plane vibrations potentially caused by minor out of track blades or thrust imbalance or pulses cannot be altered with mass changes. Also, when the aircraft is on the ground, the airframe's interaction with the ground through the tires and landing gear can dampen resonant vibrations in the airframe. Once you are inflight, the prop pitch is going to be significantly difference than what is was on the ground (when you achieved perfect prop balance) and may cause resonant airframe vibrations from prop thrust impulses impinging on the airframe. Remember that most dynamic prop balance is done at fine RPM and max RPM, on the ground, which isn't what you get in flight. It is quite typical for airframes to have RPM specific resonant / natural vibration frequencies despite the prop being in perfect "in plane dynamic balance". The fix is to use vibration dampening materials in the cabin or operate outside the resonant natural frequency. Sometimes an inflight vibration is reported to come and go over time, think about the changes going during that time, burning fuel / mass dampening changes / temperature changes / speed / trim changes. They all have effects on vibration dampening and what you feel.

I fly an RV-9A that has significant resonant vibrations in he floor and dash at certain prop and power settings, yet its prop is balanced to 0.07 IPS. I put a case of bottled water on the co-pilots floor board and the vibration goes away...but its had to carry passengers then.
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  #7  
Old 10-26-2017, 12:17 PM
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airguy airguy is offline
 
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Quote:
Originally Posted by WA85 View Post
I fly an RV-9A that has significant resonant vibrations in he floor and dash at certain prop and power settings, yet its prop is balanced to 0.07 IPS. I put a case of bottled water on the co-pilots floor board and the vibration goes away...but its had to carry passengers then.
Thread drift - but try the turn-down tips on the exhaust pipes to route the blast downward away from the belly skin, worked for me.
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Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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  #8  
Old 10-26-2017, 06:25 PM
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Bill.Peyton Bill.Peyton is offline
 
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Thanks for all the responses. Just a note, this vibration just started about 20 hours or so ago. Prior to this it was smooth as can be and had been dynamically balanced to .0233 IPS.
I just returned from the Hartzell facility. Not a bad drive from St. Louis. These guys are unbelievable to deal with. Talk about customer service!! They allowed me to witness the entire operation. I showed up at 7am and they promptly disassembled the prop. Matt, the tech support engineer looked at the grease that had leaked into the hub and thought that there was a good probability that this could be the issue. I'm not so convinced. The grease that is in the cavity where the #2 is written should not be there. What I am convinced of is that following the Hartzell prop servicing procedure of pumping in grease on the leading edge zerk fitting until it comes out of the trailing edge zerk fitting hole, with not more than 6 pumps, will yield an over lubed prop. In the photos below you can see the grease that has leaked by the inner o-ring seal and found it's way into the center section, which should normally be void of grease. This prop has been serviced 6 times in the 690 hours it has been on the RV using this method.
[IMG]IMG_1337 by Bill Peyton, on Flickr[/IMG]
[IMG]IMG_1338 by Bill Peyton, on Flickr[/IMG]
[IMG]IMG_1339 by Bill Peyton, on Flickr[/IMG]


In addition to re-sealing the hub, Hartzell checked the overall static balance, blade alignment, blade angles and leading edge blade alignment and all were well withing spec. Hopefully I will get to test this out this Saturday. If it is not the culprit, then on to another idea which Airguy posted. It reminded me of what happens when the nose wheel not have enough break out force, it will cant sideways in the prop wash. This might be causing some amount of vibration for a period of time could be another place to look. I have installed new engine mounts, so hard to imagine that's the issue. Ill be sure to post the results of the prop. reseal
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Last edited by Bill.Peyton : 10-29-2017 at 07:46 AM.
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  #9  
Old 10-26-2017, 06:49 PM
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Ironflight Ironflight is offline
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Bill -

I think that the way you stated the Hartzell greasing protocal might be just a little misleading. You said "pump it in the leading edge until it comes out the rear, but no more than six pumps". That probably logically gets you to the same place as the actual recommendation, which is "put in six pumps - but stop if it comes out the rear fitting before you get to six". I have serviced three of these props for a dozen years now, and have never had grease come out the rear fitting with six pumps. There are many people who think that you are supposed to see grease coming out the rear - and this is just wrong .... it will definitely be overgreased, and you risk popping seals.

Like I said - I think that logically you get to the same place, but stating the "until it comes out the rear" first might empower those who think they should pump until the hub is "full".

Paul
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  #10  
Old 10-26-2017, 06:59 PM
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Bill.Peyton Bill.Peyton is offline
 
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Paul,
You are correct with your wording, which is better stated than mine.

I usually see grease exit the trailing edge fitting hole prior to achieving six pumps, except for the last service, which took the total six pumps. The service tech who re-assembled the prop suggested nor more than two pumps should really ever be needed unless grease has been leaking from around the seal. He also noted that they have seen a couple of props come in for overhaul with the entire inside cavity completely filled with grease.
Bill
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