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Vibration.

Rhythmic vibration

Wrote a explanation of problem and it disappeared. “Gallop’ rhythm every 3 seconds. Engine data showed change in RPM every three seconds. We dynamic balanced the prop which was already good. We changed the spark plugs and the injector nozzles with no change. We put on a new governor at the condition inspection and the problems seem to go away for six hours and is now re-turning. Lycoming people have no clue. Prop people say to check the engine mounts. Has anybody had the same vibration problem and found the cure?
 
Honestly, my brain jumped to a fuel system problem given the cyclic nature.

What are the other parameters doing at the same time (fuel press, oil press, manifold, etc.)? What kind of systems are installed? Type of fuel? Does it get rough when the rpm changes? More info will get you to a solution quicker. Let us know.
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If the engine is truly "surging" in RPM, I would start with the prop governor as it is the most likely culprit. You can get surging from being too lean, but suspect many other symptoms to appear as well. Maybe the gov is not getting enough oil from the engine or is bleeding off too much pressure between it and the prop, due to bearings or other issues. Could also be a linkage issue where it is rhythmically moving the lever back and forth.

I would not consider surging RPM to be a vibration issue, unless the vibration is moving the cable and causing the surging.

Larry
 
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Thank you Guys. More food got thought.
I have tried switching tanks, electric fuel pump on and off. No change.
Oil pressure and temp are good and steady.
We took off MT governor and replaced it with PC?5000. Thought vibration was gone for about 6 hours, (hard to discern if turbulent), but now is returning. I am ignorant about the workings of a governor concerning oil pressure, linkage etc. Could I have an oil pump problem even though pressure to gauge is fine? Maybe looseness in the prop linkage? What about the counter balance weights in the prop itself?
I’m hoping it’s nothing too serious so that I can continue to chase the rabbit down the hole without a catastrophic event.
 
John, it might help if the group knew what engine, ignition, fuel system and RV you have. Do you have an electronic fuel flow transducer? Is the FF changing? How many hours on the engine? Morning Sickness? Is the exhaust system secure? Glass panel - any data download available?

Counterweights - aerobatic prop?

It could be something simple like a high fuel pump suction, or elevated fuel temps, or leaking prop piston o-ring, or the like. Knowing the system will help narrow down the suggestions.
 
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Thank you Guys. More food got thought.
I have tried switching tanks, electric fuel pump on and off. No change.
Oil pressure and temp are good and steady.
We took off MT governor and replaced it with PC?5000. Thought vibration was gone for about 6 hours, (hard to discern if turbulent), but now is returning. I am ignorant about the workings of a governor concerning oil pressure, linkage etc. Could I have an oil pump problem even though pressure to gauge is fine? Maybe looseness in the prop linkage? What about the counter balance weights in the prop itself?
I’m hoping it’s nothing too serious so that I can continue to chase the rabbit down the hole without a catastrophic event.

If a governor change immediately eliminated the problem, even if for a short time, that is further evidence that the governor is the source of the problem and something is preventing it from governing the RPM in a smooth fashion. I would continue down that path. It could be the governor itself or it could be problems with oil delivery to the gov or from the gov to the prop. counterweights have nothing to do with the governor, but the nose bearing and cross tubes do. There is a recent thread with someone having similar problems and the types of tests necessary to chase it down. I do believe that a linkage problem could also exist, though likely not a simple looseness issues, as the governor has a spring trying to force the arm to the fine position.

Lary
 
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Thanks again everyone. I have N772ME an RV-10 2012 with the Lyc IO-540 and Hartzell Scimitar prop from Vans with over 300 hours. Slick mags. Dynon Classic with FF. I have noticed in the past occasional drops in FF, but have not noticed it lately associated with this vibration. In fact, electric field pump on and off, switching tanks made no change to the rhythmic vibration.
I will check exhaust system, wheel pants and more with the governor. Curiously the vibration stopped after governor was changed at the condition inspection where cowl, wheel pants etc were removed and replaced.
 
Cause of vibration found!

I removed nosepant and vibration ceased. I replaced nosepant brackets that were cracked (mostly found on 10s on grass strips). Replaced nosepant and vibration recurred. I sent out nosewheelto anti splat for wheel bearing mod, balancing snd trueing. All reinstalled and no vibration! I will also replace nose gear bushings as there is still too much play.
In conclusion, an out of balanced and out of round nosewheel with a freewheeling wheel bearings was allowing the wheel to spin (only when the wheelpant was installed) and cause the shake repeatedly enough to feel it in the cabin. It was unsettling to me, but not noticeable to most passengers.
 
Antisplat

I can’t say enough good about Al and the guys at Antisplat. Changing out the wheel bearings, along with balancing and truing, eliminated my free wheeling nose wheel rhythmic vibration. (Also less shimmying on roll out.)They even gave me a better tire and tube at no charge. They are very quick with their one day turn around. Got it back in plenty of time for a Bahamas trip.
I hope this helps somebody else so they won’t go through Prop Balancing, governor change, spark plug change, Injector nozzle change etc.

I have also installed the Antisplat gust lock and it’s a super must have device.

I will install the oil separator and ez drain at the next oil change.

BTW the wheel chocks are nifty.
 
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