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Bad vibration with a perfect balanced propeller and almost new engine

  • Rodrigo Echenique
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2 months 2 days ago #17796 by Rodrigo Echenique
Rodrigo Echenique created the topic: Bad vibration with a perfect balanced propeller and almost new engine
Hi,
I have a Dynamic WT9 plane with a Rotax 912 100 hp with 166 hrs, I flew the plane and suddenly the engine started to vibrate worst and worst every time.
I did all the things that friends told me that can be the cause, I know very well this carburators because they are used in the BMW motorcycles so I did a complete check and changed the diaphragms and did the syncronization using a two bar mercury instrument, I changed the spark plugs, checked all the rubers and hoses, I built a special bench rig to balance the propeller to 0.1 gram at the tip of the blade, I set the pitch of the blades with a laser at 20 mts to be super acurate, I did everything and the engine vibrates a lot.
My propeller is a Woodcomp three blade constant speed in flight ajustable, I changed the propeller with a Bolly Magna carbón fiber super light ground adjustable propeller perfect balanced too and the engine vibrates very bad too.
Some friends told me that the problem is in the gearbox, but an engine with only 166 hrs can have a damaged gearbox?
In case it is the gearbox how can be solved?

Regards from Mexico!

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2 months 2 days ago - 2 months 2 days ago #17802 by Alexander Kluge
Alexander Kluge replied the topic: Bad vibration with a perfect balanced propeller and almost new engine
First and foremost, I am not an expert at all.
Is there any variation with rpm or with load? You certainly did check the carb compensation tube for damage (mine had been burnt)? Is it as bad as with a failed ignition module? Compression? Stuck valve? Engine mount?

By determining the frequency of the vibration, you could prove the gearbox or propeller hub to be the culprit when the frequency is simply related to rpm/2.43. If the frequency is related to rpm, the cause might be anything but the last wheel of the gearbox or the propeller... I don't know if the "sirometer" is precise enough, but it's cheap.
Last Edit: 2 months 2 days ago by Alexander Kluge.

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2 months 2 days ago #17803 by Alan Arnold
Alan Arnold replied the topic: Bad vibration with a perfect balanced propeller and almost new engine
Try moving each carb around by pushing on the air filter. Up and down and sideways. An air intake leak could cause this sudden onset and a cracked carb mount fitting might be the problem.

Alan

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2 months 11 hours ago - 2 months 11 hours ago #17822 by Rodrigo Echenique
Rodrigo Echenique replied the topic: Bad vibration with a perfect balanced propeller and almost new engine
When I did the syncronization of the carburators I removed the equalizer tube between the two carburators and also I removed each carburator to check the floats and all the internal parts of the carburators and the rubber flanges where in perfect conditions, not any crack or any leak, I also did a leak test and I don´t have any air leak in the system.
Regards
Juan Manuel
Last Edit: 2 months 11 hours ago by Rodrigo Echenique.

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1 month 3 weeks ago #17879 by Kevin Stewart
Kevin Stewart replied the topic: Bad vibration with a perfect balanced propeller and almost new engine
I have seen a similar issue to yours (although at 700 hours) and it turned out to be a damaged inlet valve. Compression leakdown tests proved normal and it was only when the cylinder heads were removed and inspected that the damamged valve was found. The seating area of the valve had become thin leaving a sharp edge around the value. Due to the wear, the valve stem protruded too far toward the rocker meaning that the hydraulic lifter could not adjust for the extra stem length. This lead to poor compression when the engine was running.

The fault developed over a number of hours until the plane was virtually unflyable. A replacement inlet valve fixed the problem.

I have had gearbox problems on mine but it only ran rough when the engine was not loaded i.e. in the cruise.

If the engine is very rough the I would be considering either an internal engine fault (valve, cam shaft, crank shaft phasing), a severe fuel problem or an ignition timing problem. I did a fault finding mind map a while ago, take a look it might give you some things to look at: www.rotax-owner.com/en/rotax-forum/3-4-s...ation?start=20#14393

Does the oil in the tank smell like a machine shop? Are there any metal particles on your magnetic plug?

Good luck, I hate these types of problems.

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1 month 3 weeks ago #17893 by Rodrigo Echenique
Rodrigo Echenique replied the topic: Bad vibration with a perfect balanced propeller and almost new engine
Hi Kevin,
My oil is clean and I checked the magnetic plug and I found very little metal particles but now I think that my oil filter collected more because I found a severe wear in the gear and clutch dogs (see the pictures)

I open my gearbox and found that the ramp dogs on the gear and the clutch have some wear in fact in the pictures you can see a groove done by the clutch dogs into the gear dogs and this is incredible because this engine have only 166 hrs and my vibration started at about 160 hrs.
In the pictures you can see that the gear and the clutch dogs have wear and kind of pitted.

All the other parts are very well so what I did was I machinned and revamped the lobes of the gear and the clutch to the same identical shape of the new gears and dogs, I did this in a 5 axis CNC milling machine, not an easy or simple job.

If I did a good job I will be able to use this gears again and the vibration will stop, if I don´t did a good job I will need to change the gear set and also the dog of the clutch but I wanted to try anyway.

I am waiting for the various shim washers I order to pack again my box to the máximum pressure and see if this cures the vibration I had.

I am sure that with the wear the pressure was less and this caused some slip and this could be the cause of the vibration.

In the mean time today I dissasemble again both carburators to see if one of the rubber manifolds are cracked and no, they are perfect!

After I repair the gear box I will do a compression check to see if it can be some valve but I don´t think so because my engine ran very well and with a lot of power, just the vibration that anoyed me.

I will let you know when I solve my problem because this can be something that can happen to others.

Best regards and Happy New Year!

JM

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1 week 1 hour ago #18264 by Rodrigo Echenique
Rodrigo Echenique replied the topic: Bad vibration with a perfect balanced propeller and almost new engine
Problem solved!!
My gearbox is like new and I have been flying some hours and it is perfect not even the slightest vibration.

Regards
The following user(s) said Thank You: Wayne Fowler

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6 days 4 hours ago #18276 by James Stroud
James Stroud replied the topic: Bad vibration with a perfect balanced propeller and almost new engine
What was the solution?

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