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912UL Engine vibration on throttle back

  • Kevin Stewart
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5 years 2 months ago #4054 by Kevin Stewart
Kevin Stewart created the topic: 912UL Engine vibration on throttle back
Gents I need some advice.

I have a 2003 Evektor Eurostar with a 912UL running a Woodcomp Klassic 3 blade fixed pitch prop. The engine has done about 700 hrs. Max static RPM is about 4800 and idle about 1400.

The engine has been well maintained and has worked flawlessly since I bought the plane 4 years ago. Just recently I have noticed a problem. If I am in the cruise at about 4400rpm the engine is smooth. When I throttle back it becomes rough and the roughness is noticeable on the rev counter with the needle oscillating instead of remaining steady. It is not a severe vibration but it is noticeable.

Sometimes the problem would go as quickly as it came. Other times it could be cured by applying power back to cruise setting.

More recently I have noticed that the vibration is not so quick to go and reapplying cruise power no longer fixes the problem.

I have looked through the various 'vibration' related threads and I see that there are a number of issues to look at. This is what I have looked at so far:

  1. Carb sync (which is good)
  2. Carb diaphrams for splits
  3. Carb pistons for sticking
  4. Throttle cables seem smooth
  5. Main needles
  6. Main jets
  7. Engine mounts

It does not seem to exhibit the problem when in the climb. It is more likely to be when off load, either slowing in the cruise or during descent.

I have not checked the prop balance or blade pitch. As far as I am aware nothing has been changed on the prop and the gearbox was serviced and reshimmed at 400hrs.

I might see if I can pick up the vibration using my iPhone microphone and trying to see the frequency of the vibration using one of the free spectrum analyser apps. That might tell me whether it is a prop blade problem or an engine problem.

Any other thoughts and suggestions would be gratefully received.

Regards,

Kevin

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5 years 2 months ago #4059 by Andre El Ghawi
Andre El Ghawi replied the topic: Re: 912UL Engine vibration on throttle back
Hi Kevin,

This sounds like an electric problem and not mechanical. You need to look at all your wires running from your ignition. You could have an either a loose connection somewhere or a damaged ignition wire which by the way is hard to detect.

I would start out scouting and looking over all the wires for lose connection or faulty wire leading to and from your ignition system and then I would look at the ignition wires.

I am almost sure that this is not a mechanical issue.

Please keep us updated and good luck.

Andre'

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5 years 2 months ago #4061 by Kevin Stewart
Kevin Stewart replied the topic: Re: 912UL Engine vibration on throttle back
Hi Andre

Why would it be an electrical fault if it seems to vary with load? Perhaps a high voltage problem might be load related but I cannot think of a mechanism that would link load to the low tension side of the ignition system.

As far as I can tell from the Rotax service info there is no link between the ignition modules and any form of load sensing. Since the only input appears to be engine speed then I would imagine that my fault would occur at particular engine speeds rather than throttle positions/loads.

I am happy to be proven wrong. In many ways an electrical problem may be easier to fix :)

Kevin

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5 years 2 months ago #4062 by Roger Lee
Roger Lee replied the topic: Re: 912UL Engine vibration on throttle back
Hi Kevin,

I'm with you and don't think this is electrical. Usually electrical is there or it isn't and usually doesn't change with rpm. Intermittent electrical issues are like chasing ghost. They're there when you don't want them and elusive when you are looking for them.

Is the low rpm vibration issue repeatable each flight? If the rpm vibration is always down in the less than 2500 rpm range then suspect something in the idle jet which you should be able to confirm with a set of vacuum gauges, not an electronic one. You can't diagnosis very well with the electronic balancer's, but gauges will tell you several things and which carb.

Here are a few things to rule in or out. First a double check on a carb sync at idle and up to at least 3500 and up to 4000 rpm, it may have changed. Check the throttle cable on the carbs and make sure both move fore and aft smoothly from idle to full throttle on the ground. I was going to say something about the gearbox, but it looks like it was done at 400 hrs., but maybe not done right? The Woodcomp prop should have the prop flange bolt torque checked and have a dynamic balance performed. Checking the prop blades for individual pitch against each other never hurts if you have not done so, but I don't think this has anything to do with your problem. Your engine mounts could be another cause if over 5 years old.

On another note your engine would be happier in cruise at 5000 rpm or more and a WOT flat and level prop pitch setting to get over 5500 rpm instead of cruising in the mid to low 4000's.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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5 years 2 months ago #4067 by Kevin Stewart
Kevin Stewart replied the topic: Re: 912UL Engine vibration on throttle back
Hi Roger,

The vibration is repeatable every flight. The engine seems to need to be up to full temperature and a reduction of power from 4400rpm to 4000rpm can be enough to bring on the fault. I have checked the carb sync at various power settings and and it is better than 0.25"Hg between sides.

When the problem manifests itself I have switched off each ignition in turn and it makes no difference to the roughness. I deduce from this that the problem is either mechanical or fuel related. The prop is the only item that I haven't really looked at closely. I'll check the blade pitch and bolt torque but I think that a closer inspection of the throttle cables may be in order.

It is a shame that there is no easy way to see the individual manifold pressures. I take it that flying with the balance tube disconnected and a couple of gauges fitted is not advisable.

The 4800rpm maximum static rpm is dictated by our national aircraft approval body when using the Woodcomp prop so I cannot increase it. I am not sure of the rationale behind the limit, I would be interested to know if anyone has any ideas.

Kevin

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5 years 2 months ago #4069 by Kevin Machniak
Kevin Machniak replied the topic: Re: 912UL Engine vibration on throttle back
Hi,

I would like to clarify from what you said that you synced the carbs at the rough running 4000rpm area ?

I had rough running in the 3000- 4000 range I did a sync at 3500, verified good sync at 5000 . This solved my problem.

One other thing I think possibly could help is dynamic prop balancing.

do you know the current friction torque numbers of your gearbox ?

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5 years 2 months ago #4070 by Kevin Stewart
Kevin Stewart replied the topic: Re: 912UL Engine vibration on throttle back
Hi,

To be fair I did the sync at about 3000rpm mainly because I was stood next to the engine with the gauges whilst my assistant operated the throttle. I don't think that it is a sync problem as the fault is not always present at the same load/speed. I feel it to be more of something sticking and not returning to its correct position.

On the subject of carb sync what would be safer would be cable adjustment at the cabin end of the the throttle cable. You could then sit in the comfort and safety of the cockpit and carry out the sync without going anywhere near the engine.

I have no idea of the friction torque values of my gearbox. I would be interested in any links to methods of measuring this as well as how to carry out dynamic balancing. The latter I believe requires some accelerometers and test kit.

Kevin

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5 years 2 months ago #4071 by Claus von Reibnitz
Claus von Reibnitz replied the topic: Re: 912UL Engine vibration on throttle back
Hi Kevin,

we had exactly the same problem with our formers club plane (a 1998 model EV-97 with a 912 UL (80 hp) w/o slipper clutch and a Woodcomp SR2000). The problem did only appear in flight while throtteling back. Carb balancing didn't fix it and we ruled out electrical problems also. Replacing the engine mount rubber grommets did help either. It came down to a propeller issue. The last thing we were investigating before I left the club was either a blade warping (wood core) or a slip within the blade fixing at the hub (this issue I have seen with several props, either through wear or excessive tolerances: just put a small piece of paper around the blade root before tightening the bolts).

Just my two cents...
The following user(s) said Thank You: Rodrigo Saldarriaga

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5 years 2 months ago #4072 by Kevin Machniak
Kevin Machniak replied the topic: Re: 912UL Engine vibration on throttle back
How to test friction torque of the gearbox can be found by going to the top of this page , click support, then engine manuals , I believe it is in the line maintenance manual but I could be wrong . I know it is one of the manuals,either line or heavy maintenance , as I follow the manual when I do it on my engine.I just don't have them available right now , there at the hangar.

Most aircraft mechanics have access to a dynamic prop balancer, some clubs even have there own or one can be had from aircraft spruce. you could try a google search of dynamic balancing as well for all kinds of info.

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5 years 2 months ago #4073 by Kevin Stewart
Kevin Stewart replied the topic: Re: 912UL Engine vibration on throttle back
Claus,

Thanks for your information, The prop is the one thing that I haven't looked at yet.

I have spoken to another Eurostar owner at my airfield and he has the same problem. He has a different prop to me, a Kiev 3 blade fixed pitch which is virtually new.

I probably won't be able to carry out any further investigation until the weekend.

Kevin

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5 years 2 months ago #4074 by Roger Lee
Roger Lee replied the topic: Re: 912UL Engine vibration on throttle back
Hi Kevin S,

You have a UL so you most likely don't have a slipper clutch in the gearbox to test. The exception would be if one had been added.

If someone has a slipper clutch in their 912ULS or UL if it was added it is an easy test. Some pre 2003 ULS engines don't have slipper clutches either so you will need to check if you don't know.
Here is an overview of the slipper clutch check.
Just measure from the center of the prop shaft out along the prop blade between 26" - 30" and put a little mark.
Take the crankshaft locking pin screw out, you can look down inside the screw hole and rotate the prop until the "V" notch comes into view which is located on the shinny part of the shat lobe. Insert the crankshaft locking pin. When inserted correctly it will be sticking out 3/4" and seated in the center of the "V" notch. The prop will now have a 15 - 30 degree free movement with some friction before it stops. This friction is what you are going to measure. Put a piece of cord around the blade where you marked it. Then using a fish scale (I like the digital fish scales) attach the scale and pull the prop through the free play. Read the scale during the pull. It will read in foot pounds. Take that number and multiply it times the number of inches you marked out on the prop.

i.e.

28" x 16.3 ft/lbs = 456 in/lbs That would be the slipper clutch friction check.

You can read this in more detail in the manuals, but this should give you a good overview.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell

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5 years 2 months ago #4075 by Kevin Stewart
Kevin Stewart replied the topic: Re: 912UL Engine vibration on throttle back
Ah, mine does not have a slipper clutch which is why I have not had to make any friction measurements in the past.

Thanks for the explanation.

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5 years 2 months ago #4077 by Kevin Machniak
Kevin Machniak replied the topic: Re: 912UL Engine vibration on throttle back
Sorry, I didn't realize it was the 80hp ul .

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5 years 2 months ago #4083 by Claus von Reibnitz
Claus von Reibnitz replied the topic: Re: 912UL Engine vibration on throttle back
Hi Kevin,

comparing the engine carrier to the original Rotax and other installations I noticed that the particular Eurostar carrier consists of rather thin and long tubes. Insofar that might from a vibration standpoint also contribute. We had repeated cracks in the exhaust mounting. As these are attached at the same point as the lower engine carrier fixing points this lead to failure of the thread in the crankcase. This warrants closely examination of any cracks developing in the carrier itself (although we didn't find any in our case).

Regards

Claus

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5 years 2 months ago - 5 years 2 months ago #4150 by Mike Brown
Mike Brown replied the topic: Re: 912UL Engine vibration on throttle back
Kevin:

I've explored vibration issues in the past with my 912 UL. In my engine installation there are two rpm bands where vibration is noticeable - 3,800 to 4,100 rpm and 4,300 to 4,800 rpm.

I guess I'll start by saying there's a known vibration problem with the 912 ULS. Rotax Service Letter SL-912-010 (attached) identifies the problem as resonance in the 3,600 to 4,800 rpm range. There are a number of contributing factors listed in SL-912-010, but Rotax does not identify a definitive root cause. Rotax does say the problem is often reduced by the installation of a slipper clutch.

Now here's what I've found. First, it's important to understand the nature of the problem. The 912-series engines are very well-balanced except for one vibration mode that's intrinsic to four cylinder boxer engines. The layout of the cylinders means that the engine produces "rocking couple" vibration that twists the engine around a vertical axis and tends to wag the prop from side to side. This vibration mode is not well understood in the Rotax community, imho. It means, for example, that the flat bed-type engine mounts found in many aircraft are not a good idea as the flat bed configuration is poor at damping rocking couple vibration. In my aircraft, which has a bed mount, there are witness marks around the Lord mounts that show the Lord mounts are moving from side to side. A dynafocal type mount is a better engine mount for the 912 as it's much better at damping rocking couple vibration.

As part of my vibration exploration on my 912 UL I installed a 912 ULS gearbox (without slipper clutch) which has a 2.43:1 reduction ratio in place of the standard 912 UL gearbox which has a 2.273:1 reduction ratio. I found the vibration was worse with the 912 ULS gearbox with all other factors the same (I did repitch the prop). I also took noise measurements with a dB meter and got a very interesting result with the 912 ULS gearbox. There was a noise peak at 4,500 rpm that was not present with the 912 UL gearbox. I interpreted that as mechanical resonance.

This result - more vibration with the 912 ULS gearbox - together with Rotax's suggestion in SL-912-010 to install a slipper clutch, leads me to the conclusion that vibration issues on 912 series engines are the result of an interaction between aerodynamic propeller loads and the inherent 912 rocking couple vibration.

So, what to do about it? In principle the answer is a well-designed dynafocal engine mount to adequately damp rocking couple vibration and a slipper clutch to break the direct mechanical connection between propeller and crankshaft.

On your aircraft, I'd start with prop pitch. There's something about the aerodynamic load on the prop blades that's exciting resonant vibration when you throttle back. We don't really know what it is so you'll have to experiment. An overpitched prop exerts more driving force on the gearbox when throttle is reduced. Your prop is overpitched, so the first thing I'd do is reduce the blade pitch to increase your static rpm above 5,000. At the same time I'd ensure that the pitch of all three blades is identical.

Then I'd suggest you inspect your engine mount very carefully. Look closely at each mount point for any damage or cracking. Look for capture washers contacting the mount where they shouldn't be. Make sure the engine mount's in factory condition. The rubber shock mounts are (may be) available in different hardnesses with different damping characteristics. Experiment with different shock mounts.

If all else fails, install a slipper clutch. This is not often done on 912 UL gearboxes as there's something about the 912 UL gearbox reduction ratio that makes the 912 UL less prone to the vibration problem. But it would undoubtedly help on the 912 UL just as it does on the 912 ULS.

Mike Brown

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Last Edit: 5 years 2 months ago by Mike Brown.

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5 years 2 months ago #4155 by Kevin Stewart
Kevin Stewart replied the topic: Re: 912UL Engine vibration on throttle back
Hi Mike,

Thanks for your info. What is interesting about my problem is that it only started about 12 months ago. Before that the engine always ran smoothly. It is very intermittent in nature and can go of its own accord.

Unfortunately I haven't had any more time to investigate what is happening since I reported last.

I cannot change the prop pitch as this is mandated by the governing body to which my aircraft is affiliated.

I'll keep you all posted with any progress.

Kevin

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5 years 2 months ago - 5 years 2 months ago #4156 by Mike Brown
Mike Brown replied the topic: Re: 912UL Engine vibration on throttle back

Kevin Stewart wrote: Hi Mike,

Thanks for your info. What is interesting about my problem is that it only started about 12 months ago. Before that the engine always ran smoothly. It is very intermittent in nature and can go of its own accord.

In that case I would focus on the engine mount. Something has changed about how the engine vibrations are damped. I would be concerned that the problem - whatever it is - is progressing so that the vibrations are now present in other phases of flight.

If you do figure this out, let us know. I'm always interested in learning why I was wrong. :) No doubt the problem will turn out to be a waterlogged prop blade or something random like that.
Last Edit: 5 years 2 months ago by Mike Brown.

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5 years 1 month ago #4428 by Kevin Stewart
Kevin Stewart replied the topic: 912UL Engine vibration on throttle back
The vibration problem has been getting slightly worse over the last few flights but new information has come to light.

Today I noticed that with the engine off but the electric fuel pump on, fuel could be seen gently seeping out of the float bowl gasket of the port carb. Removing the air filter and looking into the venturi revealed fuel coming up past the needle.

My initial thoughts are that the float needle is not shutting off the fuel completely when the float bowl has reached the correct level. This in turn causes the bowl to become pressurised forcing fuel up the main jet turning my carburated engine into a fuel injected one. This will make the mixture rich on one half of the engine. This probably explains my vibration.

The float needle and seat look OK as far as I can tell. Does anyone have any experience of how best to proceed from here?

I will probably print out the relevant pages from the Rotax manual and check the fuel level in the float bowl, etc.

Kevin

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5 years 1 month ago - 5 years 1 month ago #4430 by Russell Hildebrand
Russell Hildebrand replied the topic: 912UL Engine vibration on throttle back
Hey Kevin,

Do you have a fuel pressure gauge on the airplane? Check to see what the fuel pressure is when it happens. Sometimes as you throttle back the fuel pressure will increase for a moment. Couple that with a warn float needle valve, you may be getting transient high pressure fuel getting through the valve causing a momentary rich mixture. As you know, these engines can sometimes vibrate pretty well creating warn float chamber parts. Just a thought....now, back to football.

Russ
Last Edit: 5 years 1 month ago by Russell Hildebrand.

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5 years 1 month ago #4431 by Kevin Stewart
Kevin Stewart replied the topic: 912UL Engine vibration on throttle back
Hi Russ,

Yes there is a pressure gauge but the reading never varies at any phase of the flight.

I have had the carbs on the bench today and fed them with fuel which I have pressurised. The one that leaked on the plane still leaked but the other one was fine.

I decided to swap over the float needles and lever arms. This cured the problem on the port carb. I then changed the lever arm back and it was still fine. I then refitted the original float needle and this time no leak! However, looking in the float bowl there was some black debris. This may have been flushed out by my experimentation. I blow through the needle valve with compressed air and rebuilt both carbs. Both now appear to be leak free.

You can hear the fuel filling the bowl initially and then it goes quiet when the floats shut off the valve. There is no longer any sign of fuel leaking from the bowl or coming up through the main jet.

I am reluctant to drive the 2 hour round trip to the plane only to find I have the same problem again but I cannot think what else to do. All of the parts that I inspected looked OK so I am in no hurry to start replacing items for no good reason.

Kevin

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