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Vibration and strange EGT readings at small RPM range. (rotax)

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Vibration and strange EGT readings at small RPM range. (rotax)

Old 11th Mar 2019, 08:53
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Vibration and strange EGT readings at small RPM range. (rotax)

An interesting issue for those who know Rotax's for the last few hours of flying, the 912ULS has developed a niggling vibration, this vibration shows up when moving from full power to max power in the take off roll, usually at about 3/4 throttle, once at full power, its ok. in cruise at 5100 rpm it is very slight, 5200 RPM is smooth as should be, just noticeable vibration. at 4700 rpm, its as smooth as a rotax should be, between 4700 and 5100 rpm, the vibration is very noticeable..
to try to fix this, i have done the following.. Balanced carbies, balanced the prop, checked prop track and pitch, all good. Leads have been tested anre are within limits, but not new. carbies have been recently overhauled by rotax. engine still starts fine, very quickly, even when cold.

so, 2nd last flight, i performed a high power mag check at 5100 rpm results were.
Mag 1 off, EGT's all changed the same amount.. back on.
Mag 2 off, EGT on cylinder #2 steadily fell from 700 deg C to about 300 deg c, back on, all back to normal.
(#3 EGT failed completely and reads 0, and still read 0 for the next testing)

so, 1 spark plug change later. (all plugs looked fine), back at cruise power, again, same test, same result. hmmm
, back on both mags, i started experimenting with RPM settings to see at what RPM the vibration was the most noticeable, and the results were interesting.

4700 rpm, no vibration,
4800 rpm, small vibration..
4900 rpm, largest vibration, but also, the same EGT drop on #2 Cylinder!! from about 700 to 500 deg C.
5000 rpm, same, still lower EGT on #2.
5100 rpm, #2 EGT comes back up to normal temps around 700 Deg. in line with other readings.

Any rotax engine guru, have an idea on what might be happening?

Thanks.
Ultralights is offline  
Old 11th Mar 2019, 10:27
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Have you tried the rec flying forum? Plenty of gurus there. PM me if you want me to give you the details of a very good guru.
Ia8825 is offline  
Old 11th Mar 2019, 11:03
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A change in EGT means a change in air to fuel ratio, have you checked closely for air/ vaccum leaks? I had a slight vibration ages ago when I owned an O320 but only at certain rpm's, was a split induction tube hose! Two plugs per cylinder not only gives redundancy it also improve the flame front across the entire piston face for better combustion which in turn effects EGT's.
Hope you can get the Rotax gurus on to it👍
machtuk is offline  
Old 11th Mar 2019, 12:09
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Perhaps check the o-ring between the drip tray and the intake manifold (obviously on both carbies). It sounds like an induction leak.

The behavior of engines when running on one mag never ceases to amaze me. Sometimes (rarely) my engine will splutter and Iíll switch off one coil only to find that a plug has fouled... but yet strangely this is noticeable in flight with both coils running. Go figure.


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Old 12th Mar 2019, 00:32
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Sounds like ignition timing....preignition to be exact.
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Old 12th Mar 2019, 00:57
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Originally Posted by Professional Amateur View Post
Sounds like ignition timing....preignition to be exact.
Timing on the one cylinder? That would suggest an ignition module has failed for one of the two plugs that it services. I guess thatís conceivably possible and only at high rpm.

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Old 12th Mar 2019, 06:13
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im not sure its an induction leak, as the effect only seams to be with 1 cylinder, the other fed by the same carby has no issues at all, unless the induction leak is at a point in the manifold past the split. apart from the rpm range, everything is fine...

what makes me think its an ignition issue is the fact that when on 1 mag, the cylinder EGT drops rapidly from cruise temps to way less than half. though the strange thing is the same happening at a set rpm with both makgs on, but the drop, while noticeable within a minute, is nowhere near as rapid as with 1 mag off.
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Old 12th Mar 2019, 08:32
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Can you swap the coils / plug leads over on those? That would confirm if it was a coil.
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Old 12th Mar 2019, 11:02
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Its harmonics most probably in the rev range you describe. I will find you the Rotax info tomorrow. Basically dont stay in the 4600 to 4900 rev range is their recommendation.
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Old 12th Mar 2019, 11:05
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Good operating practices
Successful operation of the 9 series engines depends on understanding its differences and treating it accordingly.

The basic difference is that this is a geared engine that is designed to run at 5500rpm. The other conventional aircraft engines are direct drive and run at about 2500 RPM.

Gearbox: anytime you have a propeller and pistons connected you can have problems, huge forces are at work and the gearbox is able to handle them but the operator must treat the engine properly in order to maximize the longevity and reliability.

�� This is not a snowmobile or chainsaw, do not use rapid throttle movements as this causes undue wear on the gearbox. Smooth and steady is the mark of an expert.

�� Avoid low idle speeds; at idle the piston pulses are more pronounced and the gearbox has to deal with a lot of pulsing. This is worse when compounded with a heavy prop. (Rotax has a limit for the propeller “moment of inertia”)

�� Do not take off or cruise at low engine speeds. The engine was designed to take off at 5800rpm and run its whole life at 5500 rpm; the ignition, carburetion and valve timing are all designed to be at their best at this rpm. Don't run the engine in the 4600 to 4900 rpm range for any length of time because of induced harmonics.

o Low piston speeds actually contribute to piston wear as the “rocking motion” duration is increased.

o High prop loading at low rpm increases stress throughout the drivetrain.
o More combustion byproducts (carbon and lead) build up in the cylinder head with low engine speeds.

�� Avoid excessive carb heat; this is not a C-150, this engine is not prone to carb ice so the teachings of the average C-150 pilot are not relevant. If safe and sensible a quick check for a normal rpm drop when carb heat is applied is all that is needed.
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Old 12th Mar 2019, 13:08
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Rotax data points for variable pitch props are 4300,4800,5000,5500 rpms respectively for 55,65,75 and max continuous power. This doesn’t seem to fit with your 5500rpm recommendation.
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Old 13th Mar 2019, 06:02
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speaking to a few people over in the USA, the general consensus appears to be an ignition issue, the give away being the egt drop to zero combustion levels on 1 mag. and the slow small drop is a result of incomplete combustion at certain rpm settings, as only 1 plug is firing in that cylinder. the reason why is at rpm either side of the rough zone allows complete combustion from the 1 plug, so, next time i fire it up, ill change the modules over, and confirm a failure, then replace the modules, ill replace all leads at same time as well
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Old 13th Mar 2019, 09:00
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Originally Posted by Sunfish View Post
Rotax data points for variable pitch props are 4300,4800,5000,5500 rpms respectively for 55,65,75 and max continuous power. This doesn’t seem to fit with your 5500rpm recommendation.

Sorry its not my recommendation, its from a Rotax document on the Rotax owners website . EG a document published by Rotax on operating recommendations
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Old 13th Mar 2019, 09:36
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Mcoates, please point me to the document concerned. I know there is a document about not pitching the prop for. full power at low rpms. is that the One? My numbers come from the latest rotax operating manuals on the flyrotax website.

So for example 4300 and 55% power is OK but I guess anything more than that power is a no no at that rpm.

As an aside, After a lot of to-ing and fro-ing it appears that as the fuel injected version runs LOP below full WOT , power is set not by MAP but fuel flow.

There is what rotax call a “propeller demand curve” on most of their graphs which I think documents the relationship you. speak of.
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Old 13th Mar 2019, 12:05
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Originally Posted by Sunfish View Post
So for example 4300 and 55% power is OK but I guess anything more than that power is a no no at that rpm.
The numbers are all governed by the Manifold Pressure. Agreed Sunfish, it is hard to work out and it's not clear on the Rotax website. Smarter people than yourself have also struggled to work out the best settings.

https://www.rotax-owner.com/pdf/ROAN%20FAQs.pdf

Given that you have sold yours, I wouldn't be losing sleep over it.

One of the biggest mistakes you can make is to fit a constant speed unit in the hope of a higher top-end speed at altitude. Simply put, it doesn't happen, unless you're turbo charged.

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