vibrations

Surely I do follow that view and that line of reasoning . The fact there’s  no mention in the more recent chart is striking. To be honest, at first I did not regard the more recent 95/98 ROZ chart a replacement, perhaps because it looks so different, perhaps because I simply liked the former chart better, I didn’t know.

Closer look at the new chart to see if I’m reading it correctly: The ISA line at e.g. 2500 revs compared to the ROZ95 line is well above the ROZ95 data which indicates potential knocking. Knocking is no good! But “ISA” means sea level (and 15C and something more). Since MAP drops with height the two values move closer. Roughly, the difference is ironed out when flying at 3000ft to 3500ft - one inch per 1000ft as a rule of thumb- and the risk of knocking is gone (for that rev.-level). If that’s not a miss-interpretation both charts more or less say the same in a very different way.

Thanks for bringing in the new chart. It made me aware of the positive effect using higher ROZ gas, too.

Good talk about cruise RPM.  So.  Moving on, anyone have any ideas that may assist him as to why the vibration is appearing in flight? 

Hi All

I had to review all the thread.  In looked at all of this there are a few things.  First, the engine is reported to have 1200 hours (?) this is a big red flag since clearly the area of resonance is in the band of power that you should not really be cruising at.  (the design was for min 4800 and ideal for 5000/5200) On power reduction from full power or max continuous power the engine will go into its peak resonance just below max torque when reducing power.   This was well known and the reason that even with a UL version we now equip them with overload clutches.  Before 1999 the 912ULS was commonly sold with a small starter and no overload clutch.  It was so prone to vibration on powering down for approach in the mid range that Rotax published directions on how to determine if the OEM engine mount was suitable for the engine design.  I have attached that SL for you if anyone is interested. After 2003 it was rare any were sold without the clutch and the HD starter. 

First, it is in your best interest to take the gearbox off and check everything.  Reshim to the maximum preload by replacement of the Belleville washers and check the splines, both on the propshaft and overload clutch.  Look carefully at the dog hub and the wear on the gearset.  If you have to change the gearset always replace the dog hub at the same time is my recommendation.  

I have flown a few P92s and they are normally very good and not prone to vibration as an inherent issue.  Since this has been flying for a while however perhaps look very carefully at your engine rubber isolators to see if they have been worn or damaged with age.  Rubber will fail just by age alone.  Hard rubber shock mounts could be an issue.

Cheers

All...sorry typo, the 912ULS was introduced in 1999.  The major changes, due to the vibration issues I noted, occured in 2003 the year of the SL.  

Just for fun, the engine mounts that were worst were the Zenair 701, Rans S6, SeaRey, to name just a few.  The long tubes and bed mounts being the worst.  Ring mounts somewhat better.  Some aircraft, Kitfox comes to mind, had very soft rubber mounts and the only way to get a 912 ULS to work with them was to remove the spacer tube and cut it almost in half to clamp the engine down more.  (model 3 and model 4 vintage)  The newer versions of all these aircraft, using the overload clutch and HD starter, solved most of these issues.  The resonance will be absorbed into the clutch.  Obviously the clutch has to be working correctly as well. 

Cheers

Peter Kl. wrote:

Surely I do follow that view and that line of reasoning . The fact there’s  no mention in the more recent chart is striking. To be honest, at first I did not regard the more recent 95/98 ROZ chart a replacement, perhaps because it looks so different, perhaps because I simply liked the former chart better, I didn’t know.

Closer look at the new chart to see if I’m reading it correctly: The ISA line at e.g. 2500 revs compared to the ROZ95 line is well above the ROZ95 data which indicates potential knocking. Knocking is no good! But “ISA” means sea level (and 15C and something more). Since MAP drops with height the two values move closer. Roughly, the difference is ironed out when flying at 3000ft to 3500ft - one inch per 1000ft as a rule of thumb- and the risk of knocking is gone (for that rev.-level). If that’s not a miss-interpretation both charts more or less say the same in a very different way.

Thanks for bringing in the new chart. It made me aware of the positive effect using higher ROZ gas, too.

Peter

I think we are saying the same but a bit differently.

I read it that with RON98 fuel, under ISA conditions (pressure 1013.2mB, temp 15C, density 1.225g/cu.m, at MSL), you won't get knocking unless your MAP is above the orange line. Above 4000rpm, you can use WOT because you won't exceed the orange line. Below 4000rpm, to avoid knocking you must reduce throttle as necessary to keep MAP below the orange line e.g. 28.3" at 3000rpm.

With RON95 fuel, you're a lot more restricted and mustn't use WOT below 5500rpm, where you are right on the blue knock line at 28.6" MAP. Interestingly it doesn't get any better at higher rpm. At 3000rpm, you would have to reduce throttle more than with RON98, to keep below 26.3" MAP.

I think that MAP is the controlling variable in this - but I'm not certain. (I dropped out of fluid mech and thermodynamics!) If that's so, then if atmospheric conditions are such that at WOT you can get a higher MAP than the yellow line, the critical rpm to allow WOT operation will move a tad higher. I guess that means on a cold high pressure day at low altitude, with RON95 you might theoretically get knock with WOT even at max revs. I'd also guess there's enough margin of error in the figures to cover you against that effect.

Phew! What do you think?