rotax 582 harmonics

[robinsm]

Can anyone help me. I am running a new blue head rotax 582 with a c box (3:1) and a brolga 68inch 3 blade prop. The engine goes well but develops a bad harmonic vibration and noise between 5600 and 5900 rpm. The cruise range I prefer for this engine is 5500 to 5800 rpm. The motor I used to have was a 582 grey head with the same box and prop with no problems in this rev range.

 

Any Ideas would be greatly appreciated

 

Thanks

 

Maynard

 

 

[Guest Maj Millard]

Mmmmm......I've run in a couple of new blue tops this year, plus put a new set of rings in one . Can't recall any with problems in the range you mention. My initial thoughts are propeller....out of balance with a blade too heavy ?..Most are running 4 blade Brolgas maybe a harmonic between the blades and exhaust pulse ? .how many hours on the engine...some of them don't really smooth out for a bit. Or have you checked that both your carb slides are adjusted the same. Remove your air filter/s, and line the bottom of the slides with the top of the opening. Use the small brass adjuster at the to to line them up......and finally drop your fuel bowls off ,(don't loose the gaskets) and make sure your main jets are nice and tight. I did have a friend who found one of his loose from the factory. Normally the blue tops run lovely. Good luck.

 

 

[Alan]

I analyse vibration spectra of machinery as part of my job. The fact that the vibration exists in an rpm band suggests it is a resonance problem. That is not to discount the unbalance offering from Maj as there may well be an underlying unbalance. Think of a resonance as a mechanical amplifier that only works at certain speeds, so a small unbalance can really offend if it hits a resonance.

 

The balance of reciprocating machines is a black art, but it is normally sorted during the design stage. But it could be that where you have now indexed your prop through the 3:1 gearbox the unbalance and there always is some (aerdynamic and mass) is in phase with a directional engine vibration. You could try moving your prop on the hub - given the 3:1 gb I would try moving it 15 degrees from where it is.

 

The other thing that can drive a vibration resonance is mounting stiffness - to move a resonance vibration frequency up either increase stiffness or reduce mass, the opposite actions moves it down. Generally you try to have all resonances above normal operation. Did you use new engine mounts with your new motor?

 

We have fixed a multi million dollar boat by adding washers that allow us to pre-compress the mounting rubbers.

 

Alan

 

 

[Alan]

Er excuse the stupidity in the previous entry about indexing the prop around. This of course will controlled by your bolt spacing.

 

Alan

 

 

[farri]

Hi Maynard,have you done anything to the prop that would change it in any way and is the exhaust system exactly the same and in the same place as it was with the grey head?.

 

Frank.

 

 

[facthunter]

Propeller check.

 

try checking that the prop tips ar tracking in exactly the same plane, Some of these composite props. flop all over the place at speed. Have someone observe it from the side when you do a ground run. Tether the plane to a car or fence. It will be a harmonic and probably exaggerated by the engine mounts having a bit more flexibility than they should. Check that the mount plate on the bottom of the engine is tight. Nev

 

 

[robinsm]

Thanks for the advice guys. This site is a great resource. The carbs are balanced using a home made manometer (cost $3.00) so that part is cool. I moved the prop around one bolt hole and surprise, surprise, the damned thing seems to be now going great. The problem seems to have been (just guessing here) an alignment of the cylinders at TDC and one of the blades. I offset the blades and that seems to have done it. The harmonic now seems to be about 4800 to 5000 and not as strong so that is not a problem. I can live with that.

 

Again thanks to Mark and the rest of you for your advice.

 

Maynard