vibrations

[egp8111]

My KFIII rebuild has about 10 hrs. on it now with no real issues but I'd like to get it running smoother. It has the 912ul with the Ivo 72 ultralight prop. I've balanced the carbs with my Carbmate and they show dead on at 4100 rpm where it likes to cruise. It's not bad just not a smooth as some 912's I've flown. I'm wondering if I need to get the prop balanced ? Any ideas or suggestions welcome.

All and all it's a lot of fun, just a dirt simple little airplane that really performs.

Thanks,

EG

[Esser]

Having a balanced prop will make the biggest difference.

[Monocock]

EG,

1. What power setting did you balance the carbs at? Just because your preferred cruise setting is 4100, that doesn't mean that's the power setting you need to sync' at. Your post suggests you did, but I might have misunderstood it.

2. Are you sure your blades are all pitched exactly the same? Some pitch tools are less accurate than others and a blade that is different to the rest will cause vibration. I prefer ones that rely on gravity and an actual pointer on a gauge to make sure that each blade is pitched identically.

3. Have you checked the tracking of the prop tips? This was the cause of my original vibration which I cured very easily. With one of the prop tips in the 6 o'clock position line up an object on a step (a paint tin or anything will do) and place this object 1mm in front of the prop tip. Rotate the other two blades past the object and any gap differences greater than 1/8 inch will certainly give vibration when the engine is running. I think it could be this as this type of vibration often doesn't show at higher power settings as the centrifugal force above 4000 rpm is enough to force the prop blades to track better. At tick over it won't show either as there isn't enough prop speed forth tracking to make a vibration. My GSC (that was sent back) had a 1/4 inch tracking difference on one blade. At 3000 to 4000 rpm it was like the engine was trying to exit the airframe.

4. It would be worth getting the prop balanced and don't forget to check the spinner too. There are clever ways of doing the spinner and if it is a 9 inch one it's amazing how a poorly balanced spinner can throw the balance out.

I hope this helps slightly.

[egp8111]

Thanks for you suggestions. I balanced the carbs at 4000rpm. The Ivo adjust all the blades together. There is no way to individually adjust the pitch on separate blades. I'm using a small bullet spinner. I'll check the tracking.

EG

[Monocock]

Hi again. Someone here will be able to clarify this but I think you'll find you really want to be synchronising the carbs at a much lower power setting than 4000. At 4000 rpm the butterflies are quite open, therefore masking any throttle cable length differences. To get a more accurate sync' I think it's supposed to be done at around 2500 rpm, and once that's done, all other power settings will look after themselves.

Can someone confirm??

[Monocock]

EG

Go here and see page 16.

http://www.conairsports.co.uk/downloads/912%20maintenance%20article%20-%20inc%20carb%20bal.pdf

[Peteohms]

My opinion. 4100 is to low rpm for cruise. Try 5000 or more. When I throttle back to 4000 my engine is not as smooth unless I ad a bit of carb heat.

[Monocock]

Yes, I agree, 4100 is very low for a sustained cruise, but my point is that syncing carbs at 4100 is not really achieving much. When the butterflies are closed right down (2200 rpm), that's the setting to sync them. Everything else will take care of itself. Syncing at 4000 is a bit like zeroing a rifle at 3 yards.

[DesertFox4]

Mine carbs. are balanced at 2500 rpms.

[cainbird]

I just returned from flying tonight and found this thread interesting. My plane (model 4 912ul 72" ivo ifa ultralight patriot) also experiences some vibrations. On my plane it seems to be between 3000 and 4500rpm. Above 4500 through the 5s it is smooth as silk. I usually cruise around 4800 and it smooth there, but sometimes I like to slow down and pull in some flaps and with the throttle around 4000 it has a noticable vibe. Pitch settings on the prop seem to have little affect. I have not ballanced my prop yet but will soon and I will report back. My ears are open to other ideas and it sounds like EG and I have a similar situation.

Cain B

[CDE2FLY]

I'm running an IVO IFA medium on a 914 and last summer met with Dorsel to balanced the carbs using the Carb Mate which definitely smothed things out - especially at idle.

Later in the fall, Dorsel brought the Dynavibe to the North East Kitfox Fly-in and we balanced the prop. This resuted in a very noticable improvement across all RPM ranges. As soon as the engine fired, you could tell there was a substantial improvement. One of the advantages of the Dynavibe is that you balance the prop and the spinner at the same time - highly recommended!

[Monocock]

Cainbird

Don't forget to check the tracking too. I was amazed at how far out one of my tips was and that caused vibration in exactly the power setting range you have mentioned.

[egp8111]

it goes past red line at 5000rpm ! 4000 is about 80mph. I balanced the carbs at 2500 rpm but the were out of sync at the higher power settings. I'm using the dual cable throttle from McCarr. I dont see how to adjust the sperate cables.

[Peteohms]

it goes past red line at 5000rpm ! 4000 is about 80mph. I balanced the carbs at 2500 rpm but the were out of sync at the higher power settings. I'm using the dual cable throttle from McCarr. I dont see how to adjust the sperate cables.

EG,

Do you mean that your tach has a redline @ 5000 rpm? My understanding is that normal cruise is 5400-5500 rpm for The 912.

[egp8111]

sorry, not the engine redline, the airplane is pushing VNE(100mph) at 5000rpm. I've check the asi against the GPS and its pretty accurate. I'd be interested to know what others with a 912ul powered KFIII are using for cruise rpm.

[Monocock]

I'd be interested to know what others with a 912ul powered KFIII are using for cruise rpm.

Rotax clearly state that 5000 is where they are designed to be in the cruise and I know of some who cruise at 5400. I didn't like to say anything but 4000 won't be doing it much good, and the gearbox will be under more strain than it needs to be. There has been a lot of past discussion on various forums about this and people have provided some interesting stoies on the Rotax owner forum about the results of low revs in the cruise over a long period of time.

Out of interest, what's your static rpm on the ground? I'm guessing it's pretty low if you're reaching VNE at 5000 rpm. I might be wrong but it sounds like your prop might be set very coarse.

[cainbird]

Monocock,
Good point! I havent checked the tracking in a while. I will check it tomorrow.

Cain B