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-   -   Prop/Engine Vibrations (https://vansairforce.net/community/showthread.php?t=166619)

pilotyoung 12-06-2018 10:12 PM

Prop/Engine Vibrations
 
I purchased my RV-13 last February with 338 hours on it. It now has 432 hours on it. This summer I started noticing that when I reduce power from cruise, around 4400 down to about 3800 I get a vibration. I think it started after I complied with the Service Bulletin on the cracks in the front of the propeller hub. I hear the vibration more that I feel it. Two other pilots who have ridden with me also think it is from the propeller or gear box. The engine runs smoothly at cruise and at idle.

About a month ago I rechecked the front hub for cracks, and found none, and reset the blade angles at 71.4 and 71.3. The vibrations continued. This last week I replaced the soft engine isolators with the new ones from Van's and replaced all the springs on the muffler.

When I flew the aircraft, it still has the vibrations. Today I rechecked the blade angles and they are still 71.4 and 71.3. I checked the torque on the bolts and they are fine. I looked all over the engine for anything loose that could be vibrating and didn't find anything.

The propeller was dynamically balanced at 105 hours.

So I am asking for ideas, suggestions on what might be causing these vibrations. Has anyone else experienced vibrations like this?

Thanks.

tomkk 12-07-2018 05:30 AM

Dynamic prop balance? Also, I had a similar problem and I found one of my throttle cables was sticking slightly in mid-throttle range. I rerouted it slightly near the carb and secured it better so it wouldn't move. That worked for me.

JBPILOT 12-07-2018 06:15 AM

Hi John --
 
Which prop hub front did you install - D1 or F ? I understand that you can install the blades 'in' a little from where they should be with the 'F' hub. I think it has been discussed that after the new hub, assuming the blades were installed correctly, there still can be a balance difference. Be sure the blades are in there most outward position, and rebalance it. I did mine, and it helps a bunch.

pilotyoung 12-07-2018 08:45 AM

Vibrations
 
I just reinstalled the original hub after inspecting it. I am planning on ordering the new style hub and installing it.

alexe 12-07-2018 08:58 AM

John,
Did you really set the blades at different angles (71.4 and 71.3), or is that a typo in your post?
Alex

DaleB 12-07-2018 09:00 AM

Quote:

Originally Posted by pilotyoung (Post 1307547)
About a month ago I rechecked the front hub for cracks, and found none, and reset the blade angles at 71.4 and 71.3.

Why tie difference? Was that just the closest you were able to get them to being the same?

Ha... Alex was faster on the keyboard than I was, I see.

pilotyoung 12-07-2018 09:59 AM

Vibrations
 
I set them both to 71.4 and after I got all the bolts finally torqued one was at 71.3 and one at 71.4. Since the tolerance is .2 I left them alone.

rongawer 12-07-2018 10:39 AM

Seeing you’ve already balanced the engine, and assuming you haven’t changed the spinner or prop pitch, it could be things like a mis-firing spark plug(s) or carburetor balance.

If you’ve taken the spinner off to inspect - be sure you put it back on in the same orientation. It’s only gram weights that make a difference and the spinner is a very common cause. I’d also look for things like missing washers.

Lastly, do another engine balance and look for 2nd or 3rd level harmonics. It could be something like the alternator - but the most likely is something more innocuous like the spinner or a washer on a spinner screw.

rvbuilder2002 12-07-2018 10:57 AM

The carbs on the ULS tend to run the engine a bit on the rich side in the mid throttle range. If you have a float that is just beginning to get a bit heavy and floating a bit lower, it could be causing that carb to be even richer but possibly only show up at mid range throttle settings.

Phantom30 12-08-2018 06:24 PM

Quote:

Originally Posted by pilotyoung (Post 1307547)
I purchased my RV-13 last February with 338 hours on it. It now has 432 hours on it. This summer I started noticing that when I reduce power from cruise, around 4400 down to about 3800 I get a vibration. I think it started after I complied with the Service Bulletin on the cracks in the front of the propeller hub. I hear the vibration more that I feel it. Two other pilots who have ridden with me also think it is from the propeller or gear box. The engine runs smoothly at cruise and at idle.

About a month ago I rechecked the front hub for cracks, and found none, and reset the blade angles at 71.4 and 71.3. The vibrations continued. This last week I replaced the soft engine isolators with the new ones from Van's and replaced all the springs on the muffler.

When I flew the aircraft, it still has the vibrations. Today I rechecked the blade angles and they are still 71.4 and 71.3. I checked the torque on the bolts and they are fine. I looked all over the engine for anything loose that could be vibrating and didn't find anything.

The propeller was dynamically balanced at 105 hours.

So I am asking for ideas, suggestions on what might be causing these vibrations. Has anyone else experienced vibrations like this?

Thanks.

I second what Scott has suggested; also, carb sync...it is amazing the difference it makes when done right in overall smoothness of engine performance. Took me numerous attempts before getting the hang of it....and not wanting to start a war on the forum (I disagree with Van’s sync instructions) PM me for phone#; and i’ll share a slick method that has worked on my -12 and many other 912 Rotax equipped a/c. The typical comment i’ve recieved after tuning their engines...i’ve never seen it run so smooth...


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