Vibration Problem
 

I finished the condition inspection and flew the airplane Saturday. There is still a slight vibration when reducing the power between about 4,000 and 3,000 rpm. Other people have told me that is just part of the Rotax engine. But the really bad vibration is gone. The engine was running very smoothly and I was able to sync the carbs perfectly. So the airplane is ready to fly to Sun 'N Fun. I am just hoping for good weather.

Hope this does not open a large can of worms - but -
 

I am part owner of a Prop Balancer. I was having problems getting a good balance. This very light plane would just dance around if you tried to get the throttle up to max static rpm's. It caused problems trying to get repeatability. I thought I would try something since time was not an issue to get it done. Here is what I did :

I set everything up UNDER the cowl. I put the reflective tape on the rear of the spinner plate. I mounted the accelerometer as close as possible to 12 O'clock position. I set the prop so the tach light hit the center of the reflective strip. I put 2 pieces of masking tape on the top cowl exterior so I knew where the correct position of the prop was, and where the accelerometer was. I ran the two cables out the side of the lower cowl leaving the short pin out so the lower cowl could bow out enough to allow the wires out from under the cowl. I ran the two cable into the cabin thru the vent on the right side and put electrical tape over it ( because it is still cool in the Midwest ). I waited for smooth air early morning, and flew the plane with the AP on so it was steady and smooth. I was able to get 3 good 1st readings, then landed and added weight as suggested. After the 3rd flight I was able to get a 'GOOD' reading. Takes a little extra work ( maybe an hour ), but it really solved the problem. Once balanced, I was able to fly and try different RPM's and just rebooted and tried it again at other RPM's. Once away from the first desired RPM balancing, it changed a little at different RPM's, but not much. Happy so far.

Great information John. Couple questions:
1. How much did the accelerometer reading (ips) change from the ?in air? reading from the balance you performed on the ground?
2. How about the weight and location of the weight between the two balances, any significant change in that?
3. Which make and model balancer did you use?
4. I?m thinking you zeroed in on the rpms that you felt the vibrations and not necessarily a typical cruise rpm that we would use on the ground such as 4900?

This is very good info. I think you hit the ball out of the park one more time!

Well Dave - I'll try - -
 

1. A bunch. Had a friend help me do the "on ground" balance, and I think we both felt it was good. Took 10.5G at spinner screws to make it say 'good' again.
2. Too much time passed. Don't recall from last time.
3. ACES 1015 Sport.
4. I did balance at 5200 rpm. That is a little higher than I many times fly. 5050 - 5150 are more common for me. Once I was in the 'good' range, I tested at 5450, and 4900. I wanted to know what happened at other rpm's. It went up to and just out of 'good' which is .07 ips upper limit with this balancer. Spent a ton of time "playing" with it, but was interesting. Obviously a person trying to make a living could not spend as much time as I did.

I would like to hear from others that have watched or helped have their RV-12 balanced. On the ground with very low, or no wind, my plane still just dances around when you get to upper rpm's. I think the tail just is not seeing smooth air, and starts the 'dancing', and it will not calm down. Dave, if you have balanced yours, can you run the rpm's up high and have the plane sit fairly smoothly ? Mine never has.

JB, At spinner screws? Is weight bolted on prop back plate?

I have balanced 5 or 6 12?s, usually with the owners at the controls, but I have done my own by myself and can?t recall the dance as you say. I did one yesterday. Will find out if Mark noticed this movement. I suppose you notice this at a static rpm check of the prop?

Hound, we typically install the weights under the spinner screws until we reach the best solution. When satisfied we then attach the weights to the web of the spinner backing plate.

Ah, thanks Dave!

Dave -
 

Maybe it is an issue with my plane. That is why I wanted to hear from others. When at lower rpm's, my plane will sit fairly firmly. Once I get to 4500 and up, it just moves. It will not sit still. Not actually vibrating, just moving around and not stable. My engine mounts seem very solid. Not at all soft. Flying, in smooth air, it is very smooth. I balanced my prop fairly early in its life. I remember the same thing then, but we got it done with a Dynavibe balancer. Dave, if you have a quiet air day, maybe you can take yours outside and video it with the rpm's at max static you can get.

First chance I get, will do. I have your phone number and will send it to you.