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Prop Vibration

RFSchaller

Well Known Member
My prop passes through a resonance around 5100 RPM just as I lift off. It lasts less than a couple of seconds and I can see a little wiggle on the edge of the spinner briefly. I checked my engine mounts and tweaked my prop blades which were 0.8 degrees different, and that improved things, but I would like to get the prop dynamically balanced. Does anybody know somebody in the Phoenix or Tucson area that they would recommend? Has anybody else seen this?

BTW I did the journal cracking test and passed. Seems strange but I can't recreate the resonance in cruise when I search for it with the throttle.

Any ideas from the brain trust?
 
This is a common occurrence on takeoff if your nose wheel is not balanced. Try doing a soft field takeoff and see if it goes away.;)
 
.8 degree blade angle difference is a LOT (even .2 - .3 is noticable) and will also cause vibration, but it will be at all RPM's
 
You guys were right. It was the nose wheel. Soft field technique got rid of it. Time to go to the aero department at JC Whitney and get a Go kart wheel balancer!

Re: 0.8 degree mismatch. I've had much worse and never noticed it. I tried checking pitch first because it was easy. I'm not condoning operation with a mismatched pitch, just pointing out it is not always obvious.

Thanks to the Brain Trust!
 
I'm not condoning operation with a mismatched pitch, just pointing out it is not always obvious.

I guess it somewhat depends on what you are comparing to.
If you have not done a dyn. balance, maybe you have not yet flown behind a 912 that is truly as smooth as it can be.

Once you have, it is easily possible to sense the smoothness difference from even .3 deg. difference.
 
That was obviously taken the wrong way.

You your self implied you thought you needed to do a dyn balance. I assumed that meant you haven't yet done so, which would mean you maybe haven't experienced the level of smoothness (can't be compared to any other aircraft engine I have ever flown) that is possible.

You then stated you have flown with an even higher level of missmatch than .8 deg and to you it made no difference.
That is contrary to my experience and that of many who have posted here on the RV-12 forum.

So as many of my posts are, it wasn't even really intended for you, but the thousands of other people who read here on the forums, and then have to attempt to determine what information is correct.
Hopefully they will believe me (and maybe others that are willing to stand up to your implied poor advice) when I say .8 degrees blade pitch difference is a big deal and will cause a noticeable difference in smoothness, and per Sensenich's documentation, is an unairworthy condition.

I am not saying that adjusting them so they axactly exactly match will assure that an RV-12 will run perfectly smooth... it may still still need a dynamic balance to be able to do that.
What I do know is that if they are not adjusted properly, even a dyn balance will not make it run smooth.
I have have adjusted prop pitch on at least a dozen different RV-12's and done a dyn balance on at least half that many.
I can detect a smoothness influence with as little as .2 degree difference between blades, just by flying the airplane. I am not saying that to brag, I am saying that so that people wont settle for less than getting it right.
If you are offended in thinking I am implying that you are incapable of detecting a difference, that was not my intent (but you your self are saying you have flown with even a higher level of mismatch and it made no difference).

My main intent of my post was that people not believe the false information you were providing.

I checked my engine mounts and tweaked my prop blades which were 0.8 degrees different, and that improved things, but I would like to get the prop dynamically balanced. Does anybody know somebody in the Phoenix or Tucson area that they would recommend?

Re: 0.8 degree mismatch. I've had much worse and never noticed it. I tried checking pitch first because it was easy. I'm not condoning operation with a mismatched pitch, just pointing out it is not always obvious.
 
After balancing literally hundreds of props, and speaking with many of the owners before and after the balance, I've concluded that the human body is a quirky machine. Some folks are extremely sensitive to vibrations and can notice even the smallest changes while others just can't feel a thing short of a prop blade missing.

I encourage all RV12 owners to have your prop balanced as I've seen a signicant improvement in the majorty of these I've done. Rotax also recommends it prior to first flight if I recall correctly and I think this is especially important with the Rotax gearbox up front.
 
Scott,

You work with and fly these aircraft everyday. I think I'm a typical owner flying my RV about 100 hours per year. I would not expect to be as sensitive to the level of vibration as someone with your experience level in the aircraft, and I accept your observation. The only data point I have on dynamic balancing is when I had it done to my Cherokee. I didn't notice any difference even though weights were added. Maybe it was close in the first place.

Rich
 
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