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06-09-2013, 08:52 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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You really should let Walt record a full spectrum.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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06-09-2013, 09:39 AM
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Join Date: Apr 2008
Location: Boerne, TX
Posts: 228
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Walt, Well I guess that answers my question. Your profiles can detect irregularities beyond the prop.
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06-10-2013, 12:06 PM
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Join Date: Aug 2007
Location: Houston, Tx
Posts: 163
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Change of direction..
Speaking strictly as a maintainer.. The key to me is the line where he feels the vibration in the stick, it disappears when manuvering or with RPM change? With nothing being found at the front end, check the other end. The stabilator and/or trim tab may be coming into resonance in the prop wash. Do something to alter the airflow over the tail and see if that doesn't change the vibration. 
__________________
Bill S.
A+P, RV Dreamer + Schemer
RV-9 preplans in hand
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06-10-2013, 12:37 PM
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Join Date: Nov 2009
Location: Jackson Ohio
Posts: 139
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Vibration
Hello, Did you check the spinner backplate for cracks?
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06-10-2013, 03:20 PM
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Join Date: Apr 2008
Location: Boerne, TX
Posts: 228
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Gentlemen, Thank you for the suggestions. Yes, we checked out all kinds of airframe causal items as well as security of the spinner backplate. Mr. Walt Aronow has generously offered to compare my dynamic prop balance readouts with his library of RV-12 data for evidence of where the vibration might emanate from.
I have researched into and received anecdotal data which indicates that the gearbox on many non-RV-12 Rotax 912ULS engines have experienced vibration problems, usually post 500 hours due to wear of the gears permitting a chatter when the load is removed from the gears--as in leveloff from climb.
Additionally, Rotax Service Letter 912-010 mentions gearbox issues as a possible cause of abnormal vibration.
Conversation with a Rotax Service Center tech reveals that harmonic resonance is a potential in any airframe, however, he has not yet encountered this problem in the RV-12. He has replaced gear sets in 912ULS engines (again, non-RV) with improvement seen in vibration cessation.
When weather permits I will try and nail down the exact RPM/flight regime which sets off the vibe/spinner movement. If everything points to the gearbox, I will either placard the plane for continuous operation at the low cruise power settings or more likely bite the out-of-warranty bullet for a new set of gears and hope they weren't made on a Monday or Friday.
Jim
Last edited by N223JH : 06-10-2013 at 03:24 PM.
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07-25-2013, 08:09 PM
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Join Date: Apr 2008
Location: Boerne, TX
Posts: 228
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Denoument
Initial trouble-shooting isolated vibration source to nose gear fairing/pant. Since I had postponed ordering and installing gear pants until an aftermarket source (RV Bits) offered suitable upper and lower main gear fairings, the vibration issue wasn't apparent until all components were initially installed, preparatory to painting. This was with nearly 200 hours on the plane.
After a series of flights with various Vans-supplied nosegear components in place, it was clear the slight vibration--seen more than felt, in a barely noticeable spinner wobble--originated in the wheelpant front-to-rear interface. Ultimately it was found that my "rough cut" of the wheel opening at the bottom of the pant halves was sufficiently crude as to set up a harmonic after a period of time at cruise power. This manifested itself in the vibration.
A few minutes with the Dremel drum sander, taking the opening to spec seems to have cured the problem.
I am grateful that this fix only cost time and did not involve a prop, gearbox or engine mount issue.
Jim
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07-25-2013, 08:19 PM
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Join Date: Aug 2010
Location: Marshall TX (KASL)
Posts: 1,783
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Wow! Wouldn't have guessed that! Please, more detail on the kind of "Initial troubleshooting" that isolated this problem! Was it luck, serendipity, or a genius application of the scientific method?
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07-26-2013, 05:45 AM
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Join Date: Apr 2008
Location: Boerne, TX
Posts: 228
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Just application of Occam's Razor, I guess. Had to ask myself if there was anything that coincided with the onset of the problem. Only significant item was installation of landing gear paraphernalia a few hours back. I guess I was so intent on recording improved airspeed and discounting the seasonal bumpiness, that the vibration eluded me until I got up in some real smooth air and saw that spinner movement.
I was saving all the "finish" trimming of non-mating fibreglas until final fitting was set, preparatory to painting. Rear portion of nosegear pant extended a little farther under wheel opening than front. Also exacerbated, I expect, by my moving the lower attachment screws up slightly for convenience in removal.
Tube structure of 12 nosegear leg seems more sensitive than solid leg of A-model RV's, IMHO. Not an excuse, just unscientific guess, based on fact that all symptoms were absent in full-power climb and when pulling G's.
Jim
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07-26-2013, 07:07 AM
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Join Date: Sep 2007
Location: Central IL
Posts: 5,516
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While we are on vibration and balance - - we hear this and that component is already balanced. Example, in a tour at Hartzell, we walked through the final balance area. Due to the excellent manufacturing consistency, a GA, BA prop sitting there only required an AN3 washer maybe 5 inches from the center to balance it. So where does the imbalance on the airplane come from?
Possible contributors to imbalance, if all prop and engine are balanced:
1. Kit spinner hardware concentricity
2. Fiberglass layup on spinner
3. Differing washers around the perimeter to align said spinner plate
4. Primer and paint thickness
5. tiny concentricity difference with prop and crank flange.
6. small torque differences causing tiny wobble of prop
7. other?
I would think stack up of imbalance from this list could overwhelm the fine balance from a Hartzell,or other factory.
This is where you live, so What say you, Walt?
Ready to learn . . . . .
__________________
Bill
RV-7
Lord Kelvin:
“I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.”
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