Mysterious Intermittant Vibration (long)

Stock ELSA RV-12, 173 hours, current on all SB's, firewall mod accomplished.

Over the past couple of flying hours I have noticed a slight change in the spinner "picture" (edge of spinner/ edge of cowl) as viewed from the left side of the cockpit in different flying regimes. F'rinstance: at idle, some movement is seen, at run-up the spinner appears motionless. It stays "frozen" all during climbout at 5,000 RPM, no vibration felt.

Upon level-off, somewhere around 5,000 RPM, a very slight "wobble" is seen on the spinner and it remains at all RPMs down to about 3500 whereupon the spinner "freezes" in appearance. During the wobble, a very slight vibration is felt in the cabin and through the stick. Were it not for the visible change in the spinner picture, I would probably have written this off to "auto-rough."

I wish I could be more specific about the RPMs but we have had very little smooth weather/VFR ceilings and the plane is usually getting kicked around a good bit.

I have carefully inspected the engine mount/torque, motor mounts and cowl security. No fluid leaks observed. I inspected and checked torque on all prop bolts. The blades are dead-even on the SmartLevel. Prop was professionally balanced about a year ago.

Here's where it gets interesting... I can stop the "wobble" by climbing or pulling positive G's in a turn. When I release the load, the wobble and the vibration slowly return. There is clearly a relationship here but I'm going to need somebody smarter than me to figure it out.

I know on some cars there is a thrust bearing on the crank that will sometimes let the crank move longitudinally a little bit but I don't know about the Rotax 912ULS. In any event, I can't see how this movement would cause these symptoms.

I have carefully examined the airframe for anything which might cause the plane to vibrate and make the spinner appear to wobble vis-a-vis the cowl front edge.

I downloaded the data from the Dynon but cannot see any anything out of line there. All the gauges are solidly in the green at all times. No alarms. Due to the fixed-pitch prop, the RPMs are constantly changing somewhat in rough air.

I welcome any and all ideas. I will be phoning Sensenich and Rotax on Monday to see if there are any precedents.

Jim
#120264

Have you had the prop balanced? I have found the RV12/Rotax can benefit significantly from getting the prop dynamically balanced.

Walt, Paragraph 5 in the initial posting indicates that the prop was professionally balanced, dynamically, I might add. Inasmuch as you are in the business, does intermittent vibration only at mid-range RPM, when not under G-load indicate to you, in your professional opinion, a prop balance problem?

If you have run across these specific symptoms, I would be very interested to know, as a second prop balance would be a quick "cure." It would be good to know, also, if a balanced prop can become unbalanced over the course of 80 hours or so, if the blades remain set at identical angles.

Thank you for assistance.

Jim

Sorry missed that part in your initial post.

Loading the aircraft up is going to force the motor harder against the mounts, so it not hard to imagine this would allow less movement of the spinner and appear to make the vibration lower.

If nothing has changed (prop, spinner, blades, blade angle) it should not go out of balance.

Have you tried to shake the engine with the prop by hand to see if there is any unexpected play?

I noticed a slight asymmetry from looking at the spinner line from the pilot seat. It looks like the engine is cocked to the port side. But other than the impression there are no other unexpected quirks. Sometimes I think I am just hypersensitive to stuff. Kind of like flying a single engine plane over the ocean: automatic rough!

Can you assign a frequency to the vibration?

Walt, thanks for the confirmation on loading causing compression of the rubber mounts. I was pretty sure this was the case and would tend to suppress vibration.

Mr. RFS, yes, checking for movement by grabbing the prop was one of the first things we did and the small amount of movement matched that of the RV-12 in a neighboring hangar. I am going to pull the cowl again and make a concerted effort to see if one of the rubber biscuits has over compressed or something. I haven't pulled more than 3 G's on this plane but I guess anything is possible.

Mr. Horton, I sure don't know how to assign a frequency to the vibration but would like to know how. It is seen rather than felt (spinner) then progresses to a barely felt vibe in the stick, then gets a little deeper. You can't see it on the panel and I was unable to feel anything by gripping the siderail.

Thanks to all. Jim

If you're inclined to fly it up here, I'd be happy to check the balance and do a vibration spectrum analysis for you. I have a number of RV12 spectrum plots to compare it to.

Thank you for the offer of your professional services.

I need to check out all the non-prop possibilities including some anecdotal information regarding gearbox problems in non-RV-12 912ULS engines. Apparently minute imperfections in gear mesh and thrust bearings can cause intermittent vibrations. I will be talking to the service centers tomorrow to see what they know about this issue. Specifically if our type of transmission has been in for work pre-overhaul hours.

Does your library of spectrum plots enable trouble-shooting beyond simple prop imbalance? F'rinstance, would it show engine miss, worn Lord mounts or...gearbox irregularities?

Typical vibration "signature" of the Rotax w/3 blade prop below, I also have 2 blade prop models. Anomalies can be detected in such things as the gearbox or other rotating components. You can see the different rotating rates of various components and their vibration level (soory but I am unable to get a better picture for posting).

You really should let Walt record a full spectrum.

Walt, Well I guess that answers my question. Your profiles can detect irregularities beyond the prop.

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.

Vibration

Hello, Did you check the spinner backplate for cracks?

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

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

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?

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

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 . . . . .