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Old 05-10-2021, 08:19 PM
00Dan 00Dan is online now
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 222
Default Chasing a vibration at cruise RPMs.

BLUF: I have a vibration I hear and really feel in my feet when the RPM is above approximately 2450 RPM.

My RV-4 has a slightly over pitched 3 blade Catto on my 150 hp O-320. The prop is on a 4Ē Saber extension and has a 20 pound crush plate on the front; Iím using a 12Ē Catto spinner with the front bulkhead removed to accommodate the large crush plate (the crush plate touches the inside of the spinner and provides a bearing surface). I recently had the a dynamic balancing performed and for full throttle static conditions, the results were great - I started at 0.454 IPS and we got it down to 0.005 IPS on the third iteration. In flight however, I still feel a vibration above 2450 RPM up to the max RPM I can achieve. I emailed Catto and Nicole recommended trying to reclock the propeller 1 or 2 bolt holes. I reclocked one hole (60 degrees) CCW (looking at the tail) and also checked the tip tracking when I did so (it was within Cattoís 1/8Ē tolerance). I left the balance weights in place and flew for an hour, and still had the high RPM vibration. When I landed I took the prop back off and reclocked it another 60 degrees CCW (120 degrees total); I havenít test flown this configuration yet.

Why am I posting this in engines instead of propellers? Based on all the above, I believe the prop can be ruled out as the cause unless this latest reclock proves to be the silver bullet. I downloaded the EMS data and reviewed the traces - I didnít see the signatures of either a bad exhaust valve or a pair of cylinders down on power from a cam/intake issue.

I want to think itís something on the engine that I havenít considered causing this, although Iím not able to positively rule out something on the airframe. I did check the obvious (loose fairings) and in the past such defects manifested as a high frequency audible buzzing rather than a low airframe rumble.

Iíd very much appreciate any insight on where to look next, especially if this latest reclock isnít the solution.
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Old 05-10-2021, 08:45 PM
00Dan 00Dan is online now
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 222

Iíve attached the balancing report, for reference.

Hereís the engine data from the flight before balancing:

Hereís engine data from the first flight post balancing:

Hereís engine data after the first prop reclock:
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Old 05-10-2021, 09:33 PM
BobRv4 BobRv4 is offline
Join Date: Nov 2012
Location: Gilbert, AZ
Posts: 142
Default Balance

If you moved the prop, you canceled your balance job.
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Old 05-10-2021, 09:53 PM
00Dan 00Dan is online now
Join Date: Nov 2016
Location: Atlanta, GA
Posts: 222

Originally Posted by BobRv4 View Post
If you moved the prop, you canceled your balance job.
It takes me 20 minutes to revert the clocking and since I havenít touched the weights installed Iím willing to experiment with it.
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Old 05-10-2021, 11:04 PM
BobRv4 BobRv4 is offline
Join Date: Nov 2012
Location: Gilbert, AZ
Posts: 142
Default Balance

Did the vibration change at all before or after the balance job? Does the vibration change with airspeed, or load on the prop? In a climb or descent. We found a landing gear shaking on a H rocket by flying formatiom at different speeds. So maybe have a trusted friend fly next to you looking at gear. How old are your engine mounts? Good luck.
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Old 05-11-2021, 05:51 AM
mahlon_r mahlon_r is offline
Join Date: Jan 2005
Posts: 1,056

When you do a dynamic prop balance you are not only balancing the prop spinner combo but also the entire rotating mass of the engine as well. So based on your results the engine and prop are not vibrating abnormally. Either you have an airframe anomaly or something is transmitting a normal engine vibration to the airframe for you to feel. Try adjusting airspeed at that same power setting and see if the change effects the vibration you feel. If it does,to me, that would suggest something airframe related. If you feel no difference, look to the engine mounts, tight engine controls, something very close to the tubular engine mount etc. that is acting as a transmitter.
Good Luck,
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
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Old 05-11-2021, 06:15 AM
MRM449 MRM449 is offline
Join Date: Feb 2015
Location: St.Ansgar, Iowa
Posts: 10

When I was in phase one with my 4 it developed a vibration just like yours. It was above 2350 RPM. I didn't have an engine monitor at that time. The higher the RPM the worse it got. I pulled the throttle back and at 2000 RPM it went away. I limped home one very nervous pilot. Spent a lot of time checking mags, plugs, harness. Pulled the carb and had it checked. Would go test fly and it was always there. Finally decided that I was going to have to fly it to find my problem but thought I would wash the plane before I flew. Always wash my wheel pants last and when I started to rub the bugs off one of my wheel pants it was loose. Tightened it up and went flying. Vibration gone.
Mike Mullenbach N449PM
Saint Ansgar, IA.
RV-4 Built and flying since 1999
VAF Dues Paid 2020
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Old 05-11-2021, 07:07 AM
lr172 lr172 is offline
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 6,537

I also have a 20# crush plate and much care must be taken when tightening prop bolts. You have to manually take the weight off so that the plate centers itself. If you don't the weight causes it to favor the low side.

If you have a traditional crush plate, I suggest you remove the 20# plate and see what happens without it.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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Old 05-11-2021, 07:24 AM
Mel's Avatar
Mel Mel is offline
Join Date: Mar 2005
Location: Dallas area
Posts: 11,172

Have you considered exhaust "pounding" on the floor? If your exhaust pipes don't have "turn-downs" at the tip, sometimes this can cause a vibration on the floor.
Mel Asberry, DAR since the last century. Over 1,000 certifications accomplished. Discount for Veterans, Law Enforcement, Fire Fighters.
EAA Flight Advisor/Tech Counselor, Friend of the RV-1
Recipient of Tony Bingelis Award and Wright Brothers Master Pilot Award
USAF Vet, High School E-LSA Project Mentor.
RV-6 Flying since 1993 (sold)
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Old 05-11-2021, 07:38 AM
Floog Floog is offline
Join Date: Mar 2018
Location: Desert Aire, WA
Posts: 36

Long shot. Wheels spinning.....? Touch the brakes.
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