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Chasing a vibration at cruise RPMs.

00Dan

Well Known Member
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 4” Saber extension and has a 20 pound crush plate on the front; I’m using a 12” 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 Catto’s 1/8” 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 haven’t 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 didn’t 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 it’s something on the engine that I haven’t considered causing this, although I’m 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.

I’d very much appreciate any insight on where to look next, especially if this latest reclock isn’t the solution.
 

Attachments

  • N443JT Dynamic Prop Balance 4-29-2021.pdf
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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.
 
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,
Mahlon
 
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.
 
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.

Larry
 
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.
 
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,
Mahlon

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.

Larry

Wouldn’t the balancing have accounted for any minor variation with this? I would think if the crush plate was off center (being heavier than the prop) I would feel it at all RPMs.


From all the good advice here I think I’ll still fly the 120 degree reclock just to humor myself, then (assuming it doesn’t help) revert to the original prop indexing and go through my engine mount and fairings to see what could be transmitting a vibration. I’ll also test if it’s prop load or airspeed which causes it.

I do have the turn downs on my exhaust tips (two pipe crossover system).
 
My rubber wing fairing gap seals underneath the wing could move about 1/16" and that was plenty to cause a nasty vibration. I "temporarily" put on some speed tape and it disappeared. I doubt this is your problem, but I would have bet a nickel that there was no way that small movement was causing such a huge racket in the cockpit when I was flying - I was wrong. I guess my point is to not assume anything about the cause of the vibration. :)
 
I chased a vibe for MANY hours. It developed at 500 hours of TT, after I leveled off at cruise altitude. The first time I returned to base rather than continue on to my destination in NC.
I Balanced the prop, Hartzell BF. Cleaned the nozzles and restrictors. Changed the plugs ($2.75 each). Sent the Pmags in for verification (sorry, but thanx Brad as they were fine). After all this Gene the local IA looked it over and said “if u’ve done all that u should look outside the motor “.
Turns out the Silicon hose from the firewall mounted oil recirculator down to the exhaust attachment would swell during full power and end up contacting the motor mount when fully heated. The engine pulses were transferred onto the motor mount. Initial fix was to shorten the hose and test. “Permanent “ fix was to create an S bend in a 5/8” aluminum tube that routed the line far from the motor mount. 100+ hours and running fine. Look for the less obvious.
 
Today I checked that all fairings and my wheel pants were secure and went flying. I was still able to feel the vibration in my feet particularly, both through the rudder pedals and feet on the floor. I tested varying airspeed at constant power and also letting airspeed drive the prop at a lower power setting. For curiosity’s sake I also tapped the brakes to eliminate any chance of it being the wheel (no effect).

I was best able to replicate the vibration with throttle position regardless of airspeed. It’s just a rumble of higher magnitude, really. Based on this I plan to reclock the prop to the point it was balanced at as a known “good” point then work my way through the engine mount and see where it may be transmitting to the airframe.
 
I was still able to feel the vibration in my feet particularly, both through the rudder pedals and feet on the floor.

I was best able to replicate the vibration with throttle position regardless of airspeed.

Sounds like you need to take a good look at the exhaust.

Is this a new issue, or has it been there all along?
 
Sounds like you need to take a good look at the exhaust.

Is this a new issue, or has it been there all along?

Unfortunately I don’t know. The truth of the matter is I wasn’t particularly paying attention before, since I was flying with a new prop and was planning to get it balanced anyway.

Is there anything on the exhaust I should be looking at besides general integrity?
 
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Grab the tail pipe, see if it can move around and hit anything. The typical way the tail pipe is suspended is subject to coming loose and letting the pipe droop or otherwise move about. The method I am talking about uses some bits of quarter inch or so tube, inserted into some short lengths of fuel hose, and held in place with screw type hose clamps.

Also, if the tail pipe is too short, you can get exhaust pulses that cause a vibration in the floor board.
 
Wouldn’t the balancing have accounted for any minor variation with this? I would think if the crush plate was off center (being heavier than the prop) I would feel it at all RPMs.


From all the good advice here I think I’ll still fly the 120 degree reclock just to humor myself, then (assuming it doesn’t help) revert to the original prop indexing and go through my engine mount and fairings to see what could be transmitting a vibration. I’ll also test if it’s prop load or airspeed which causes it.

I do have the turn downs on my exhaust tips (two pipe crossover system).

My vibrations change a bit each time I loosen the prop bolts. Granted, I don't have any significant vibration issues, but Sam at Sabre was clear that it is important to equallize the weight of the 20# plate when tightening the prop bolts. I assume this is because he has had vibration issues from his customers.

With prob balancing, you are adding a few grams at positions 6-9"" out from the center of mass. 20 lbs is A LOT of weight. If it falls .025" off center, due to gravity, consider the impact relative to a few grams of balance weight. Sams advice made complete sense to me and I follow it.

Larry
 
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Remove the 20lb crushplate. I had one on my -6. I felt a vibration at higher rpms and i balanced the prop with my dynavibe. I even sent the crush plate back to sabre and they sent me a “close tolerance” bolt holt replacement. I tried every trick sam offered about tightening the bolts etc. I was never able to make it run smooth. I removed it and added 10lbs of dive weight to the alternator bracket. This helped the cg issue and eliminated the vibration.

The best fix, that i latter did, was to remove everything forward of the flywheel and convert to a whirlwind rv200 c/s. Oh so smooth and the performance difference was eye watering.
 
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My vibrations change a bit each time I loosen the prop bolts. Granted, I don't have any significant vibration issues, but Sam at Sabre was clear that it is important to equallize the weight of the 20# plate when tightening the prop bolts. I assume this is because he has had vibration issues from his customers.

With prob balancing, you are adding a few grams at positions 6-9"" out from the center of mass. 20 lbs is A LOT of weight. If it falls .025" off center, due to gravity, consider the impact relative to a few grams of balance weight. Sams advice made complete sense to me and I follow it.

Larry

Remove the 20lb crushplate. I had one on my -6. I felt a vibration at higher rpms and i balanced the prop with my dynavibe. I even sent the crush plate back to sabre and they sent me a “close tolerance” bolt holt replacement. I tried every trick sam offered about tightening the bolts etc. I was never able to make it run smooth. I removed it and added 10lbs of dive weight to the alternator bracket. This helped the cg issue and eliminated the vibration.

The best fix, that i latter did, was to remove everything forward of the flywheel and convert to a whirlwind rv200 c/s. Oh so smooth and the performance difference was eye watering.

I’ll check that the plate is centered when I revert the clocking. As far as removing the plate, I’ll explore that if I don’t find anything on the mount or exhaust.
 
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I’ll check that the plate is centered when I revert the clocking. As far as removing the plate, I’ll explore that if I don’t find anything on the mount or exhaust.

I will add that my 6 has less vibration and flys better with the std plate. However the 20 pounder significantly increase my baggage carrying capacity, so I accept it. All of the vibration issues are around idle and nothing noticeable at 2700 RPM.
 
A couple of airframe things to check for RV4 drumming. Sometimes several of the pop rivets along the lower edge of the cheek cowls break. It's subtle, because the paint will hold the heads in place and everything looks OK, but there will be noticeable drumming in flight.

Also, the rivets on the bottom, aft of the exhaust pipes take a pounding and can work loose. If it happens to enough of them, the there will be a lot of floor vibration. Look for smoking rivets on the underside in the oil/exhaust stain area, any should be replaced with -4 rivets or cherry max.
 
Grab the tail pipe, see if it can move around and hit anything. The typical way the tail pipe is suspended is subject to coming loose and letting the pipe droop or otherwise move about. The method I am talking about uses some bits of quarter inch or so tube, inserted into some short lengths of fuel hose, and held in place with screw type hose clamps.

Also, if the tail pipe is too short, you can get exhaust pulses that cause a vibration in the floor board.

This looks suspect...
 

Attachments

  • 027EED31-18A8-47D5-A350-DF71808FD519.jpg
    027EED31-18A8-47D5-A350-DF71808FD519.jpg
    207.2 KB · Views: 119
This looks suspect...


After finding this I adjusted how my engine mount was shimmed and retorqued all the bushings. This opened up the gap between the exhaust pipe and the mount. A static test (I grab it and see if it can be made to touch) determined the pipe can no longer contact the mount, where previously it could.

A test flight showed much improved vibratory signature. Obviously the standard four cylinder signature remains, and I noticed some type of harmonic through the airframe at approximately 2500 RPM (it was probably there before but not detectable), but it’s all much more livable.
 
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