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Tracking Down Vibration

Geico266

Geico266

Well Known Member
Specifically for -3's, but all comments about eliminating vibration are welcome.

Patient: 1996 RV-3, 0-235, Sterba cruise prop 66 X 68, Brand new Slicks, harnesses, fine wire plugs, new carb, comps are low 70's, dynamically balanced 6/08.

Diagnosis: The vibration is under full power only, stops when throttle is pulled back. I have switched F&R mags in flight on & off with no difference in vibration. It just seems to be more than there should be, but not enough to ground the plane.

Treatment: I found a few areas where the cowling was touching the air box and I modified it to clear, but I just found it is still touching a little. The spinner may be touching under load as there are some marks on it, but not enough to worry about?


Is the -3 just prone to more vibration that other RV's?

Any comments greatly appreciated.
 
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I'm assuming the engine mounts are original. I did notice they looked over compressed. The 0-235 is not a conical mount so it is prone to more vibration.

I have heard rotating the prop 90 degrees is a good cheap place to start. Anyone try that?
 
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Do you have access to a prop balancer/spectrum analyzer?
The harmonic order and magnatude would be useful in tracking down the problem.

You can take the transducer from a prop balancer and mount it anywhere on the airframe, in the cockpit, clampped near your butt since thats where you are feeling it, and go fly. Recording these results will point you in the right direction.

Its going to be hard to deduce from your butt feel over text and email to find the problem. Plot some data and we are all over it.
 
Kahuna said:
Do you have access to a prop balancer/spectrum analyzer?
Click to expand...

Yes, I do! It's a nice ACES system. I could hook it up and go flying and record the difference vibration readings at different power settings. The only trick is getting the guy that owns the balancer to let me fly with it! ;)

To other -3 owners, How much vibration do you feel?
 
The ACES box is exactly what the doctor ordered.
Come back with plotted data and we will go from there.
 
I've experienced similar on Continental IO-240's in Katana's. Light wooden props, relatively course pitch, high power, low airspeed/climb and worn motor mounts. Almost a thumping like you're feeling the power pulses but its the motor hitting the hard limits of the mounts. Lowering the nose and increasing the speed and rpm made it go away.
Just a thought...
 
mvs163 said:
I've experienced similar on Continental IO-240's in Katana's. Light wooden props, relatively course pitch, high power, low airspeed/climb and worn motor mounts. Almost a thumping like you're feeling the power pulses but its the motor hitting the hard limits of the mounts. Lowering the nose and increasing the speed and rpm made it go away.
Just a thought...
Click to expand...

Yes, this is sounding familiar. The 0-235 with the cruise prop will only turn 2600 RPM straight & level. Red line is 2,800 RPM. I wonder if having the prop trimed down 2-4" would help. It's due for an overhaul anyway. Sterba said he would do it for $50.
 
Just change the mounts if they are 12 years old. Even if it's not the main problem, it still is a problem.
 
Baffle touching?

Geico266 said:
Specifically for -3's, but all comments about eliminating vibration are welcome.

Patient: 1996 RV-3, 0-235, Sterba cruise prop 66 X 68, Brand new Slicks, harnesses, fine wire plugs, new carb, comps are low 70's, dynamically balanced 6/08.

Diagnosis: The vibration is under full power only, stops when throttle is pulled back. I have switched F&R mags in flight on & off with no difference in vibration. It just seems to be more than there should be, but not enough to ground the plane.

Treatment: I found a few areas where the cowling was touching the air box and I modified it to clear, but I just found it is still touching a little. The spinner may be touching under load as there are some marks on it, but not enough to worry about?


Is the -3 just prone to more vibration that other RV's?

Any comments greatly appreciated.
Click to expand...

I recently changed engines from a 320 to 360 on a plane (not an RV) which included a new cowl. When I first started flying it, I noticed a vibration that only happened after 2500 RPM, and got worse the higher the power setting. That didn't make sense to me, as my old engine ran very smooth at higher RPMs. I checked all the usuals, including changing the prop 180 degrees. I was looking for any place the cowl might be touching, particularly near the airbox and exhaust pipes, but couldn't find a problem.

Then after a few hours, I noticed some black smudging on my top right cowl where the baffles met the cowl. I thought there was enough clearance there, but evidently not. What was happening under higher power, is the engine twists on the mounts against the rotation of the prop, and that side was too tight against the cowl.

I then snipped another half inch off my baffles on that side, and the vibration disappeared.

Just something to check on yours...

Walter
 
Mounts? Which one?

Geico266 said:
I'm assuming the engine mounts are original. I did notice they looked over compressed. The 0-235 is not a conical mount so it is prone to more vibration.

I have heard rotating the prop 90 degrees is a good cheap place to start. Anyone try that?
Click to expand...

You say NOT conical so it is prone to more vibration? Actually dynafocal is better than conical.

New engine mounts can make a big difference. If you had a conical mount with horrible smashed down rubber bushing / washer set-up, changing it with LORD CORP mounts will be night and day. Van sells them and they are not cheap. Confirm they are LORD MOUNTS, accept no substitution.

This applies to Dynafocal as well, NEW LORD MOUNTS will help. The change will no be as dramatic as the conical just because the basic dynafocal engineering is pretty good. Dynafocal mounts from LORD are superior, materials, design and manufacturing, in my opinion.
 
A recent tour of the Barrett shop convinces me that some vibration can not be cured because it is caused by a crank shaft not balanced to limits inside the manufacturing tolerances. The acceptable manufacturing tolerances of crank shafts can result in considerable variance from one engine to another from the very beginning because of this acceptable tolerance range.

That being said, if an engine that has been running smooth becomes less smooth, something has changed and it was not the crank shaft. I know from experience with a normally very smooth Subaru that any part of the engine structure touching the cowling can make a huge difference.

 
Walter Klatt said:
I was looking for any place the cowl might be touching, particularly near the airbox and exhaust pipes, but couldn't find a problem.

Walter
Click to expand...

I did find an area on the airbox that was touching pretty good and modified it, but not quite enough for total clearancve. It still rubs just a little. This just might be it! Stay tuned!

Thanks Walter!

Thanks George! I'm gonna put on new mounts too.

Thanks all who have responded. I really apprciate it! I really do! Sometimes these things just mess up my mind and I need to talk them through with guys smarter than me.
 
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I modified the airbox and adjusted an exhaust tube that could have been touching. The plane is MUCH smoother now.

Thanks for all your input guys.
 
richvidaurri said:
I have been looking around for a kit plane and I have decided on the RV-3B. It's the perfect airplane for the flying I do - or it will be if I could install a larger engine.
Click to expand...

Rich it would be best if you started a new thread about this subject.

To answer your question though, you may want to look into a Harmon Rocket. They are built for speed and performance and basically are RV-4's.
 
did this just start

Did this vibration just start or was it there from day one?

I had a vibration on my RV-4. It was fine after first flight and during the test period until I installed the wheel pants and leg fairings. I was told to look at all the things mentioned above but eventually found that it was air seperation at the trailing edge of the canopy vibrating the canopy skirt. A thicker rubber gasket in that area solved the vibration.

Slower speed no vibration...higher speed vibrations....

What I am suggesting is look at anything that could vibrate not just the usual suspects. but, ie. trim tab, flight controls, antennae, fairings etc. It may not be an engine problem.
 
Chris,

I have found the faster I go (higher RPM) the smoother it gets. 2,650 RPM and it smooths out. I think the prop is just too big for this engtine. All I can get is 2650 max. Red line on the engine is 2800 RPM.
 
spinner backing plate cracked

Doubt this is it, but I picked up a vibration a few months back that turned out to be a cracked prop spinner backing plate (around 3 of 4 bolts). This area is hard to see if you have a close cowl to spinner fit, and a fmr neighbor actually found it. The old two sets of eyes was certainly validated that day. Spinner coming loose from prop would be very bad.
 
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