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RV-7 Vibration

pilot28906

Well Known Member
Hello all,
When in cruise I get a vibration change that appears to come from the cowling or engine area. At times it happens with a change in power and at other times for no apparent reason. I thought I had stopped it by tie wrapping the drain tube running down the firewall and tightening the exhaust brackets and getting the exhaust as far as possible from the firewall. However the vibration came back. Just finished the annual inspection and trimmed some baffling that appeared to be rubbing the top cowling but on the first flight the vibration came back. It is more of a frequency change than a new vibration.

Anyone have a similar experience? RV-7, standard cowl, Catto 3-blade prop, IO360-M1B engine, standard Vetterman exhausts.

Thank
John:confused:
 
Hello all,
When in cruise I get a vibration change that appears to come from the cowling or engine area. At times it happens with a change in power and at other times for no apparent reason. I thought I had stopped it by tie wrapping the drain tube running down the firewall and tightening the exhaust brackets and getting the exhaust as far as possible from the firewall. However the vibration came back. Just finished the annual inspection and trimmed some baffling that appeared to be rubbing the top cowling but on the first flight the vibration came back. It is more of a frequency change than a new vibration.

Anyone have a similar experience? RV-7, standard cowl, Catto 3-blade prop, IO360-M1B engine, standard Vetterman exhausts.

Thank
John:confused:
Might check the rubber strip that goes along the wing root.
 
Baffles or exhaust rubbing on something is my first thought.

Something that startled me a time or two is/was the resonance of the floorboards which changes substantially depending on where my feet are placed.
 
I have an app on my android phone called "Resonance". Joel Graber told me about it. I think it was intended to survey floor vibration in architecture, but it works pretty well in the airplane. Displays and records frequency and amplitude for the X, Y, and Z axis.

The idea is to identify the frequency of the vibration you're feeling, because frequency offers a clue as to the source. For example, firing events for a 4 cyl at 2400 would be 80 hz. Propeller out of balance at 2400 would be 40 hz. A loose wing root strip would be some random frequency not related to RPM.

Location can also offer clues. For example, assume 40 hz as above. If the left cockpit rail shows lots of amplitude in the X axis but the right rail does not, chances are the vibe is simply an air beat on the fuselage and canopy from the clockwise rotating prop. If the amplitude is pretty much the same on both rails, it may be an out-of-balance prop.
 
Something that startled me a time or two is/was the resonance of the floorboards which changes substantially depending on where my feet are placed.

Then fly for hours over the open ocean. That vibration gets exponentially worse! :D
 
Hello all,
When in cruise I get a vibration change that appears to come from the cowling or engine area. SNIP

Thank
John:confused:

A vibration, or a noise? It is not easy to localize the source, other than the obvious 40/80hz ones Dan mentioned (prop, etc.). Dan's (always good) advice on the app is good. I use Spectroid on my Android phone, although I've never used it on the plane. These planes are essentially one big rattle/vibration.
 
Thanks all, I will try the suggestions and one of the apps. The frequency change is startling at times. I will report back.

Thought about trying the vetterman exhaust extensions but didn’t see them on their web site; can these be ordered by phone?

Thanks again.
 
mystery vibes

I would think the wing root fairing seal would change with speed. I was feeling a high frequency vibe that changed with speed. Did a good once-over, maybe twice, and found the backing plates for main wheel pants were both cracked. The fairings weren't installed so they were at the mercy of the wind. Took them off (in two pieces) and the vibes disappeared. My point is it could be something sort of obvious...hopefully.
danny
RV9
 
Have you tried stepping on the brakes when it happens. I had my tires rotating on a previous airplane that would start and stop at times.
 
I haven’t thought about the wheels, I will try the brakes to see if it helps and look at the wheel fairings and wing root gap seals. Thanks for all the great suggestions.
 
Someone posted this on VAF before, could have been Walt.

Vibration Checks

Check prop spinner mounting bulkheads, both front & rear for cracks
Check prop blades for looseness & track
Check front of cowl for spinner rubbing
Check cowling mounting hinges for broken loops, especially bottom sides
Check baffling for rubbing on cowling
check baffling for cracks, side panels
check baffling for cracks around lower cylinder skirts
check (carbureted or vertical FI) air filter mounting plate for cracks
check alternate air door for looseness
check oil cooler if mounted on baffling for cracks
check crank breather tubing for rubbing
check engine mounts for wear, engine sag
check air snorkel for rubbing interference with cowling, or rubbing on lower side cowling
check for anything rubbing on engine mount
check loose hot air baffle
check gear leg fairings for looseness
check gear leg rear edge hinges for looseness
check wheel pants for looseness or wear
check w/shield fairing for separation from plexi
check canopy skirts for looseness or dynamic vibration
check all wing access covers for looseness
check gear mounting covers (rv-8) for looseness
check flap to fuselage fairing for looseness
check flap hinge & aileron bearings for wear

I've found all these conditions on various RV's over the years & the owners common complaint was vibration, weird feel, or noise during flight.

& yes the prop balance could cause a varying harmonic. The tail pipe hangar would be the first thing I would look at, I've rotated the pipe around to get the bend in a better angle in some cases & have trimmed the cowl in some others to gain more pipe clearance on others. In a couple, I've revamped the tail pipe hangar arrangement to better float with the engine & not hang from the engine mount.
 
Thought about trying the vetterman exhaust extensions but didn’t see them on their web site; can these be ordered by phone?

This may or may not help.

I didn't have any belly skin cracks but wanted to reduce the floor vibration. Vetterman sells turndown extensions and straight extensions. I already had the turndown exhaust, but they were short. So Clint at Vetterman's sent me two short, straight extensions. They are each attached by one bolt. Hint: Put the bolt in per the instructions so the bolt is pointed toward the center of the airplane and not visible to a bystander standing next to the airplane (not like I did).

i-FdnrsLm-L.jpg


i-dmrKFRw-L.jpg


And the instructions:

i-cBQQXrp-XL.jpg
 
I have an app on my android phone called "Resonance". Joel Graber told me about it. I think it was intended to survey floor vibration in architecture, but it works pretty well in the airplane. Displays and records frequency and amplitude for the X, Y, and Z axis.

The idea is to identify the frequency of the vibration you're feeling, because frequency offers a clue as to the source. For example, firing events for a 4 cyl at 2400 would be 80 hz. Propeller out of balance at 2400 would be 40 hz. A loose wing root strip would be some random frequency not related to RPM.

Location can also offer clues. For example, assume 40 hz as above. If the left cockpit rail shows lots of amplitude in the X axis but the right rail does not, chances are the vibe is simply an air beat on the fuselage and canopy from the clockwise rotating prop. If the amplitude is pretty much the same on both rails, it may be an out-of-balance prop.

And for iPhone users, there is an app called, appropriately enough, Vibration, which does the same thing. A Fast Fourier Transform (FFT) for the math geeks. Useful for all sorts of things.
 
Next time move your feet to various places on the floor while observing. On my 6, I find different vibration and sound patterns based upon where my feet are placed. Drumming of the bottom skin is a real thing on these planes. Not sure if it is exhaust or turbulent air coming out of the cowl or both. I do know that many have reduced drumming by extending the tail pipes. Big issue on the 10, as you can see how long the pipes are now compared to the original version. Some builders inadvertently cut them down and while it might help with drag, it makes a lot of noise and vibration.

Larry
 
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