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What's Causing This Weird Vibration?

R

RV7ator

Well Known Member
Since my OP last April, http://www.vansairforce.com/community/showthread.php?t=170636 I've additional info. Ideas, anyone?

The weirdness: a vibration suddenly kicks in at an IAS between 90 and 110 mph during the first of the day's take-offs. Nothing subsequent.

The Whirl Wind 74RV spinner has broken - thrown out a chunk - twice within 700 hours, and the third is showing damage. The spinner bulkhead is chafed on on only one side, and after a few hours the screws can again be snugged down because enough material has eroded away from spinner and bulkhead.

What on earth would cause an engine to break spinners!

I've heard of vernatherm, governor, pressure spring, sticky valve guide notions that seem to address the obvious link between oil and under cowl temperatures with the vibration; some sort of resonance interior to the engine that excites the prop/spinner combo. But, my goodness, all seem like a stretch. My gut still says engine vibration isolators are not doing their job (They are the Lord mounts with the secondary internal cone limiting start-up motion.)

A 200RV prop Whirl Wind loaned to me hasn't altered the vibration phenomemon. Curiously, Whirl Wind has stepped away from stroker 360s (and anything else with greater than 9.6:1). From their website: "(I)0-370/375 NOT SUPPORTED"

So much would seem to implicate the airframe, but I've dismissed that mainly over the temperature issue - first takeoff only, and sometimes not at all if temps are already elevated - and magnitude is linked to power.

Any further ideas, brain trust?

John Siebold
Boise, ID
 
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I would be looking for contact between the baffling and the cowl. You can typically find witness marks on the cowling interior surface if contact is being made, though it may be very light if the contact is only occassional. I don't see how spinner based vibration can be variable based upon tem\p.

Larry
 
The weirdness: a vibration suddenly kicks in

The weirdness: a vibration suddenly kicks in at an IAS between 90 and 110 mph during the first of the day's take-offs. Nothing subsequent.

First flight only: I'd suspect a tire that has taken a flat spot from sitting and/or is out of balance. Apply the brakes smartly as soon as it happens, and see if it stops. Some more sophisticated aircraft automatically apply the brakes after takeoff to preclude resonant vibrations as the wheels spin down.

Cheers, David
RV-6A A&P
 
Didn't see these points covered in previous thread;
Has prop been dynamically balanced?
Were spinners replaced or repaired?
Has prop been balanced after each spinner repair?
Has prop been balanced without spinner installed to determine if spinner is the problem?
Has plane been test flown with spinner & bulkheads removed? (caution for cooling changes!)
Has the prop bushing been confirmed in correct clocking locations per Lycoming SBs?
Has prop pitch been confirmed between blades? Blade stops?
Hope you find the culprit soon, if not, I'd seriously think of switching to a prop with more mass to absorb the 375 torque pulses (if all else checks out)
 
A friend of mine installed a WW prop on his Lancair 360. Had nothing but problems with it. Mostly it kept losing the prop seals and leaking grease. This happened over and over. WW said it was a power pulse problem with hopped up engines, this 360 has 10:1's, EI's, etc. They gave him a balance wheel to help stop the pulses to the hub but this didn't help, the engine needed the weight of heavier blades to run smooth. He dumped the WW prop and went back to a Hartzell prop. Problem solved.
 
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Dr. Dan and Dr. Mahlon

RV7ator said:
Since my OP last April, http://www.vansairforce.com/community/showthread.php?t=170636 I've additional info. Ideas, anyone?

The weirdness: a vibration suddenly kicks in at an IAS between 90 and 110 mph during the first of the day's take-offs. Nothing subsequent.

The Whirl Wind 74RV spinner has broken - thrown out a chunk - twice within 700 hours, and the third is showing damage. The spinner bulkhead is chafed on on only one side, and after a few hours the screws can again be snugged down because enough material has eroded away from spinner and bulkhead.

What on earth would cause an engine to break spinners!

I've heard of vernatherm, governor, pressure spring, sticky valve guide notions that seem to address the obvious link between oil and under cowl temperatures with the vibration; some sort of resonance interior to the engine that excites the prop/spinner combo. But, my goodness, all seem like a stretch. My gut still says engine vibration isolators are not doing their job (They are the Lord mounts with the secondary internal cone limiting start-up motion.)

A 200RV prop Whirl Wind loaned to me hasn't altered the vibration phenomemon. Curiously, Whirl Wind has stepped away from stroker 360s (and anything else with greater than 9.6:1). From their website: "(I)0-370/375 NOT SUPPORTED"

So much would seem to implicate the airframe, but I've dismissed that mainly over the temperature issue - first takeoff only, and sometimes not at all if temps are already elevated - and magnitude is linked to power.

Any further ideas, brain trust?

John Siebold
Boise, ID
Click to expand...


Did you try Dan Hortons advice? In your OP, you say it is most evident in the rudder pedals. Is it floor boards under your feet or in the actual rudder pedals?

Spinner breaking? That sounds serious. It is difficult to understand how a dynamic balance would say all is well if it is shaking so bad as to fatigue and break the spinner.

Perhaps try a different balance man & machine? I would be afraid to fly it by the symptoms revealed.
Perhaps balance your main gear wheels/tires?
 
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F1R said:
Spinner breaking? That sounds serious. It is difficult to understand how a dynamic balance would say all is well if it is shaking so bad as to fatigue and break the spinner.
Click to expand...

I had one plane that came in with a heavy vibration at low rpms but was able to balance smooth at hi cruise rpms. Eventually we found the spinner itself was the culprit (chromed aluminum spinner with a big glob of chrome pooled near the front tip but offset). In that case a new fiberglass spinner fixed it all up.

Maybe this or something similar is happening.
 
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