AP Brain Trust Challenge: Very Subtle Shudder/Vibration

Sinistar

Sinistar

En-Route
Joined
Sep 9, 2016
Messages
3,734
Display Name
Display name:
Brad
Cessna 182P Stock O-470. 800SMOH, 300 since top end, 2300TT. The prop hasn't be overhauled since the early 1990s. Mags were IRAN and timing adjusted 280hrs ago.

First Noticed:
Near the end of a 3hr leg on a long'ish letdown my passenger (pilot+mechanically savvy) and I noticed this very subtle shudder/vibration. He was wearing a older non-noise canceling headset and was hearing it. I was wearing a sound canceling headset and was feeling this same thing in the seat. We were slowly descending (-300fpm) a total of 6000ft. The prop was still out, leaned for cruise and still quite a bit of power. Ground speed was probably about 170mph during that period. As I started bringing power back, carb heat in, etc it went away. I did notice during that period that the RPM needle was just barely fluctuating (about +-10rpm) - like the needle was barely moving. On the ground no oil lost, no leaks, prop blades find, etc. On the way back it never did it.

First obvious guess was prop governor so I called the mechanic and prop shop and they both said it seemed okay to try it out again to learn more, keep it local, etc.

I did a second local flight about a week ago. This flight was to learn more. I was able to repeat it in cruise. It seems it needs to warm up and either level flight or descent where air speeds are highest. I didn't notice anything in the climb out. I did not see oil pressure fluctuating during the event nor did I see any MP fluctuations. CHT's and Oil temps look normal as well. Unfortunately no Engine Monitor so going by 48yr old gauges, sound and feel.

Got the call today and the prop shop said the governor tested out great on the stand and they do not recommend IRAN, overhaul, etc. They said a possibility its leaking in the prop itself but didn't seem at all sold on that.

The A&P said it could obviously be a lot of things but was first to admit he had no definitive reason. While the lower cowling is off I am having them install new plugs since that was recommended for our next annual. I am going to try another local test flight afterwards and get more data.

The shudder/vibration is best felt vs heard. The duration of frequency (or harmonic) is probably like 1...2 seconds.

No known oil leaks and not adding oil, No leaks around the prop. I definitely can not spin or twist the blades during pre-flight - neither could the A&P. The prop is old so it has its share of tiny little nicks and scratches but nothing that has alarmed two different A&P's at unrelated locations.

I would say that even with this subtle vibration it is still smoother in cruise than the Dakota and Cherokee 180 I have ridden in lately - my only two data points. But tells me if its a balance issue it is more subtle than gross.

Any ideas would be greatly appreciated or similar post links would be greatly appreciated!
 
Sinistar said:
The A&P said it could obviously be a lot of things but was first to admit he had no definitive reason.
Click to expand...
Sinistar said:
I was able to repeat it in cruise. It seems it needs to warm up and either level flight or descent where air speeds are highest.
Click to expand...
This is the problem--too many variables at this point. One route to try is to determine if the source is aerodynamic or mechanical. Aero vibs tend to cycle slow and mech tend to cycle much faster. Sometimes trying several high power ground runs at different headings vs wind can help separate vibs between the two sources. And what do you mean by "The duration of frequency (or harmonic) is probably like 1...2 seconds."

If nothing can be found that's obvious, maybe look into having a vibration analyzer put on your aircraft and run a vib spectrum to try and isolate the frequency of the vibration. Most prop balance equipment can be used to run a spectrum which I've used in the past to chase odd vibs and quirks.
 
Thanks for the information - getting help on a starting point is a great first step so much appreciated.

At first I was going to have the prop re-balanced and it is still an option but both the AP and myself are thinking to wait and bit and see if a local flight or two can narrow down the source. A dynamic balance is only about $250 which is pretty cheap in the grand scheme of things so I will definitely try that at some point if other more obvious stuff doesn't pop up.

You mention aerodynamic and how is more likely to cycle slow. That "Duration frequency...." was my way of saying the very low level vibration seems to repeat about once every second or two. You can actually feel it cycling/repeating about once every 1...2 seconds when its happening. Which is giving me an idea!

When taking off in the hotter summer DA's, even down at our lowly 1000msl the ground speeds at takeoff are a bit higher. You can tell it a bit from where you rotate. Usually in the 182 it is before the 1st 1000 footer. But I can also tell by the wheels / wheel pants. The first two summers we didn't have the wheel pants on so I don't recall experiencing it. But this summer on a hot day they shudder/vibrate pretty good. I can tap the brakes but it doesn't stop immediately. Take off on a cool morning the next day...doesn't do it. So maybe this is a wheel pants thing. And maybe its a front wheel pants thing since toe brakes can't stop that wheel! They have to come off this fall anyways so maybe I will pull them off before the next test flight. Or maybe if I can repeat in cruise I can yaw a bit to see if it goes away or something like that.
 
MauleSkinner said:
The thing that jumped out to me is that you were leaned for cruise, but descended away from the altitude for which you were leaned. Maybe it was too lean?
Click to expand...
Seems like that would result in engine roughness, rather than a subtle vibration.
 
Dynamic prop balance with an analyzer that does harmonics will be a great starting point.. Engine vibrations have different hz than prop vibrations.
 
I also fly a 182P and I have some irritating sense when something feels different. I recently had an issue with a slowly growing vibration. When this happens, if there is nothing obvious on the outside that could cause it, it tends to be mostly resolved with a prop balance. In fact, I just had my prop balanced for $200ish and it smoothed things out.

However, as others have said, your issue could be something entirely different but before heading down the long list of expensive options it seems like an inexpensive prop balance would be a great place to start and see if it alters your vibration.

And for the record, I'm not a fan of surprise vibrations.
 
Ryanb said:
Seems like that would result in engine roughness, rather than a subtle vibration.
Click to expand...
If there's one plug going bad, it will stop firing when the mix gets too lean for it. It might still make a tiny spark, but that's not enough when the mix is leaning out. The vibration will come as that cylinder takes longer to combust and its pressure peak is later. The OP should get it to start the vibration in the descent, and just push the mix in some and see if it goes away.

Ignition is far more often responsible for performance issues than fuel systems are. Magnetos just make it worse. We're all used to electronic ignition in our cars that develop twice the voltage of a mag and make really fat, hot sparks, and doing it for 300,000 miles without a hiccup. The old-design Champion plugs have also made a lot of people mad. Much money spent chasing the problem and throwing parts at it.
 
Amen to the old Champion plugs causing issues. Several years ago, I had issues on my Jacobs that turned out to be three plugs with internal resistance over 50,000 ohms.
 
I've done 2 local test flights after they put the governor bsck in. They also installed new tempest plugs. No change.

Now the new plugs are noticed...during startup its like the engine is gonna rip off its mounts...not that bad but you get the drift... more aggressive during I initial start. Almost no mag drop...so new plugs acting like new pluhs. But new plugs did not alter the subtle vibration during cruise.

On the last flight I tried aggressive lean to nearly full rich and it remained the same. I tried aggressive slips both ways to see if something airframe related might change...nada.

I've tried many different rpm settings and I think I found a spot where the frequency changed a bit but very subtle.

One thing that did effect it was slowing way down in cruise until the rpm can't be maintained. I forget the MP. Once its not getting enough power to maintain constant rpm it goes away.

The wife's taking it for a spin as I type this. Maybe she'll figure out the vibration...that should open the door for a few jokes!

The next flight will be with wheel pants removed for winter flying. Don't expect it to change much.

I think my next step is a dynamic balance. Given the cost vs possibility it is probably the next best $$$ towards troubleshooting.
 
Sinistar said:
One thing that did effect it was slowing way down in cruise until the rpm can't be maintained. I forget the MP. Once its not getting enough power to maintain constant rpm it goes away.
Click to expand...
By this statement do you mean that you unloaded the prop and were basically gliding?
Sinistar said:
I think my next step is a dynamic balance.
Click to expand...
A couple tips:
Before you perform the dynamic balance check the security/torque of all the major engine items like exhaust system, prop bolts, alternator, engine mounts, etc. Especially check the rubber engine mounts for condition as they are technically a vibration damper and are tuned to isolate inherent engine vibrations from the cabin in the lower frequencies. If you find something loose perhaps fly again and check it.

Also, if your prop balance check doesn’t turn up any issues, might see if they can perform a complete vibration spectrum check of the engine on the ground and in flight to see if you can determine the vibration freq/magnitude. Then with those readings you can compare to factory data and possibly narrow down the source of the vibration. Have chased a few stray vibrations down with this method. Most prop balancers have this capability provided they have enough accelerometers and cables to rig it up.
 
Figured it out!!!

TLDR: It was the nose wheel and or nose wheel pant.

@Bell206 - your first response regarding aero frequencies being lower and mechanical frequencies being higher was spot on!

When something seems wrong up front and related to the prop I think having the governor looked at was a good first, safe place to look. As I updated a few posts afterwards the governor was fine. The A&P said fly it some more and try narrow it down - which from the outside might seem unsafe bet he didn't seen anything engine or serious in the prop so that's just what I did. I made a few flights to understand exactly how to make it happen. No big trips just experimenting. Eventually my wife took it out and we both agreed on the sound, the location and frequency.

When they re-installed the governor we upgraded to tempest plugs since they were overdue and that didn't effect it either.

My next step was to to change oil (since it needed it). I tipped it back on its tail and did everything I could to get all the oil out. Then I fully topped it off. I hate doing that because its all going on the belly...which you will see in our aerial picture thread I haven't posted yet. I was wondering if being fully topped off would at least change something with oil flow up in the hub, etc...nope.

So on Saturday I removed all the wheel pants for winter and to make the annual easier. I also scrubbed the belly.

I learned from my dad that when you jack up a tire and before you remove it give it a good side to side pull and you might get lucky and catch a bearing going bad or find out the lug nuts weren't tight, etc. So I did this to the nose wheel before removing it since it has to be removed to take off the wheel pant. I noticed the slightest amount of side to side play. And who did that...me! When I put the nose wheel pant on this spring I was afraid to over tighten and pinch the fork too tight on the wheel bearings. You could say it was a 1/2 turn of the nut to loose. That half turn comes from finding the next cotter pin hole. So now I had found at least something.

Then it gets a little humorous.

The wife wants to fly it first since we will be landlocked into our hangar for the next month while they redo all the asphalt. So I am thinking she will come back with this huge smile that it has been fixed. Instead she hands me this old Bose A20 headset I had asked her to try. She's like: "These suck." Don't worry Bose fans...its not a bashing thread. Basically they were turning off after about 5 minutes...but the power LED was slow blinking green meaning good battery....i figured that out later.

So she full stopped and switched to the Zulu 3 and was off again.

Then she lands again and sits their idling for awhile - WTF??? Turns out this monster Bee crawled out of the wing air vent when she was turning base to land LOL! She somehow killed it between base and final - not bad!. One leg was all that was left and about as long as the fingernail on my thumb. We left it stuck to the inside window as her trophy LOL!

That's what she gets for going first :p.

So she just does another couple pattern laps and needs to head home. I am like" "What about the vibration!". Her: "Oh between the headset and bee I pretty much forgot to check it out". Aarrgghh :mad:

So now its my turn for my last flight for the month. Fingers crossed there was only 1 monster bee.

I take off, climb right up to 4500, level off, lean to stumble and back a bit, close the cowl flaps...wait for it....nothing but the buttery smooth we were always used to :) There were scattered clouds with bases at 4500msl and tops at 6000msl. So I just buzzed around them for an hour!!!

So technically it could be any of the 3 wheel pants or that slightly loose nose wheel axle bolt. I can reverse the order next year and figure it out. My theory is that at the leveled off cruise speed of about 135kts IAS, the nose wheel starts spinning fast enough that it wobbles, then slows down and repeats.

Oh yeah the Bose A20 problem. I had been letting a friend use them while he has been time building. He finally bought Zulu's from pigpen so gave me back the A20s. Last night I discovered that the (+) contact in the battery compartment has a layer of corrosion on it and wouldn't make good enough contact. So I fixed that and they are good to go as well...but I still like the Zulu 3's better
 
455 Bravo Uniform said:
Nice find.

I wonder if a GoPro style camera could have also confirmed that.
Click to expand...
I bet it would have. Lately I have just been running (2) inside the plane but I do have a exterior mount and should have thought of that.

I'm still kicking myself for first assuming its the prop governor but just not enough experience (thankfully) to know how they fail and it seemed like the right place to start. Oh well.
 
Actually if I would have had the GoPro running I would have got the wife's epic bee kill on camera....dang!
 
You must log in or register to reply here.
Back
Top