jlwild wrote:An out in left field thought. Could the vibration be due to elevator or trim tab flutter?
Unlikely. Elevator or tab “flutter” would ordinarily progress rapidly to destruction and probable loss of the airplane. (and one of the reasons to be wary of excessive “play” in the tab during pre-flight inspections.)
Also, flight control flutter usually occurs at higher airspeeds…and the low RPM at which this is being described would be at low speed.
George, the reason I mentioned this long shot possibility is Gareth asked the question "has anyone any idea what could be causing an RPM-related vibration"...
I have had one in flight experience with that what was related to RPM and airspeed. In 2016, on the last leg of a long flight from Kennesaw, Georgia to Del's "Mountain Airframe" shop in Mena, Arkansas, I felt and saw a low vibration at 2450 RPM 40 miles out from Mena. I felt the vibration in my seat and "saw" it in my clear half filled quart water bottle in the passenger seat. There were clearly visible small vibration waves in the bottle. Reducing RPM from 2450 to 2000 the vibration went away. Increasing vibration it came back. At the time i was +/- 40 miles out from Mena. So, I made a a long long gentle low RPM decent, with no turns, for a straight in landing. Del was there to meet me and I mentioned what happened. Del quickly found the issue. One of the tail feather hinges was worn and had visible play! So, we added changing out worn bearings to my to do list. Time on the airframe was 4909 hours. This may not be what Gareth experiencing but it is something to check out. Jim
Jim Wildharber, Kennesaw, GA
Past President TIC170A (2010-12) and Georgia Area Representative
'55 170B, N3415D, SN:26958, O-300D; People's Choice '06 Kelowna, B.C., Best Modified '07 Galveston, TX, Best Modified '08 Branson, MO.
Hopefully Gareth will check back in on this interesting thread and give an update. He mentioned that he has checked the airframe, presumably hinges, inspection plates, windows, anything that can move, etc. He suspected it was “harmonic” and that leads to questions on condition of the dampeners on the crankshaft that are supposed to move in synch with power strokes at various points during rotation. Some suggestions provided on how that might happen. He also reported the engine was consistently smooth which would probably eliminate the crank as a cause. Or not?
His post indicated some questions about the position of the propeller on the crankshaft flange. The suggestion of trying a different mounting position seems like an easy troubleshooting step. In any case, making sure the propeller is mounted right is a good place to start.
Long ago, I experienced a “new” vibration after reinstalling the same propeller on an 170
(O-300) after an overhaul by Pacific Propeller in Stockton. Before the overhaul, everything was smooth at all power settings/rpm’s. After overhaul, a vibration was felt passing through a certain rpm (can’t remember the number). My mechanic, who normally maintained helicopters rechecked every aspect of propeller installation and performed dynamic balancing using the Chadwick system. This involves adding weight to the propeller hub via washers on the attach bolts until the system indicates balance. As I recall, the Chadwick directs you to the position needing more or less weight. After about an hour, the needed weight was established and I recall him installing precise shapes/weights on one or two of the bolts. Problem completely solved. This mechanic did opine that using this approach might somehow allow the prop to cure an imbalance elsewhere. He implied that as long as the result was smooth, nothing to worry about.
There was mention the OP previously performed dynamic balancing on this airplane so I’m not sure this has anything to do with the current problem, but it’s something to consider. After reading all the suggestions here, I hope he has some helpful advice in eliminating vibration.
Thanks all for your various suggestions and stories.
I am (understandably I think) leaving the harmonic balancer possibility to last as it involves pulling at minimum the exhaust on one side and removing cylinder 1 or 2 to have a look and see if the balancer is stuck. It is unlikely I would be able to measure much so this would be a "rule out stuck balancer" inspection.
As regards airframe suggestions: I rebuilt this aircraft over about a 2-year period including repairing the rudder and replacing most hardware. The elevators are definitely not visibly fluttering (I’ve looked) and there is no play in any of the control-surface or trim tab hinges. All cables are at the prescribed tension and handling is not an issue. The way the problem has always been RPM related and not speed related makes me think engine, not airframe.
Jim mentioned the water bottle. There is an airgap at the top of my compass (need to fill it one of these days) and I can see the vibration in the liquid surface exactly as he describes.
Some months ago, I removed and rotated the prop 180 degrees. Like Richard’s it is a D crank, thus 6-hole and indexed so there are only 2 possible orientations. No joy!
AvGas is US$10.72/US gal so in school we generally ran the 172s around 2200 to save fuel when tooling around going nowhere in particular during lessons. So I guess that was my go to RPM. The more fundamental issue is I need to know why it is vibrating rather than avoid a particular RPM. It is a symptom of something not right. Avoiding the symptom will not find the problem and I need to trust the aircraft before bringing passengers along. If it was an engine (like the IO-470 etc) that has a notorious RPM to be avoided, I would have no problem doing that. However, the reputation of the O-300 is underpowered but beautifully smooth.
We did run an analysis with the dynamic prop balancing rig and found relatively low levels of imbalance (I can’t remember the number). During this exercise, we noticed rough running so abandoned the exercise to chase that (again).
Here's where we are:
1. Checked the crank paperwork. We rebuilt this engine around an overhauled (yellow tag) crank in which the bushings were replaced on the counterweights and the counterweight blades.
2. The aircraft flew for several hours without this problem. It was then intermittent and eventually became constant.
3. I replaced the valve springs early in the saga
4. I replaced the mags, harnesses and plugs – in my mind ruling out the ignition system
5. I purchased a rebuilt Marvel-Schebler carb from Aircraft Spruce. A crack opened in the flange to which the airbox attaches while I was installing the stud, it was barely more than hand tight. MS informed me their QA does not let such faults happen and therefore I must have cranked on the stud and broken the flange and they refused to cover it under warranty. As a result I put the old carb back on. MS agreed they would replace the bottom casting of the carb for $250 and I would have to pay shipping of $100 each way. I did not opt for giving them more money as I was properly hacked off with their piss-poor service.
6. Checked the valve guides on 2 of the pots and found them way out of spec. Given the cost of owning an aircraft that I can’t use was mounting up, I bit the bullet and replaced all 6. Still had the vibration.
So to yesterday:
Having had the flange on the rebuilt carb repaired by a specialist welder locally (no one panic: remember permit to fly aircraft – think FAA experimental) I installed it yesterday. Adjusted the idle speed stop and the idle mixture and it was struggling a bit at idle.
As it warmed up, I noticed #6 CHT 30 degrees C cooler than the others. Step 1, rule out probe problem, so I checked the thermocouple connections at both ends and all good there. Shut down and measured temp with laser thermometer next to the top spark plug holes and all 6 were within 5-10 degrees C of each other. Warmed it up again. Shut down, crawled under and measured next to the bottom plugs on #4 (140C) and #6(98C); exactly as the engine monitor was telling me. The temp sensors are the ring type that replace the copper washer on the bottom plugs.
Pulled the plugs: bottom plug was fouled; top plug was a little sooty. Cleaned them both and swapped top and bottom plugs when reinstalling. Warmed up again and got the same temps as before.
I ran up to 1600 and did a mag check. Running on R mag, runs fine 50-75 RPM drop. Running on L mag, rough as f***! Ran up to full static power, just short of 2200RPM. O-300C with 1C172/EM 7653 should be making close to 2300RPM. (2230-2350)
At this point I noticed that #5 temp is now starting to lag 1-4 similar to #6.
It also dawned on me that the RPM in the engine monitor has acted up since I put the new mags in. Sitting at 1100RPM warming up it will periodically flash down to 800ish for a split second and then back up to 1100. This happened about 5 or 6 times each minute and frequency of that is now increasing. I read the mechanical tach as a general rule, so I had put that to the back of my mind as something to sort out later. However, that signal comes from the P lead on the L mag!
I have concluded that the L mag is faulty. I will contact Champion tomorrow to see if further troubleshooting is required. The mags have 9 hours on them, so I’m hoping warranty will not be disputed, although after dealing with Marvel-Schebler on the carb, I’m not holding my breath.
I had similar mag problems a while back. Mags only had 300 hours on them, sent them back for rebuild, still had problem with right mag. I noticed that if I wiggled the mag switch while doing a run up I got a slight fluctuation.... I disconnected the P-leads and did another run-up...Bingo! No problem. I replaced the mag switch and took the old one apart and could see carbon tracking in it. Worth a try. Also Bendix was having problems with some of their points recently. The company that rebuilt my mags (Premier in Troutdale, OR) recommended just dressing the old points as they were having too many problems with new ones.
