The International Cessna 170 Association

Hi all,

I am still searching for the cause of the harmonic vibration in my 170. Previous discussions led me to valve springs which turned out not to be the problem. Here's a quick update:
1. Replaced the valve springs
2. Replaced the ignition system: mags, harness, plugs. (Mags were hosed).
3. Checked airframe for anything that could be causing the vibration ... nothing
4. Measured valve guides and found a couple to be waaay out of spec: replace all six cylinders.
5. Checked the engine mounts - all in good condition and tightened correctly
6. Measured the prop flange with dial indicator. Runout at the flange = .0023" tolerance = .005"
7. Checked prop tracking <1mm (39 thou) difference between the blade positions at the tip.
8. Measured the pitch angle of the prop blades (on the aircraft with electronic inclinometer so slightly approximate) ~0.5 degrees difference at 2/3 station.

The engine runs beautifully, no roughness at all. However, the vibration in flight is unchanged. 2350+ RPM it is there but not too obvious (lower amplitude). 2150-2200RPM where I like to cruise it is uncomfortably obvious - harmonic resonance!

I'm left with 2 things we can think of:
A. The prop (1C172/EM7653) or the crank flange is indexed incorrectly (6 hole -D crank in -B engine = O300-BCC) - seems unlikely except there is a pen mark on the inside of the hub where someone wrote "bottom". With the #1 cylinder at TDC on the compression stroke is NOT at the bottom, it's about 45 degrees off the bottom. Note 6 hole prop so the old clocking document for the 8-hole doesn't apply (I think). NOTE: The propeller, engine and crank came from different sources.

B. A harmonic balancer is stuck - yes I should have checked this while I was changing the cylinders, but of course I didn't! This also seems unlikely as the crank was refurbished and the bushings replaced on the balancers and blades.

The question: Has anyone seen a stuck harmonic balancer on an O-300? If so how perceptible was the vibration it caused in the airframe?

Ongoing question: Has anyone any idea what could be causing an RPM-related vibration that persists after everything listed above?

As always, input appreciated!

Gareth.

Well I can’t imagine that scenario “ stuck damper “. Worn pins and bushings certainly and if really worn bushings metal to metal contact in the vertical plane and a vibration but in my experience, its felt in deceleration. Don’t know why that is and whether there’s any trend as in others observing this. The weight normally slides from side to side in the groove between the shaft and weight. There’s two weights on the shaft.
I posted the wear before George suggested a possible prop strike but I think its wear over time with worn components.
Replacing the bushings requires tooling and unlikely something that would be undertaken in a “ field overhaul”.
I had a crank I sent out for overhaul and the weights were condemned due to wear in the bores, so I know this stuff wears.
Again I only noticed a vibration in deceleration, subtle around 1800RPM but I now fly a lycoming and the vibration is anything but subtle. The Continental is such a smooth motor even when it becomes a senior, so I understand your concern.

Because the vibration occurs and varies with RPM, it is might be related to the propeller. I recommend seeing if your mechanic can arrange for propeller balancing using a Chadwick system. Others may chime in pro or con.

https://www.avionteq.com/Honeywell-Chad ... r-Analyzer

https://www.expaircraft.com/PDF/SmoothPropeller.pdf

gobrien wrote: ... The engine runs beautifully, no roughness at all. However, the vibration in flight is unchanged. 2350+ RPM it is there but not too obvious (lower amplitude). 2150-2200RPM where I like to cruise it is uncomfortably obvious - harmonic resonance! ...

Critical Service Bulletin CSB 09-11A is for larger Continental engines but I wonder if engine RPM might be a factor in your vibration problem. 2200 is bottom of the green arc on the tachometer so I'm probably barking up the wrong tree here. ECI Service Instruction 89-5-1 and Chapter 8 --"Troubleshooting"-- in Continental's Standard Practice Maintenance Manual offer suggestions on troubleshooting, most of which I think you've looked into. If it were me, I'd try and get the engine dynamically balanced. Chances are the guy doing it will have suggestions on further troubleshooting. Edit: Looks like Harlow beat me to the same suggestion. Great minds think alike! But I see from your Feb. 7 2023 entry that you may have already done this. What were the results of that?

Thanks Gary for the Continental manual birddog.
From the manual

The complete crankshaft /counterweight assembly is dynamically balanced and the counterweights are matched in pairs with a maximum weight variation of two grams. If either counterweight is damaged, both counterweights in the matched pair must be replaced as a pair on that crankshaft cheek, even if only one counterweight is unserviceable.

The weight in the top pic I’m guessing by the pic has wear in excess of the .005 limit. Anyone need some paperweights for the office ?
But back to the topic, (for inspiration, might tinfoil my head) …The vibration occurs in steady state above 2400 RPM , but does it occur at all altitudes ? Does making a throttle adjustment to vary the RPM from the lower RPM vibration band to the upper band have a corresponding increase in vibration and finally what’s it like with wide open throttle ?

My question is: Why do you like to cruise at 2150-2200..?? It’s not the recommended RPM…. 2450 is the recommended….

I once operated an airplane which had a “harmonic” that drove us nuts. It would only do it in climb….(possibly because of C.S. prop it changed pitch to maintain the selected rpm when the AOA changed during level-off.)

We spent a ton of money trying to figure that out.

One day an inspection cover under the cockpit was discovered to be the culprit. The tinnerman nuts which the PK screws that held it had lost their “clamping” forces. A technician replaced them without even knowing …because he’d used an elect screwdriver that stripped them out…. so he inserted new tinnermans….the inspection cover quit smoking at the edges (probably held tighter to the fuselage.)

Useless info, perhaps….but demonstrates that “harmonic” sounds are not necessarily harmful…and that this may be a red-herring. I believe you should avoid cruising at that RPM.

To quote the great philosopher Henny Youngman, "I told my Doctor 'it hurts when I do this' the Doctor said 'don't do that' "

Try taking the prop off and reinstalling it 180 degrees out. That was accidentally done on my O300-D at one annual which caused an unusual vibration which disappeared when turned 180 degrees back to where it was originally.

My 170B behaves the exact same way. At climb power and cruise (2450) engine/prop is relatively smooth. When I pull back power in the pattern to 2100 on the downwind leg (I'm not in a rush...) I feel an increase in vibration. I had my propeller dynamically balanced and it improved smoothness across the board. My mechanic told me "You've got a 30 year old prop mounted to a 30 year old engine. It's got compression and isn't making metal. Fly the damn thing." So I've been following his advice.... (while always watching for a place to land )

Fishsticks wrote:My 170B behaves the exact same way. At climb power and cruise (2450) engine/prop is relatively smooth. When I pull back power in the pattern to 2100 on the downwind leg (I'm not in a rush...) I feel an increase in vibration. I had my propeller dynamically balanced and it improved smoothness across the board. My mechanic told me "You've got a 30 year old prop mounted to a 30 year old engine. It's got compression and isn't making metal. Fly the damn thing." So I've been following his advice.... (while always watching for a place to land )

He told you that in the 1980’s…..??

GAHorn wrote:He told you that in the 1980’s…..??

Haha. In my case the engine and prop were last overhauled in 88.

An out in left field thought. Could the vibration be due to elevator or trim tab flutter?

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.

Agree, think Reno Galloping Ghost.

Unfortunately the lease on my hangar will not be renewed and I’m losing my hangar of 25 + years so I’ve been moving things and losing place of things but I did measure the crankshaft counterweight bores of an engine I’m presently parting. One set of bushing bores in the crankshaft flange, measured right in the middle of the allowable wear limits. The other set of holes were .008 over the limits and out of round about .001, so new bushings will be required should the crank pass further inspection. I’ve got the counterweights somewhere and will measure and report their bore measurements when I get a chance. Just another slap of reality or maybe something else but I’ve recently seen advertised an overhauled 0-300 in Atlantic Canada for 40K.