C150 vibration: should I just get over it?
 

So I've a C150J with the standard 0-200. 1,400 hours on the engine. Just finished an extensive (and expensive) annual inspection. New Slick mags, prop balanced, spinner backing plates inspected, etc.

And she vibrates. Not as much as she once did (think East German tractor), but just enough to keep me constantly on edge. I can feel it through the rudder pedals, and even through my rear end. It's worse at some power settings than others. Completely unnoticeable on the ground.

By all the rules, the engine is fine. Invariably starts immediately from cold. Compressions in the 70s. Drinks the usual quantity of oil. Idles happily at 500 RPM. Mag-drops about 60-70 RPM each side on preflight. T&Ps solidly in the green in the air. The only thing out of the ordinary on the instruments is that at cruise power, the tachometer needle lazily wanders through an arc of about 25 RPM. I don't know if that even signifies anything the engine's actually doing, or whether it's just indicative of the shortcomings of an ancient instrument.

If it didn't feel so off-putting, I'd be happy as Larry. But I've flown other C150s that are as smooth as silk. Mine always feels as though it's going over a very slightly rough road.

Bearing in mind that I don't have a ton of money to hunt down the endless possibilities, if this isn't a safety-related issue (it isn't, right?), ought I just to resign myself to the quirks of what is now a vintage aeroplane?

If you're feeling vibration through the pedals or your butt, that's probably too much. You mentioned that the prop was balanced, I've had experience in which the spinner was removed to allow the balance weights to be placed on the prop hub, or spinner back plate. then the reinstallation of the spinner was what actually through it back out. If you suspect this, have the spinner removed, and fly it that way - is it more smooth?

If not, or no spinner, this variable will not be the factor. Go back to the balancing agency, and ask for warranty on their work. A 150 can be silky smooth.....

Vibration can come from one of two sources, the rotating parts such as the engine and propeller or aerodynamic flutter somewhere in the control surfaces due to loose hinges or rigging.

Since the engine checks out okay, the prop has been balanced and no vibration is noticed on the ground, my best guess would be wear and tear on the airframe is allowing something to flutter in the airstream and cause the vibration. It is a pretty old aircraft, so quite a lot of bits and pieces will be showing signs of wear and not being quite so tight as they once were.

Since you can feel the vibration through the rudder pedals, the rudder hinges and control rigging would probably be something worth checking. Prop wash past the rudder in the air would alter depending on engine power setting, so that kind of matches your description of the problem.

Since the aircraft has just passed an annual inspection, I wouldn't have thought it was a serious safety issue otherwise it should have been spotted. You did mention it to the engineers?

I'm not an aircraft engineer, although I do have extensive fault finding experience in other fields, so take this post with a large pinch of salt.

Oh, I've been complaining about vibration since I bought the aircraft. At that point it was really atrocious. Turned out it then had an airscrew that wasn't for a C150 at all, and one blade that was an eighth of an inch shorter than the other into the bargain.

It's better now, though still not terrific. But all the obvious things that one would think of, including the rigging, have been checked within the last five tacho hours. The annual inspection was, as I say, extremely thorough. Hence it's probably an un-obvious thing.

Have you checked the tracking of the propeller? Basically, all you do is put something on the ground (such as a paint tin) so that it's JUST touching one tip of the prop, and then pull the prop through half a turn (check mags off!!) and see if the other blade tracks true.

Any more than a few mm of difference will cause vibration.

I had it on an aircraft once. I tried a different prop and it was instantly solved.

It could be as simple as the engine mounts. Cheap to change. Worth a try.

Does the engine shake or is it running rough, how do the Lord mounts look like? It could be mechanical shaking and if the mounts look cracked, change. If this is one of the O-2/3xx with crankshaft dampers, they might be not free to move. I had this once in a O-300-D, which was running occasionally rough all time and after 1800 hours on OH we found that one of the dampers was probably not moving correct since last OH - after, it was silk like satin.

GOULI's suggestions make sense to me.
Ever so slight age-related wear together with prop-wash over the rudder might be getting close to it.
We had a similar problem years ago with a high time 182.
Does the vibration alter during rudder input?

On the brighter side, until you do get it sorted, you might find the girls queueing up to go for a ride with you. :E

Blind Squirrel

There is an existing thread somewhere here - which I can't find - from an American with the exact same problem. He solved it by noticing that his propeller stopped at a different angle, after shut down, to other similar machines. He took the prop off and moved it round the hub to match the others and vibration was cured...

First, I seem to have missed when it vibrates. Only in the air? On the ground moving or when on the brakes? If on the brakes, what happens at revs when you cycle the controls?

If it happens in all conditions then it's more likely to be in the engine compartment. Or prop.

Blind Squirrel

Found it !!

http://www.pprune.org/private-flying...sna-150-a.html

The OP states that the prop was balanced as a part of the maintenance accomplished. If so, I think he's looking for warranty on that service, unless it's something else.

The prop on an O-200 is not "clocked", you can install it in whichever position you like. The O-200 is not equipped with crankshaft dampers. If the engine were "clock" sensative, the propeller would be dowled to only one of two positions on the crankshaft. That's not to say that reinstalling it might not help - it could. But if it does, you'd have to wonder why, something must not be perfect witht eh crankshaft or flange, and that's a little worrying!

If something else, "flutter" of the rudder is very unlikely, as it is a balanced flight control. If the rudder, or something else back there is horribly loose or defective, that certainly should receive immediate attention, but is unlike the source of airframe vibration. If airframe turbulence is hitting the tail and shaking it, again, something would have the be wildly out of place to cause it. I have experienced that, but while carrying something external to the aircraft, which obviously caused it.

It might take a little chasing, but a maintainer who can balance a prop, should be able to make the aircraft fairly smooth, or state why they cannot.

Many thanks for the suggestions, all. But that's what's scaring me -- especially that terrifying thread unearthed by ETOPS. There must be at least two dozen possible causes in total: each equally plausible. Checking out all of them would probably cost as much as the aeroplane...

Does your 150 have spats fitted?

No, BARAB. Pain in the backside if youi're operating off wet grass.

Have you checked your engine bearer for cracks?

It is worth "indexing" the prop so that it is in the righte place for prop swinging (I know it has a starter), and if it already is then try rotating it 180 degrees.

Another option is to get the prop, and spinner dynamically balanced (adding small weights to the spinner back plate. Not expensice.

Russell.

Does the vibration vary with revs - if you try a different setting do you get a significantly different vibration mode?

Reason I ask is that one machine I regularly fly vibrates lightly in some flight conditions because of turbulence off the canopy area hitting the vertical tail. It doesn't of course vary with engine speed, though higher revs cause a slightly different vibration mode, presumably as the flow breaks away differently.

It's possible to see the T-tail oscillating gently when you crank your head around. It's not always present; some times it seems to be set off by a little adverse yaw, so if your feet are skewed on the pedals or you have an aircraft with a slight fuselage mis-rig, that may cause it. Many old 150s have become slightly twanged over the years, so that could be a cause. I have mostly flown Pipers, not Cessnas, but at least one Pawnee I flew regularly also had a tendency to shimmy, as did one well worn Cub.

You should also check for induction leaks. Seen the same issue solved with just one new gasket more than once. A can of WD40 and a good mechanic can diagnose in short order....

There is a correct index position for the Cessna 150 propeller. Fig 14-1 page 14-3 also para 14-6 page 14-2 of the 100 series maintenance manual.

"On the Model 150 the propeller """MUST""" be installed to trail 30 degrees after top centre of the crankshaft"