Smoooooooooth??

[LarryT]

Originally Posted by Walt

Done a bunch of FP balances (which usually come out very nice) but none with a Landoll balancer installed, so I really don't know how it would react.

From what I know about the balancer which isn't much (fluid coupled inertia ring?), it leads me to believe I would be chasing a moving target.

That's what I thought (you are also correct that it is a fluid coupled inertia ring). The engine seems pretty smooth to me as is, but I have had floor vibration I think was exhaust related. Putting tailpipe tips on helped greatly. Work on cockpit floor stiffening and damping should further improve the situation.

LarryT

[Walt]

Not sure how much is involved in removing the damper but I think the "ideal" thing to do would be to balance the prop first without it and then reinstall it.

[DanH]

[quote=Walt;480633]Good stuff here, I am going to follow up on the lower RPM spikes to see what I can find out.[?QUOTE]

Human perception of vibration can be misleading. If your instrument does not detect an increase at 1800 (or whatever) after balance work, it's probably not because the instrument is lying.

First, we perceive lower frequencies more readily than higher frequencies, even when the actual vibratory amplitude is the same. The limits of perception are something like 0.005g at 5 hertz to 0.03g at 50 hz.

Second, think about signal to noise. You can easily mask a particular vibratory frequency with another frequency (or a stew of frequencies). Take away the masking frequency and suddenly the other one is apparent.

Third, it may not have anything to do with rotating balance. For example, it could be propeller blade pass frequency slapping the canopy (60 hz for a two blade @1800), or the recip motion of one heavy piston (also 60hz @1800).

Added together, you can have a vibe more easily felt, no longer masked by rotating vibration, and won't register in a measurement of rotating vibration. Throw in a little human nature on top of human perception, and the pilot blames the vibration guy

[Sam Buchanan]

[quote=DanH;480970]

Originally Posted by Walt

Good stuff here, I am going to follow up on the lower RPM spikes to see what I can find out.[?QUOTE]

Human perception of vibration can be misleading. If your instrument does not detect an increase at 1800 (or whatever) after balance work, it's probably not because the instrument is lying.

First, we perceive lower frequencies more readily than higher frequencies, even when the actual vibratory amplitude is the same. The limits of perception are something like 0.005g at 5 hertz to 0.03g at 50 hz.

Second, think about signal to noise. You can easily mask a particular vibratory frequency with another frequency (or a stew of frequencies). Take away the masking frequency and suddenly the other one is apparent.

Third, it may not have anything to do with rotating balance. For example, it could be propeller blade pass frequency slapping the canopy (60 hz for a two blade @1800), or the recip motion of one heavy piston (also 60hz @1800).

Added together, you can have a vibe more easily felt, no longer masked by rotating vibration, and won't register in a measurement of rotating vibration. Throw in a little human nature on top of human perception, and the pilot blames the vibration guy

Excellent points, Dan.

I don't know the origins of the vibration characteristics I observe, just that I observe them.

Your point about vibrations being more evident once a masking vibe is removed might explain why I saw "more" roughness at 1800 rpm. After the balance routine was performed, the transponder visibly vibrated in the tray which was not evident prior to prop balancing. But this occurs only during a quite narrow rpm range. Maybe a secondary vibration had been neutralizing this vibration previously. Great care was taken during the engine overhaul to purchase pistons with matching mass.

Anyway, dynamic prop balancing is a good thing and I recommend it for everyone.

[Snowflake]

Anyone know where dynamic balancing services can be found in the Vancouver, BC area? Hopefully someone with similar experience to the original poster on aircraft?

I wonder if this would be a good thing to do if you're running a FP Sensenich on an O-320... With the 2700rpm limit maybe running it smoother would be a good thing?

[Sam Buchanan]

Originally Posted by Snowflake

Anyone know where dynamic balancing services can be found in the Vancouver, BC area? Hopefully someone with similar experience to the original poster on aircraft?

I wonder if this would be a good thing to do if you're running a FP Sensenich on an O-320... With the 2700rpm limit maybe running it smoother would be a good thing?

Rob, my RV-6 has a 160hp O-320 with FP prop (2600 rpm restriction). Dynamic balancing is definitely recommended for FP prop.

[Walt]

I agree Dan, here is some great info on Vibration perception and sources, the Spectrum analysis of Paul Dyes engine identifies these areas so if we had a spiking 60hz imbalance we would see it on our analysis.

Here is some great info on Vibration (quoted from the vibration experts so I don't have to type it all ):

1/2 Order Vibrations It turns out that if all of the pistons produce nearly identical combustion pulses, the 1/2 order vibration will be very small, .1-.3 IN/S. When any one cylinder produces less power than the rest, the 1/2 order vibration will increase from .3 IN/S on up to over 1 IN/S for a misfire. Mechanics know very well what can cause one cylinder to be weak. Plug misfire, bad plug, plugged injector, broken ring, leaky valve, low compression, bad magneto, worn cam, collapsed lifter, etc. Any of these things can cause a higher than normal 1/2 order vibration.

1/2 order vibration are especially troublesome as they can be felt in the cabin by the pilot. Low frequency vibrations are not well isolated by most engine mounts, and the vibration will shake the entire aircraft. If not taken care of, a 1/2 order vibration can loosen rivets, hinges, and pivots all over the airframe, as well as causing premature pilot fatigue. This is a serious safety issue.

1P Vibrations 1P vibrations are usually dominated by propeller imbalance. Happily we can correct this with propeller dynamic balance. 1P vibrations can also be caused by unequal piston mass. Pistons and cylinders are often replaced 1 jug at a time, and once in a great while the wrong weight piston is used due to error. In horizontally opposed engines, this will produce a 1P vibration in the horizontal plane, but little vibration in the vertical plane. An out of balance prop will produce nearly equal vertical and horizontal vibrations. If the rear of the engine has a high 1P vibration that is not corrected by balancing the prop, piston mass imbalance is the most likely cause.



Above you can see the Paul's 1/2 order vibration of .145 IPS @ 1173.8 CPM/19.5 Hz which is considered to be quite acceptable.

Paul's 2X blade rate 4691.3 CPM/78 Hz was .089 IPS is less than the "average".

Quote from DSS on 2X vibration results:For unknown reasons we have found it is not uncommon for an engine to produce a 2P or 2-1/2P vibration in the range of .5-1.0 IN/S. These readings are taken on the ground at typically 2500 RPM.

Bottom line is we "should" be able to see if there is going to be a problem at a particular harmonic.

[scsmith]

I had my Whirl Wind 200RV and IO-360-A1A combination dynamic balanced a month ago by Kregg Victory at KRHV. We did the balance at 2200 rpm.

In addition to the smoother cruise, one thing I noticed is that the throttle-up acceleration of the engine is MUCH smoother. As you open the throttle and the engine picks up RPM, it does so much more smoothly. So this would seem to be opposite of some of the other experiences reported here.

The cruise smoothness is most apparent on cross-country flights when you are droning along for a long time. I notice less fatigue (not that I feel much fatigue flying anyway, but...) The background buzz that you feel when you grab the structure is gone.

[chrispratt]

I took my RV-8 to Walt Aronow (52F) on Saturday (Nov 13) for a prop balance which it had never had.

As it turned out, mine was like Doug's -- no need to add any weights. It was already within the target zone. I was almost disappointed, but happy that mine was operating at peak for the nearly 400 hours I have on the airplane.

Chris