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Smoooooooooth??

Ironflight

Ironflight

VAF Moderator / Line Boy
Mentor
I have been a believer in Dynamic Prop Balancing ever since I had the Val’s prop balanced shortly after she was flying. I frequently have pilot passengers remark on the smoothness of the engine in cruise, and I think that the balancing is a large part of this. I have often wondered if the balance might shift over time with wear, but haven’t had a chance to just check it, and still don’t know for sure. But I WAS able to take advantage of a great offer by Walt Aronow up in Fort Worth to get Louise’s prop balanced today, and learned a bit about how it might shift.

Louise had her prop balanced about four years ago during an annual, and told me then that she really couldn’t’ feel any difference afterwards. Shortly thereafter, she had a strange grove appear in the prop hub that might have been caused by the spinner (even though it had plenty of static clearance), and had it dressed out by a prop shop. This entailed removing and replacing the prop, as well as some material removal near the root of one blade. Ever since then, we have talked about getting it rebalanced to see if there had been a significant shift, but never quite got around to it until Walt’s discount for this month ($150, with $25 of that going to VAF as a donation) appeared to us to be an offer we couldn’t refuse. That plus it was a gorgeous day to fly across Texas, and Louise was out of town, so I would be the one to do the flying!

Doug met us when I arrived after an uneventful trip up from Houston, and offered to take a few pictures as we went.

http://picasaweb.google.com/VansAirForce/PropBalance_WaltPaul#

Cowl removal was quick, and before we got started, Walt volunteered to grease the prop since we had things opened up. After that, he began to set up for the tests. I could tell right away that he had the latest equipment for the job – compact, rugged, and simple to attach. The last time I had the Val done b a traveling balancing fellow, his equipment looked like it had been salvaged from a castle laboratory in Transylvania, right down to an X-Y plotter and an oscilloscope. Walt’s gear looked like it could land on a beach with Navy Seals and not be damaged. A small accelerometer was bolted to the engine spine, and a light sensor attached to the top of the Number 2 Cylinder rocker arm cover screw. A patch of reflective tape on the back of the prop blade for the light sensor to detect the blade finished things up. Walt taped the cables along the leading edge of the wing so that he could stand well clear, and we were set to go.

After that, my job was simple – run the engine up to cruise RPM and hold it there while took a ten second vibration sample. Full aft stick for this, please (and a rope tied to the tail wheel and secured to Walt’s truck as well…)! After each of the four runs, Walt did some magic and calculated the weight and location of the required balance weights, which are nothing more than AN bolts, nuts, and washers attached to the flywheel holes. The system tells him how much weight to attach to which positions, and he goes through his hardware collection using a sensitive gram scale to find combinations that give him what he needs. Each run was better than the last, and we ended up with a balance of 0.024 inches/second – about as close as you can actually measure. And there was no question that the engine was smoother after the balancing. We started out at about 0.24, so that was an order of magnitude improvement.

The whole thing took about two hours, which gave us plenty of time to drive over to Justin, TX with a gang of the 52F regulars for a great Chicken Fried Steak (That phrase is always capitalized in Texas for some reason…) and a bunch of pilot talk before I saddled up for the smooth trip back to Houston. Clearly an improvement well worth the cost – even if you factor in the fuel for the trip. But hey, it was a nice day, so who am I kidding – I would have been flying anyway….

Thanks Walt for a great job at a great price – and to Doug for the photos!

Paul
 
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A few friends of mine had their's dynamically balanced and both of them agreed that in cruise they could tell it was smoother but anything less than cruise, not so good. One says at lower RPM his shakes bad and the other removed the weights cuz he could not stand the shake at less than cruise RPM. $$$ down the tubes for him!

Sure wish there was a way to smooth em out at all RPMs...

I had mine done at the time and out of the gate mine was close. The weight added to mine was very very small. I could not tell much difference at any RPM.
 
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Paul,
Thanks for the writeup. I went in and added some comments to the pics so folks would have an idea of what some of them were showing.

Brantel:
I've never heard anyone say that after balancing they had a higher vibration at a lower RPM. When the airplane is run up to cruise RPM I watch the vibration all the way up and I always see a decrease in all ranges, even the idle its improved. Not saying it didn't happen just that I don't understand the dynamics of that particular problem. Actually if a customer had the problem we could check it at that RPM to try to figure out what was going on.

I will say this, if I ever balance a prop and the customer honestly feels like it didn't improve his (or her) vibration and is not 100% satisfied, I will refund his (or her) money in full. I don't want anybody to ever feel like they are out their hard earned $$$ and got nothing for it!

Most of the airplanes I've done have a vibration of over .2 IPS (some as high as .6), at .2 and above you will almost definately notice an improvement when we are done.

If you're between .1 and .2 IPS then you "should" notice some improvement, anything less than .1 IPS to start and you likey won't feel any improvement. Of course I tell customers this up front so they will know what we are starting with and what kind of results to expect when we finish. However, even if you can't feel the vibration improvement that doesn't mean it's not there, you just can't feel the very small changes

I've had only one plane that I've checked that intially came in at .035, I improved it a bit to about .028 just for the heck of it with no charge to the customer, I told him up front I couldn't really improve it enough to make a significant difference, but we did it anyway
 
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Walt said:
I've never heard anyone say that after balancing they had a higher vibration at a lower RPM. When the airplane is run up to cruise RPM I watch the vibration all the way up and I always see a decrease in all ranges, even the idle its improved. Not saying it didn't happen just that I don't understand the dynamics of that particular problem. Actually if a customer had the problem we could check it at that RPM to try to figure out what was going on.
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The prop on my RV-6 has been balanced twice in the eleven years the plane has been in service, once when I first began flying it, the other following an engine overhaul. I highly recommend dynamic balancing and it made a dramatic difference in the smoothness of my engine.

However, I can attest to the increase of vibration at lower rpm than cruise even when cruise vibration has decreased. The second time my prop was balanced, some roughness the engine had at ~1800 rpm got slightly worse, and a rough range at ~1300 rpm became noticeable. The person doing the balancing suggested I fly the plane and if I wanted any adjustments he could move the range of "roughness" to a different rpm. But I was pleased with the cruise smoothness (0.024 ips) and didn't request any changes.

So, increased roughness can happen at lower rpm even when cruise vibration is reduced. I don't know why it happens, but I considered the smoother cruise to be worth it.
 
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Good stuff here, I am going to follow up on the lower RPM spikes to see what I can find out.

I think most people would agree that engine RPM is generally maintained at or above cruise RPM for 95% of the flight so it only makes sense to improve that RPM range more than anywhere else.

I do see vibration decrease with higher engine RPM, this is why I usually recommend balancing at 2350-2400 (or best static for a FP which is usually around 2100-2200). Anything above 2100-2200 RPM and the vibs keep getting lower (I checked one at 2600 and it was better there than it was at 2350). I use 2350 for hartzell as it takes around 22-23 MP to get that RPM and most Hartzells have the 2350/22MP restriction.
 
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Ironflight said:
I frequently have pilot passengers remark on the smoothness of the engine in cruise, and I think that the balancing is a large part of this. I have often wondered if the balance might shift over time with wear, but haven?t had a chance to just check it, and still don?t know for sure.
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I have had people comment that my plane ran smooth in cruise and I agreee when I ride in other planes. I do detect I have an increase in vibration down at 2100-2300 rpm then I do at 2600 rpm.
A question on the balance process. You ran up to cruise rpm. I have fixed pitch prop so how do I get to cruise rpm when sitting static for a dynamic balance? Will I then balance at my max static rpm of 2200 rpm (I have Sensenich 85" pitch cruise prop)?

I have wonder if my engine was getting rougher over time so I down loaded Vibration App for my iPhone and just hold it to the instrument panel flying at different rpm to get a quick vibration spectrum. I will try and remember to do it every so often to see if I see a trend.
 
plehrke said:
I do detect I have an increase in vibration down at 2100-2300 rpm then I do at 2600 rpm.
A question on the balance process. You ran up to cruise rpm. I have fixed pitch prop so how do I get to cruise rpm when sitting static for a dynamic balance? Will I then balance at my max static rpm of 2200 rpm (I have Sensenich 85" pitch cruise prop)?

I have wonder if my engine was getting rougher over time so I down loaded Vibration App for my iPhone and just hold it to the instrument panel flying at different rpm to get a quick vibration spectrum. I will try and remember to do it every so often to see if I see a trend.
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For fixed pitch we generally do them at 2100-2200 or max static RPM, as you noticed the vibration improves as you go higher and we see the same thing here: higher RPM's = smoother. For C/S balancing low cruise is typically 2300-2400 (for 4 cyl) so this is where we do the balance.
 
FP w/ Landoll harmonic balancer

Walt-

Have you balanced an engine/FP prop combination that has a Landoll balancer? Does it even need to be done?

LarryT
 
LarryT said:
Walt-

Have you balanced an engine/FP prop combination that has a Landoll balancer? Does it even need to be done?

LarryT
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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.
 
Thanks for the reply

Walt said:
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.
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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
 
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.
 
Walt said:
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 ;)
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DanH said:
Walt said:
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 ;)
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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.
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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?
 
Snowflake said:
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?
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Rob, my RV-6 has a 160hp O-320 with FP prop (2600 rpm restriction). Dynamic balancing is definitely recommended for FP prop.
 
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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.

d97f8gdfgrh.jpg


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.
 
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off-cruise vibration

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.
 
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
 
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