Smoooooooooth??
 

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/VansAirF...ance_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

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.

Walt, I'm next. Let's do it next week....

dr

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

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.

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.

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.

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

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.