Dynamic Propeller Balancing

How to keep the Cessna 170 flying and airworthy.

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brian.olson
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Dynamic Propeller Balancing

Post by brian.olson »

As a noob aircraft owner myself, this is written primarily for all the other noobs that will follow me in coming weeks/months/years.

Summary: I had 62C's propeller dynamically balanced two weeks ago, and it was a positive experience/outcome.

Detail: I've owned 62C for approximately six months now and any maintenance activities I've invested in have primarily been to increase the longevity of the aircraft, not to fix broken stuff. 62C has been a tight flyer with no complaints. I did notice a very slight "buzz" in the control yoke (and you could see it, faintly, looking at the shoulder straps of the passenger seat) extending up into the yoke-mounted iPad holder which seemed to change based on RPM. Not a "vibration" per se, just a "buzz" - and nothing I was overly concerned about

After researching dynamic propeller balancing, I was pleased to discover that Sensenich Propeller Services has one of its two maintenance facilities just off the field at the Gainesville, GA airport, a short one-hour flight north of my home field. On a beautiful Thursday morning I flew 62C north and met one of the techs at the airport, taxied over to the end of the hangars where they typically do run-ups, and spend the next two hours running the tests.

Equipment consisted of a single accelerometer mounted under the cowl near the front of the engine, an optical sensor taped on the top of the cowl using 500-mph tape, and a piece of reflective tape affixed to the back of the prop. Cables from both sensors were taped to the side of the aircraft and run out to the computer unit. It took approximately one hour to have all the equipment secured and ready for the test.

We ran three tests, which involved me running the engine to full static power (on ground, chocked, w/brakes) for approximately 15-20 seconds while the computer analyzed the RPMs and the vibration, after which I slowly brought the engine back to idle and shutdown. The first test showed a reading of .2 IPS (inches per second). According to the tech, the average reading for aircraft which haven't received balancing before is .4-.5. He also mentioned that there was general agreement by both engine and propeller manufacturers that anything below about .3 is undetectable by a human, though it can still be a source of concern over the life of the engine. The computer analyzed the results and recommended a weight and location to affix a "test weight" to the spinner. The weights were in the form of washers, and affixed using the standard screws holding the spinner in place. The two remaining tests were performed equally, with the computer making minor adjustments to weight and location.

Once the final weight and location recommendations were received, the spinner was removed and a bolt and washer permanently affixed to the spinner back plate. The tech had to drill a hole to accommodate the weight. A fourth, and final test, was performed once the spinner was reassembled showing a final reading of .02 IPS, which was below the typical final reading of .05 IPS according to the tech.

I received a 337 in the mail a few days later for the logs and have a logbook entry indicating the work that was done.

In the several flights since that time I have noticed that slight "buzz" completely gone. I also am left with peace of mind that this was another step in keeping overall engine vibration to a minimum, and life of the engine to a maximum.

For folks with more experience than me, please feel free to suggest edits to clear up any misinformation in the above - I'd like to make this as useful as possible.

Photos below show the process that I have outlined above.
Attachments
IMG_0316 1.JPG
IMG_0315.JPG
IMG_0314 1.JPG
IMG_0317 1.JPG
Last edited by brian.olson on Wed Feb 28, 2018 5:07 pm, edited 1 time in total.
Brian
1950 170A
N5762C s/n 19716
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GAHorn
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Re: Dynamic Propeller Balancing

Post by GAHorn »

Great Report, Brian! Maybe you could submit it to headquarters or Bruce F. for publishing in the 170 News for others to enjoy..?

A dynamically balanced prop will extend the life of avionics, windshields, cowlings and other accys which will easily pay for the trouble and expense. Kudos to the service-provider for documenting their work. Many shops that do this work are not certificated repairmen/shops and do not offer documentation. (Not necessarily a reason to reject them...just a note to avoid the surprise.)

What were you charged?
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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brian.olson
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Re: Dynamic Propeller Balancing

Post by brian.olson »

George - I paid $300 for this. My research shows this was on the high side, but having it done by a propeller shop, by a guy who regularly balances props on King Airs, pistons, etc., I felt like it was worth the cost.
Brian
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Re: Dynamic Propeller Balancing

Post by GAHorn »

I agree. The documentation adds sufficient value, IMO. Back when I owned a 206 I had the prop balanced by Jordan Propellor in SAT. What was not asked... was discovered after the process...no documention. (The work actually being performed by the father of one of Jordan's prop-techs and not actually "by" the shop. The price at the time was $150 and I learned from that experience to ask up-front about the documentaion. BTW, technically anyone performing maintenance on an airplane must "log" it. That person expected me to do that... 8O

Hope this is informative for others.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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nippaero
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Re: Dynamic Propeller Balancing

Post by nippaero »

Nice write up Brian.
1952 170B
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Re: Dynamic Propeller Balancing

Post by marathonrunner »

I balance propellers with an Aces balancer. I have had it for decades and get it certified annually. Anyway, unless their instructions have dramatically changed, the way the final balance weight is attached does not appear correct in the photo. I could be incorrect. When I balance them with that kind of spinner the hole is drilled on the edge where the spinner screws attach. It is then countersunk and a countersunk structural screw with the washers and nut are attached. The spinner then fits over it. On a Lycoming I use the flywheel holes and do attach it the way it shows in the photo. The reason is the light aluminum backing plate may allow the longitudinal attach to fail but being under the spinner will be structurally more sound based on the effect of the spinning and weight. The arm has to be figured into the equation for the final balance weight and is measured in grams.

Once balanced they tend to remain that way indefinitely unless you remove and replace accessories or have to file the propeller. That is another whole forum as if you remove a substantial amount of material from one blade you need to remove a like amount from the other to maintain balance. I forget how many tons each blade is "throwing" when rotating but I believe Lloyd Nelson of Pacific Propellers, now deceased, told me it was in the neighborhood of 18 tons roughly.
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Hineywheel Bill
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Re: Dynamic Propeller Balancing

Post by Hineywheel Bill »

I'd like to have mine balanced at some point. I feel very, very little vibration but I'd rather not feel any, or at least as little as is possible.
Owner of:
1954 C170B N1921C S/N 26066
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1948 C170 N3815V
1951 C170A N1490D
1946 C140 N76447
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nippaero
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Re: Dynamic Propeller Balancing

Post by nippaero »

I balance propellers with an Aces balancer. I have had it for decades and get it certified annually. Anyway, unless their instructions have dramatically changed, the way the final balance weight is attached does not appear correct in the photo. I could be incorrect. When I balance them with that kind of spinner the hole is drilled on the edge where the spinner screws attach. It is then countersunk and a countersunk structural screw with the washers and nut are attached. The spinner then fits over it. On a Lycoming I use the flywheel holes and do attach it the way it shows in the photo. The reason is the light aluminum backing plate may allow the longitudinal attach to fail but being under the spinner will be structurally more sound based on the effect of the spinning and weight. The arm has to be figured into the equation for the final balance weight and is measured in grams.

Once balanced they tend to remain that way indefinitely unless you remove and replace accessories or have to file the propeller. That is another whole forum as if you remove a substantial amount of material from one blade you need to remove a like amount from the other to maintain balance. I forget how many tons each blade is "throwing" when rotating but I believe Lloyd Nelson of Pacific Propellers, now deceased, told me it was in the neighborhood of 18 tons roughly.
Thanks for sharing your method. I was wondering what the best way to "attach" the weight to a spinner like the 170 was.
1952 170B
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canav8
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Re: Dynamic Propeller Balancing

Post by canav8 »

Tim is correct. No weight should be attached to a spinner backplate ever. The basic reason is by putting the weight on the backplate you require more weight to do the same job as having it out on the flange.
Here are the references regarding propeller balancing. The FAA considers these as acceptable data.
Advisory Circular 20-37E is the guidance for Propeller Maintenence.

For Dynamic Prop Balancing check into these publications
Chadwick-Helmuth Publication No. AW-9511-2, entitled “The Smooth Propeller”,
and ACES Publication No. 100-OM-01, entitled “ACES Systems Guide to Propeller Balancing”.
52' C-170B N2713D Ser #25255
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lowNslow
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Re: Dynamic Propeller Balancing

Post by lowNslow »

The instruction in the manual “ACES Systems Guide to Propeller Balancing” mentioned above gives instructions for installing weights on either the flange or the backing plate.
The McCauley Propeller Owner/Operator Information Manual also shows weights attached to the backing plate.

Also note that AC20-37E states:

"(c) For aircraft or propeller manufacturers that provide procedures for dynamic balancing of the propeller in their maintenance manuals or instructions for continued airworthiness, propeller balancing is not considered a major airframe alteration."

McCauley's manual includes procedures for dynamic balancing so no 337 required.
Karl
'53 170B N3158B SN:25400
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nippaero
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Re: Dynamic Propeller Balancing

Post by nippaero »

Good info Karl. I've been researching this subject. I am trying to find out "Who" is authorized to perform a dynamic balance. Does it have to be an A&P?

I noticed the AC says:
(h) The dynamic balancing procedure and the propeller’s return to service must be accomplished by an appropriately rated person, in accordance with the previously stated criteria.
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Re: Dynamic Propeller Balancing

Post by lowNslow »

It would require at least and A&P, and possibly a A&P with IA authority if it did not meet the requirement of a minor alteration mentioned above (this should not be a problem if you are using a McCauley prop).
Karl
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heltg
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Re: Dynamic Propeller Balancing

Post by heltg »

Just a few interesting notes. When researching sources for dynamic balance, ask if they can provide a spectrum analysis. As George mentioned, a spectrum analysis can reveal an "unhappy" engine. If everything looks good, at least you will have a baseline with which to compare in the future. It becomes an overall health record for your engine. It can potentially provide early warning of mag, induction, plug, lead, injector problems.
Also of interest is Hartzell Standard Practices Manual (SPM) 202A which authorizes nutplate installation at 12 equally spaced locations (30 degrees) on the front of the bulkhead. This allows for balance weights to be installed without spinner removal.
In addition, anyone with skull cap or no spinner, be advised that dynamic balance may not be an option, as there is no place normally to attach weights. In this case, it would still be advisable to have a balance check done along with the above mentioned spectrum analysis.
If Central PA is you area, I can guide you to factory trained people to provide these services. I'm a Training and Support Rep for a leading manufacturer of this type of equipment.
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Bruce Fenstermacher
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Re: Dynamic Propeller Balancing

Post by Bruce Fenstermacher »

heltg wrote:.....
In addition, anyone with skull cap or no spinner, be advised that dynamic balance may not be an option, as there is no place normally to attach weights. In this case, it would still be advisable to have a balance check done along with the above mentioned spectrum analysis.
If Central PA is you area, I can guide you to factory trained people to provide these services. I'm a Training and Support Rep for a leading manufacturer of this type of equipment.
Does Tom use your equipment? :D

I've been wondering about balancing a prop with a skull cap. Always wondered if it worth doing and using additional washer(s) under prop bolts to achieve better balance. Admittedly, there is very little wiggle room on a bolt combination that will work and still have enough bolt threads out the back of the prop flange and not bottom the bolt threads or to much bolt out the back to hit the cowl.

I'm guessing between you and Tom, we may find out the answer to my questions when you get your '48 flying with a skull cap.
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heltg
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Re: Dynamic Propeller Balancing

Post by heltg »

Bruce, this IS Tom. I have successfully balanced under a skull cap, but as you guessed, there usually is not enough thread to allow for much weight. I keep a pile of "modified" AN970 washers which are trimmed to allow skull cap clearance, but the number one can install is still dictated by the bolt length.
And the 39V project has been slowed to a snail's pace over the winter---no heat in the garage.
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