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Bob Axsom

Well Known Member
In other threads and at other sites I asked about the possible benefits in speed from dynamically balancing the prop at 2700+ rpm. My thought was "if I can get everything working together smoothly then the minimum energy will be used in internal opposition of forces and the power to the prop will be greater resulting in higher pitch and more speed. The responses have not been encouraging so I wonder about the methodology and the effect. Here are some of the feedback and some of my thoughts:

- Noticed no difference in performance after the dynamic balancing (interpretation - it may have seemed smoother but the plane flew no faster)

- The dynamic balance is done at only one speed and the speed I can get my engine rpm up to on the ground with my fixed pitch prop is far below what I fly at and balancing at race rpm is impossible (interpretation - my prop is constant speed but the pitch sitting on the ground at 2700+ rpm is much less than the pitch in flight at this rpm so the dynamic balance effort is at best a gross adjustment with no fine tuning for race applications and could introduce degradation in performance and reliability if the balance or physical integrity are improperly affected by the effort)

- The reduction in vibration after dynamic balancing was very noticeable and good for all things from me to the airframe and the avionics (interpretation - this doesn't effect speed and is unimportant to me - I'm satisfied with the vibration levels from a comfort and reliability perspective)

What are the expert thoughts and experience?


Bob Axsom
 
Perhaps not repeatable, but it happened....

An attempt to balance a prop dynamically saved my butt not long ago. The balance itself was not a factor but the problem it turned up was.

Balance on the ground was perfect, but in flight vibration was very noticeable much to my consternation and the guy who did the balance job. This was repeated twice indicating something was really screwed up.

Well, after a bit of sleuthing the prop installation over a period of days, I noticed by moving a blade fore and aft, the prop was not tight against the engine flange!! It could be moved enough to see day light between the prop flange and the engine flange and this with the bolts torqued to about 60 foot pounds. (?)

Further investigation revealed the bushings in this mounting system were about .004 over sized in length, the bushings were bottoming out before the flange faces met each other. This happened due to an innocent mistake with bin labeling at the source of the bushings.

No question the prop would have departed the airplane in time had I not gone for the dynamic balance job. Beyond that, dynamic balancing is a compromise of sorts. A prop and engine can be balanced for only a specific rpm. Having done this 4 or 5 times in the past, I have found the best overall smoothness is achieved by balancing at a relatively slow rpm, say 2100, rather than doing it at 2500-2600 as with a constant speed set up. A fixed pitch balance at 2100 will be ok at the higher rpm's. So far the Catto has been so smooth out of the box, I have not had it dynamically balanced.

dd
 
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When I had my former c182 motor and prop replaced it was prop balanced. The gear was installed, wires taped to secure, and off we flew. I thought this was how it was done until hearing it was also tested as a ground run up. While in flight we recorded data at different settings. Why not find someone to do it this way. Then you can check it at the condition you find most applicable.
 
Properly done, dynamic balance of the engine and prop can only help with the longevity of the engine and even prevent rattling things loose in the airframe.

I wouldn't expect you'd be able to notice the improvement in power output with even a perfect balance job. Just not a major contributor to overal power output.

A good balance job could be done at some fraction of full speed. Turbomachinery (in the shop) is done at a small fraction of full speed. There may be some advantage to balance at higher rpm for a plane (mine seems to have a critical around 2400), but the danger increases exponentially with high rpm ground runs.

There's another variable however, and that's the aerodynamic one. Machinery imbalance is fairly easy to correct. Air flows are erratic and unpredictable, particularly on the ground - and they'll cause mechanical vibration.
 
What it is suppose to do.

- The reduction in vibration after dynamic balancing was very noticeable and good for all things from me to the airframe and the avionics (interpretation - this doesn't effect speed and is unimportant to me - I'm satisfied with the vibration levels from a comfort and reliability perspective)

This is what Dynamic balancing is designed to do not to give you better performance.

After I did mine it was very noticeable how much smoother it was.

On most planes you will notice a smoother running airplane, some planes are very good to start with and can be improved but will not be as noticeable.
I checked one plane and told him up front that it was already as good as it could get, no charge.

I now offer this service:
http://home.newwavecomm.net/bobbyhester/dynamic_prop_balancing.htm
 
I got my prop balanced, albeit on a 172. I sometimes feel like there is something wrong with the engine because it's so smooth. It's kind of like riding a Harley; the power pulses let you know it's making power. For comfort, and longevity of the parts associated with an airplane, it's worth the money.
 
Bob,

Been playing phone tag with my friend at GS Prop. I will get in touch, and will also ask John Parker at American Air Racing his opinion (he does DB's as well).

I imagine they will have similar responses...that the DB is to smooth it out. I will see if they (especially John) thinks there is a speed bennie. (And apologies for the delay in data collection...been splittin' duties between Mr. Mom and Hangar Rat).

It'd be nice if you could correlate smoother operation more speed. Maybe at the extremes of roughness to smoothness it would actually manifest itself as such. And maybe there is a very small incremental speed increase if you go from good to better (that's very subjective, of course!). Of course, every increment counts, but there's a dollar-to-benefit factor to consider! It's probably like so many things, do it and test. Then you get to ponder if it was the balance...or the fuel vent fairings...or the temperature that day...or the ahhhhhhhhhh!!! :)

Cheers,
Bob
 
Insight GEM g4 monitor

This GEM product has been out for some time, but I have not seen an actual user report yet.

It is unique in that it measures your engine/prop vibration while in flight, as well as being a usual CHT/EGT monitor...

g3_vib.jpg


...and it also does a balance analysis...

g3_prop.jpg


Has anyone seen this feature in action?
 
Hartzell Confirmed "no Speed gain"

My questions to Hartzell:

Question: Last October I purchased the propeller through Van\'s Aircraft
as a one time upgrade of my Hartzell propeller with F7666-4 blades. I
race my RV-6A in Sport Air Racing League cross country air races. I
have been wondering about dynamic balancing and a couple of friendly
competitors from Canada told me they flew to your facility to get this
done. My questions are: 1) will I gain speed from this operation? 2)
are you the best facility to do this work? 3) how much does it cost? 4)
When can I schedule a flight in to get the work done if I decide to go
forward with this?

Bottom line of the confidential response - No speed gain will be achieved.

Bob Axsom
 
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