Question on dynamic prop balance

I worked with a local shop on prop balancing and I'm wondering if it's correct.

I'm running a Hartzell constant speed/blended airfoil prop and the plane seems to have a slight buzz between 2300 and 2500. When I asked the shop about it they said they balance all props at full power and if it's in balance It'll be good throughout the full range.

Anything above 2500 feels smooth and the full power balance run only produced a .07 and was told nothing needs to be done.

I was thinking about putting a bolt on the ring gear as the solution that was given on the machine was adding 6 grams at 42 degrees but with bolt holes at 30 and 60 degrees I'm not sure if I should do anything.

Where is the buzz felt? I just balanced my similar prop after 300 hrs and the feet buzz did not go away at all. Must be that it is aerodynamic.

birddog486 said:

I worked with a local shop on prop balancing and I'm wondering if it's correct.

I'm running a Hartzell constant speed/blended airfoil prop and the plane seems to have a slight buzz between 2300 and 2500. When I asked the shop about it they said they balance all props at full power and if it's in balance It'll be good throughout the full range.

Anything above 2500 feels smooth and the full power balance run only produced a .07 and was told nothing needs to be done.

I was thinking about putting a bolt on the ring gear as the solution that was given on the machine was adding 6 grams at 42 degrees but with bolt holes at 30 and 60 degrees I'm not sure if I should do anything.

Click to expand...

My practice is to ask the owner what normal cruise rpm they operate at & balance the prop there. Reasoning is to minimize pilot fatigue from the constant shaking. So if he normally does cross country at 2400 rpm, that's the rpm I'd use for balancing & it should be the smoothest running.

Fixed pitch prop balancing is different as static rpm runs only get up to about 2100 rpm. Higher rpm comfort becomes a trial & error effort.

As for the flywheel bolt weight solution, one AN4-7 & AN365-4 weigh about 12 grams so you will have to come up with something different.

An AN3-3, AN365-1032, & AN960 washer comes out to 6 grams. You could drill the spinner backing plate at 42 deg & be right on the money for your situation

Make sure that anything you do is reversible. The dynamics could be exciting something on the airframe.

jcarne said:

Where is the buzz felt? I just balanced my similar prop after 300 hrs and the feet buzz did not go away at all. Must be that it is aerodynamic.

Click to expand...

Mostly feel it and see it on the glare shield over the instrument panel. It's not any kind of shake. More like an extremely high frequency vibration.

The guy doing the test did mention it could be aerodynamics causing it.

He's mostly correct that as long as it's balanced, it's balanced, but that really only applies to the physical mass imbalance of the rotating assembly. If the prop blades arent exactly the same due to manufacturing differences or a slight difference in pitch control between the blades, you can get a vibration from the aero loading. This vibe will change with rpm and blade pitch, which is why you normally balance at the cruise rpm.

birddog486 said:

I worked with a local shop on prop balancing and I'm wondering if it's correct.

I'm running a Hartzell constant speed/blended airfoil prop and the plane seems to have a slight buzz between 2300 and 2500. When I asked the shop about it they said they balance all props at full power and if it's in balance It'll be good throughout the full range.

Anything above 2500 feels smooth and the full power balance run only produced a .07 and was told nothing needs to be done.

I was thinking about putting a bolt on the ring gear as the solution that was given on the machine was adding 6 grams at 42 degrees but with bolt holes at 30 and 60 degrees I'm not sure if I should do anything.

Click to expand...

I have a Catto fixed pitch 3 Blade (pusher) that has been problematic to balance. I use a Dynavibe stopping at 4 RPMs going upscale from idle to max static. I found I was getting different balance solutions at each RPM. I set the balance for cruise, but when I got down to pattern RPMs (1800-1900), I got a very noticeable vibration.

For a different reason, I re-clocked the prop 180 degrees and re-balanced. The indicated imbalance was now consistent and smooth at all RPM.

My thought moving forward is to do the initial test with multiple RPMs up to max static. If the results are erratic, re-clock the prop until the indicated imbalance is similar at all RPM, and then add balance weights.

I know we don't have the same kind of configuration... but maybe this info might help.

BTW, The Dynavibe website has a lot of good videos about balancing. the new equipment they sell is a lot better than the old and is more capable in determining the cause of the vibration using PSD style analysis. May want to check that out... a lot of good info.

Best,