Angier,

Some background to start from:

Props are first statically balanced...think of mounting it on a crankshaft that has very little rotational friction, and adding/subtracting weight from the hub or blades until it doesn't rotate to a heavy-part-down position in the balancing fixture. This is normally done by the manufacturer/rebuilder, and requires specialized tools/experience.

When the prop is installed on an aircraft and the spinner/nuts/washers/etc added for a "real world" application, the balance is affected. As these things are not added symmetrically WRT the plane of rotation, some amount of "wobble imbalance" is also introduced. Both can be significantly mitigated by "dynamic balancing." This is a good way to "fine tune" the balance of the rotating components as a whole, and  as they are installed on your engine. The simple dynamic balancing we typically do for props will be optimum only for a specific configuration, RPM and prop loading (pitch/power setting).

A typical GA prop balancer consists of an accelerometer that mounts temporarily to the engine crankcase, a strobe or rotational position sensor, and a computer to determine the approximate weight and rotational position to add it, as determined during a series of engine runs at the desired RPM.

My MT prop/hub assembly came with a single fender washer and #10 x 3/8" bolt added to the spinner back plate during static balancing at the factory. After test runs at 2300 RPM, I determined that the bolt alone, without the washer, gave me an out of balance reading of 0.04 inches per second (IPS) at 2300 RPM. 0.07 is considered well balanced for this prop. I then replaced two #10 stainless spinner mounting screws with 1/4" longer versions--at the indicated position--to achieve 0.03 IPS. That was about as good as I could do, and the engine is very smooth during cruise. It really doesn't take very much weight.  Call me if you want to discuss.

Bob

-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Greenbacks, UnLtd.
Sent: Thursday, August 12, 2010 8:20 AM
To: lml@lancaironline.net
Subject: [LML] Re: Prop Balance

Could one of you describe the process of dynamically balancing a  
typical prop?
My old brain cannot visualize how weights are attached.

Angier Ames
N4ZQ

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