Prop balance after overhaul

[MikeS]

That's the way Prince does it too. Sterba uses a little lead plug near the hub. Interesting question . . . how others do it.

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> "You would have to use a lot, and I mean A LOT of paint on a propeller to get it out of balance."

I disagree there. It takes hardly anything at all to take a prop from 0.00 IPS to something less perfect, especially if someone is painting their tips.

Throughout the history of aviation pilots and mechanics learned to accept a certain amount of vibration as "normal" on recips. Until the low cost digital equipment showed up sometime after the turn of this century, the mainstay of prop balancing equipment was the very expensive Chadwick-Helmuth which we now know was awful. What balancers using that equipment thought was acceptable in fact meant it was simply "better than it was before" . . . nowhere near as well balanced as can be done with current low cost digital balancers. Chadwick of course has also gone digital and I imagine still costs big bucks based on their name and reputation. They were the only game in town for many, many years.

Those that were really good with their old Chadwick equipment could do a half-assed job, depending on the operator. "Prop" balancing has always been somewhat of an art and remains so today even with our easy-to-use equipment. As someone mentioned, some jobs are easy as pie. Others however can turn a sane person into a drooling madman . . . we've all seen them . . . sometimes hanging around the ramps and runways howling and gesticulating at the airplanes that go by. T-6's and Helio Couriers especially seem to set them off. Since the advent of TSA and the arrival of easy-to-use digital equipment I don't see those poor fellows nearly as often as I once did. But even with digital stuff it can still sometimes put a person over the edge. That's why prop balancing remains somewhat of an art. It's not difficult to master though. You just need your own equipment. And do some reading. For anyone interested in the subject I've attached a very easy to read and understand primer on the subject.

 

[coloradobluesky]

When I painted mine I could tell the difference with three coats and that was just on the back of the 2 blade Hartzel on an 0-360 Lycoming A1A engine.

 

[nrpetersen]

I had to rebalance a McCauley prop on our then-new 172M It had a heavy blade which I identified using a single wrap of duct tape near the spinner. It took maybe 5 coats of paint on the back side of the light blade to get smoothness. It was then refinished by Maxwell prop shop and their magic. Cessna paid for it under warranty. It ran smooth for 33 years.

If you can identify unbalance as a light blade, one advantage of using paint - unlike something around the spinner, it isn't going to have any surprising consequences (DAMHIK)

 

[Ted]

Only balance a prop when the engine vibrates excessively. Its possible to balance a prop with paint, BTW. At least to some extent.

This is not good advice. It's not always inherently obvious when your prop is vibrating significantly. The benefits of a properly balanced prop via dynamic balance are truly remarkable from the left seat.

When I had the 310 done, its vibrations weren't "excessive" by any means. Although the vibes were within limits, we made them 1/5 of what they were. I found the plane quieter and my body less fatigued on long trips.

 

[MikeS]

> "It's not always inherently obvious when your prop is vibrating significantly."

Very true. My most recent reminder of this was a Franklin 0-235 installed in a Cavalier SA102 I was rehabilitating in order to ferry it. Franklins have some kind of crankshaft vibration dampener which I'm still very interested in learning about. It's a component built-in to the crank somehow . . . not something external like the harmonic dampeners we see on car engines. It may work the same way as a harmonic dampener but it's somehow incorporated into the crank itself. I remain very interested in learning more about what it looks like and how it works. I just haven't gotten around to do more digging yet.

When I did the prop balance on this plane I saw how well this Franklin vibration dampener worked. The engine/prop felt very smooth before I put the sensors on it . . . yet it took a HUGE amount of weight to get the balance down below .07. I even had to buy longer prop bolts so I could put a bunch of heavy steel washers underneath the nuts on the bolts where the weight was needed. Darndest thing I've ever seen. However Franklin does that, it sure seems to me (without knowing more about it) that Lycoming and Continental ought to be using this technology on their crankshafts.

I'm still mystified how an engine/prop can "feel" relatively smooth yet still be as drastically out of balance as this Cavalier was. Needless to say, it was smooth as silk once the dynamic balance was done. I couldn't believe how much weight it took.