Dynamic Prop Balancing.

[Paul462]

Amigos,

Last Tuesday we spent five hours (and $400) dynamically balancing the front and rear engine/propeller installations of our 1968 T337C. Prior to this exercise the plane ran fairly smoothly, without any excessive vibrations. We used a Mircovib II, state-of-the-art digital balancer, and obtained print-outs of the results. The engines/props were balanced at cruise power setting of 25 in/2400 RPM.

Both engines/prop installations started out around .25 inches/sec; the front installation was reduced to .018 in/sec; the rear to .012 in/sec (in improvement of greater than a factor of 20!). There should have been a noticeable reduction in cabin vibration.

During test flight yesterday the airplane shuddered during the take-off run, kind of a low-freq vibration (we never felt that before), and then during climb-out at 28 in/2600 RPM there was a new, sharp vibration at about the engine frequency. During cruise vibration levels appeared somewhat greater than usual - the 496 mounted atop my glare-shield vibrated slightly, which hasn't hapened before.

Any thoughts, everybody? Has anybody dynamically-balanced their planes, and what success did you have? Why would a dynamically-balanced plane vibrate more AFTER than BEFORE !?

[tropical]

I use to do Dynamic balances with an Aces 2020 Probalancer.

There can be several factors. First, the person doing the balance may not be doing it correctly.

Second, what is the TT of your engines, time (years) since overhaul and were the engine mounts changed?

Faulty engine mounts, worn out magnetos will cause problems, as will a faulty ignition harness. Also, did the person doing the balance check the prop track?

So will loose and misaligned control surfaces and gear doors. Go up and at cruise without changing power settings dive and climb and see if the vibration changes.

[Pete Somers]

Paul
I think that you should have had the props removed and in a prop shop for a static balance first. Then carried out a dynamic balance on a known good prop, but it does sound like the readings were wrong and the weights wrong (easy to do)

Regards
Pete

[tropical]

Typically a prop not in static balance will not balance dynamically. Every time you place a solution weight the clock angle/IPS will change, but not go down. When I did dynamic balances I always started with a questionare on the airplane. You can have the props dead on balanced but a bad engine mount will create havoc. Magnetos, ignition leads, plugs, etc can also affect vibration levels.

Also track is something very few mechanics ever check when installing props. The prop and spinner can be balanced but if not in track will create some interesting vibration.

[Paul462]

Thanks for the input, guys! To get a second opinion, Mary & I flew the plane yesterday, and she gave it the "yoke-mounted GPS test". Previously, her yoke-mounted GPS vibrated so badly that during climb-out that she had to hold it still with her hand to read it - now it doesn't vibrate as much during climb-out, although at cruise it's about the same.

We didn't experience the take-off shudder from last week during either take-off yesterday.

So maybe I'm just noting different vibes - at least from Mary's point of view there's some improvement during climb-out!

Overall, I don't think there was any cabin improvement, but maybe it's not as bad as I initially thought - I was hoping for a big improvement, which didn't happen. We'll check prop tracking during next annual.

Paul

[tropical]

Originally Posted by Paul462

We'll check prop tracking during next annual.

Paul

Prop track takes just a few minutes to check.With the plane parked in the hangar, place a block on the floor just behind the front prop. Let one blade just barely touch the block. Make sure mags are off, then turn the prop and bring the other blade down to the block. If they are in track, then the second blade will be where the first blade was in reference to the block. If not, let's say you have an 1/8 inch difference, then you have a track issue. Sometimes track can be corrected by moving the prop a bolt hole or two left or right on the flange.