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| AOPA Comanche Balanced |
The AOPA Comanche "Win a Twin" flew in after attending Oshkosh 2004 so it could have both engines professionally balanced by Aviation Resources.
(AOPA's Tom Horne (left) Jim Barker (right))
From the AOPA website:
http://www.aopa.org/pilot/twin/0408.html?PF
Prop balancing, and more...
After AirVenture, I flew N204WT to Aviation Resources at its location on the Cumberland, Wisconsin, airport (UBE). There, Jim Barker and his team set to work balancing our/your propellers. Using dynamic prop balancing procedures, it was learned that the left propeller was out of balance by a factor of .315 inches per second (IPS). In other words, the prop disc was wobbling about its axis at a rate of about a third of an inch. That's not much, but it's enough to make the engine and prop vibrate and set up unhealthy stress paths in the propeller blades and engine crankshaft. Abnormally high vibrations can also cause horsepower-robbing friction and abnormal component wear especially of the alternator mounting brackets and exhaust stacks.
By bolting 29.6 grams of nuts, bolts, and washers to a strategic spot on the crankshaft's flywheel, the vibration level was reduced to .003 IPS. That's a 10-times reduction in vibration levels. Barker took about three hours to do the work, using a special tachometer and vibration sensor, plus proprietary computing equipment.
Then it was on to the right engine, and that's where the value of prop balancing kicked in to the max. Barker saw a .539 IPS out-of-balance condition indicating unusually rough vibrations. And in fact, you could see the out-of-balance prop by looking at the spinner as the engine ran. The spinner tip wobbled as it spun around at 2,100 rpm during the ground runs used for balancing trials.
Barker's computer said to place a whopping (a relative term in prop balancing) 96.78 grams of weight to correct for the imbalance. From experience, Barker knew that such a large weight recommendation meant that the odds were that the prop would never balance properly. Something was wrong with the right engine's prop setup. Barker found out, and you won't believe what it was.
The prop was installed incorrectly. It was mounted 90 degrees from its proper location. Turns out, you don't just bolt a prop any old place on the crankshaft flange. You use a reference, or index, bushing to align the blades. In this case, the index bushing was aligned with the top dead center mark on the flange.
The prop was removed, reinstalled in the correct position, then dynamically balanced after three ground runs. All went well, and the right prop's final vibration reading was .040 IPS not bad at all.
Aviation Resources also addressed some other maintenance issues tightening the right engine's alternator belt and number-two cylinder's upper spark plug and replacing exhaust gaskets on the right engine. The oil was changed on both engines as well.
In all, it turned out to be a great maintenance stop. It was the first time in 45 hours that the cowlings had been removed and the new powerplants and props had been inspected. Guess 50-hour inspections really are good ideas.
And I could tell the vibration levels were way down on the four-hour, 30-minute nonstop flight back home to Frederick, Maryland. For the trivia-minded, I averaged true airspeeds in the 174-knot range at my 9,000-foot cruising altitude, saw sustained groundspeeds at 190 knots, burned 81.9 gallons, and logged 45 minutes of actual in some of the bumpiest clouds I've ever encountered cumulus leavings of an aggressive cold front that pushed through the Midwest the day before.
101 Airport Avenue, Hangar 7
Cumberland, WI 54829
Hangar: (715) 822-5787 Office (715) 822-3413 Cell (715) 491-1303
Fax: (715) 822-5492
Hours: M-F 8-5
Let's Fix It!!!
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