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I had the propeller dynamically balanced on my Cessna 150 today. The guy
attached a reflector tape to the back of one blade. He then duct-taped an
optical sensor to the cowling, looking at the tape. This allows him to
determine the location of the prop blade relative to the vibration sensor,
which he called a "velocity sensor" (I think it was actually an accelerometer)
bolted onto the engine. To attach this sensor to the engine, he used the
excess thread length of one of the studs on the engine case up near the prop
flange. A fixture gets attached to that stud without needing to remove the
nut.
After all that, he gets a measured disturbance correlated to the "time since
blade passage", which if you have a constant RPM, gives you disturbance
correlated to propellor angle. Both sensors fed into a gizmo a little smaller
than a laptop computer which he laid on the passenger seat. He then ran the
engine while the aircraft was tied down.
The disturbance is measured in "inches per second", hereinafter called IPS.
Less is better. The initial run had 0.14 IPS. That's not bad, he says. He
said that many folks don't even try to correct below 0.10, and some outfits
don't even worry if it's as high as 0.20 IPS. He says he's generally not able
to get below 0.05 IPS.
I wanted the airplane to be as smooth as it was before the prop was repainted.
(note to y'all -- the leading edge of the prop was getting a little raunchy.
They stripped off all the paint, made the leading edge totally smooth, and
repainted it at the annual inspection.) It wasn't too bad, but it vibrated
enough that the writing at the bottom of an approach plate was unreadable. Up
near the clip it was readable. So it was basically not possible to fly an
approach without one hand on the chart, turning a normally three-handed
operation into a four-handed one. Not good.
When all was said and done, the prop balanced out at 0.03 IPS. This was
accomplished by adding 3.3 grams to the spinner backing plate. This was the
combined mass of a half-inch #8 bolt, a small metal stop nut, and one #8
washer. That's all. In this business you measure fasteners to one-tenth of a
gram. It was truly astounding how such a small mass, located only at the
spinner edge, could make such a difference. And to think I used to fly with
bird poop on only one blade!
The whole process took 90 minutes and cost $150. It's incredibly smooth now.
It was worth every penny.
By the way, a tachometer calibration is a by-product. Mine was reading 90 RPM
low. No wonder I get book speed on a 1972 machine!
- Rob Wolf
rwolf99@aol.com
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