Prop Clocking Matters

I went to Lycoming school several years ago. One of the memorable items
mentioned was that if an owner re-clocks the prop to make hand-propping
easier, cracks in aluminum brackets and filament failures in instrument
bulbs most likely would result. The instructor mentioned that this was due
to a high frequency vibration (one that the pilot may not notice) that would
result from an out-of-balance condition. His moral to the story, if these
conditions exixt on your plane, check the index position of the flange
master dowel to the prop. Your combination was out-of-balance enough to
notice before failures began.

Dale Alexander

"Kyle Boatright" wrote in message
. ..
I have always clocked the prop on my RV-6 so it stops at 10:00 and 4:00
when viewed from the front. The idea being that this is the best
orientation for hand propping if I ever needed to do that.

During my recent condition inspection, I mistakenly reinstalled the prop
with either a 120 degree lead or a 60 degree lag from normal, depending on
how you look at things. On my trip to SnF, I noticed that there was
considerably more vibration than normal, particularly at higher power
settings and rpm, but otherwise things were fine. I noticed the same
thing when I flew earlier this week.

Due to an in-process installation of a 12V outlet in the cockpit, there is
an unclipped zip tie visible during flight which has not been clipped.
With the prop normally clocked, the tip of the zip tie hardly moves. When
the prop was indexed differently, the end of the zip tie shook like a
double jointed hooker on dollar day. OK, maybe not that bad, but I felt a
real need to work that line into my narrative...

So, today I reindexed the prop. And we're back to a very smooth
engine/prop combination, which makes me a happy camper.

It made up for the fact that the videographer (me) completely screwed up
the recording of a prop stopped glide test I conducted earlier this week
when I went to altitude and shut down the engine for about 5 minutes of
glider time.. I had great intentions of filming all of the instrument
readings (ASI, VSI, etc) during the test and having the prop visibly
stopped in the background of the video. Unfortunately, I must have missed
when I went to push the record button, because I had zero, zip, nada,
nothing, when I went to review the recording.

I can tell you that 80 knots indicated is the minimum sink speed on my
RV-6 with the prop stopped, and the sink rate is 700-750 fpm at that
speed. Also, it requires 140 knots indicated to get the prop windmilling
again to restart the engine.

And, with the prop stopped, the engine/prop combination is extremely
smooth.... ;-)

KB

"Dale Alexander" wrote in message
...
I went to Lycoming school several years ago. One of the memorable items
mentioned was that if an owner re-clocks the prop to make hand-propping
easier, cracks in aluminum brackets and filament failures in instrument
bulbs most likely would result. The instructor mentioned that this was due
to a high frequency vibration (one that the pilot may not notice) that
would
result from an out-of-balance condition. His moral to the story, if these
conditions exixt on your plane, check the index position of the flange
master dowel to the prop. Your combination was out-of-balance enough to
notice before failures began.

Dale Alexander

It really is not my specialty, and I do not know in the case of aircraft
engines, or for current production automotive engines, but it was common for
manufacturers not to "zero balance" a lot of the older automotive engines.
That means that the flywheel was heavier on one side because it acted as
either all, or more likely part, of one of the crankshaft balance weights.

Just a little "food for thought."

Peter

"Peter Dohm" wrote

It really is not my specialty, and I do not know in the case of aircraft
engines, or for current production automotive engines, but it was common
for
manufacturers not to "zero balance" a lot of the older automotive engines.
That means that the flywheel was heavier on one side because it acted as
either all, or more likely part, of one of the crankshaft balance weights.

That is now part of the function of the harmonic balancer, on today's auto
engines.
--
Jim in NC

On May 4, 5:01 pm, "Morgans" wrote:
"Peter Dohm" wrote

It really is not my specialty, and I do not know in the case of aircraft
engines, or for current production automotive engines, but it was common
for
manufacturers not to "zero balance" a lot of the older automotive engines.
That means that the flywheel was heavier on one side because it acted as
either all, or more likely part, of one of the crankshaft balance weights.

That is now part of the function of the harmonic balancer, on today's auto
engines.
--
Jim in NC

Sometimes. Many of them aren't heavy on one side. The
"harmonic" part comes from the fact that it's two pieces of metal, an
inner hub and outer ring, with rubber between them. The rubber-mounted
ring (pulley) dampens the high-frequency vibrations created along the
crankshaft, or "ringing," that can cause catastrophic crankshaft
failure if its amplitude happens to increase at some resonant RPM.
It's there to "detune" the crank. Short, stiff cranks usually don't
need them.

Dan

Dan_Thomas_ wrote

Sometimes. Many of them aren't heavy on one side.

I'm not surprised. The engines I am most familiar with are weighted, but I
don't have much exposure to a lot of engines. I should have used another
weasel word like "some of."

The "harmonic" part comes from the fact that it's two pieces of metal, an
inner hub and outer ring, with rubber between them.

Right, which is why I said "part of the function."
--
Jim in NC