Prop Clocking Matters

[Kyle Boatright]

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

[Marc J. Zeitlin]

Kyle Boatright wrote:

> 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.

Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
though). Van's lists the stall speed as 49 mph (43 kts). I would have
expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
kts (92 mph) seems really high. My COZY has a stall speed of about 70
mph (61 kts), give or take, and the min. sink speed is around 80 mph
(70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
sink is around 550 fpm.

Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
interested in seeing the #'s.

Thanks.

--
Marc J. Zeitlin
http://www.cozybuilders.org/
Copyright (c) 2007

[Kyle Boatright]

"Marc J. Zeitlin" > wrote in message
...
> Kyle Boatright wrote:
>
>> 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.
>
> Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
> though). Van's lists the stall speed as 49 mph (43 kts). I would have
> expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
> kts (92 mph) seems really high. My COZY has a stall speed of about 70
> mph (61 kts), give or take, and the min. sink speed is around 80 mph
> (70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
> sink is around 550 fpm.
>
> Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
> interested in seeing the #'s.
>
> Thanks.
>
> --
> Marc J. Zeitlin
> http://www.cozybuilders.org/
> Copyright (c) 2007

A couple of things might come into play...

1) 80 knots was the indicated airspeed, and is subject to whatever errors
are present in my ASI and pitot/static system at that speed.

2) An RV-6 has a pretty low aspect ratio and therefore gets really draggy as
you approach stall speed. This may tend to increase minimum sink speed.

I didn't try to evaluate the glide outside a range of about 60 knots
indicated to 90 knots indicated.

KB

[Dale Alexander]

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
>
>
>
>

[[email protected]]

On May 4, 4:55 am, "Kyle Boatright" > wrote:
> "Marc J. Zeitlin" > wrote in m...
>
>
>
>
>
> > Kyle Boatright wrote:
>
> >> 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.
>
> > Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
> > though). Van's lists the stall speed as 49 mph (43 kts). I would have
> > expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
> > kts (92 mph) seems really high. My COZY has a stall speed of about 70
> > mph (61 kts), give or take, and the min. sink speed is around 80 mph
> > (70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
> > sink is around 550 fpm.
>
> > Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
> > interested in seeing the #'s.
>
> > Thanks.
>
> > --
> > Marc J. Zeitlin
> > http://www.cozybuilders.org/
> > Copyright (c) 2007
>
> A couple of things might come into play...
>
> 1) 80 knots was the indicated airspeed, and is subject to whatever errors
> are present in my ASI and pitot/static system at that speed.
>
> 2) An RV-6 has a pretty low aspect ratio and therefore gets really draggy as
> you approach stall speed. This may tend to increase minimum sink speed.
>
> I didn't try to evaluate the glide outside a range of about 60 knots
> indicated to 90 knots indicated.

Any short-winged airplane will sink pretty good when slow. My
Jodel stalls at around 40 but it's sinking so fast by the time I get
there that the stall speed is irrelevant. If I get below 60 indicated
the bottom falls out pretty quick.
Prop indexing has something to do with the prop's orientation
during the power pulse. Most of these opposed engines have a slight
rocking moment around the vertical axis, caused by the offset of the
opposing cylinders, and if the prop is more or less horizontal during
the biggest push, it will prevent most of that rotation just because
of the location of the blade masses and felt vibration wil be
minimized. Having the prop at 10:00/4:00 has the prop passing through
the horizontal during much of the power stroke.
That's what I understand, anyway. Vibration analysis defies
intuitive thinking so I might be all wet. Something I don't understand
is the idea that old guys have told me: if the alternator belt on your
6-banger Continental with its three-bladed prop keeps flipping off,.
rotate (re-index) the prop 180 degrees and it will stop doing that. It
works, too: on a 185 we used to have, the alternator belt regularly
flew off until we rotated that prop. I didn't think a three-blade prop
would have any sweet spots, but as I said, vibration analysis defies
intuitive thinking.

Dan

[Peter Dohm]

"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

[Morgans[_2_]]

"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

[[email protected]]

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

[Morgans[_2_]]

<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

[Kyle Boatright]

"Kyle Boatright" > wrote in message
. ..
>
> "Marc J. Zeitlin" > wrote in message
> ...
>> Kyle Boatright wrote:
>>
>>> 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.
>>
>> Huh. That seems fast, for an RV-6 (sink rate seems reasonable,
>> though). Van's lists the stall speed as 49 mph (43 kts). I would have
>> expected min. sink speed to be somewhere around 1.2 Vs, or 51 kts. 80
>> kts (92 mph) seems really high. My COZY has a stall speed of about 70
>> mph (61 kts), give or take, and the min. sink speed is around 80 mph
>> (70 kts), with a max L/D around 95 mph (83 kts). Sink rate at min.
>> sink is around 550 fpm.
>>
>> Do you have sink rates for speeds from Vs up to 120 kts or so? I'd be
>> interested in seeing the #'s.
>>
>> Thanks.
>>
>> --
>> Marc J. Zeitlin
>> http://www.cozybuilders.org/
>> Copyright (c) 2007
>
> A couple of things might come into play...
>
> 1) 80 knots was the indicated airspeed, and is subject to whatever errors
> are present in my ASI and pitot/static system at that speed.
>
> 2) An RV-6 has a pretty low aspect ratio and therefore gets really draggy
> as you approach stall speed. This may tend to increase minimum sink speed.
>
> I didn't try to evaluate the glide outside a range of about 60 knots
> indicated to 90 knots indicated.
>
> KB

I look up the old 1993 CAFE report where they extensively flight tested an
RV-6a. That airplane's minimum sink speed was determined to be 70 knots, but
that was using their "zero thrust" testing method. With that technique, it
is as if the prop doesn't exist - it neither provides thrust or drag. Not a
terribly good measure, if you ask me, because getting rid of the prop in
flight isn't a user selectable option on most airplanes.

In addition to instrumentation differences (CAFE does a very good job
installing and calibrating the instruments in their test aircraft), my
airplane has some physical differences from the test airplane. First, mine
has the stock cowl, which may be draggier than the test airplane. My
airplane has lower drag gear leg fairings and wheel pants than the test
aircraft. In addition, the test aircraft had a nosewheel, as opposed to my
taildragger.

What does all of this tell us?

Dunno.

KB