Prop Indexing

Why are fixed-pitch props on PA-28s indexed to the 10-4 position from
the front?

a) To help hand propping?
b) To minimize vibration?
c) To improve performance?

My money is on (a) but others disagree.

In rec.aviation.owning Mike Granby wrote:

: Why are fixed-pitch props on PA-28s indexed to the 10-4 position from
: the front?

: a) To help hand propping?
: b) To minimize vibration?
: c) To improve performance?

Interesting question and one that I've had since we got our PA28-180. When we
got ours, it was indexed so TDC was at about 9-3 (from the front). Two other guys'
cherokees were at about 7-1. I liked ours since it was easier to pull through as
though hand-propping. A year ago my mechanic decided it wasn't correct, so we
reindexed ours to be at 7-1. At that time, we consulted the service manual, which
states 8-2... equally incorrect at both locations. We decided to reindex ours to the
lower location anyway. After doing that, a test flight seemed to have less vibration
than we'd had in the past.

So, this brings up more questions:
- Is the 8-2 position listed in the service manual supposed to be from the *cockpit*
view or the front view? If from the cockpit, then your 10-4 assertion is equivalent.
- If my plane (now in the lower, hand-propping-hostile location) is incorrect, why did
the perceived vibration decrease?
- What the f*sck IS the correct orientation?


To propose an answer to your original question, I recall someone (powerflow?)
claiming performance benefits. Maybe it was for a different aircraft, but they
claiming that propwash pulses timed with air intake strokes resulted in a slightly
higher obtainable manifold pressure. Seems possible, as even 1/2" of MP is noticable
in-flight. Still not sure I buy into it though.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

To propose an answer to your original
question, I recall someone (powerflow?)
claiming performance benefits.

LoPresti claim their Howl Cowl works better with a three-blader that
pulses the air in time with the engine, but I don't really buy it,
either...

This question was posed at our retired engrs coffee shop group this AM
where such weighty issues are discussed ad nauseum.

The consensus was that probably minimum-engine prop dynamic structural
stresses (stresses other that the engine output torque and engine
output torque pulses - i. e. due to gryroscopically induced moments
applied to the crankshaft) will occur if the prop is vertical at the
point of maximum yaw velocity.

How that translates into a rough running system given the engine mount
system is not obvious.

I didn't personally experience the roughness of the original 172M that
had the improperly indexed prop, so I can't verify that the roughness
was first order as I originally stated. I just assumed it was.

At any rate, I still put my money on the prop orientation being
optimized to the second harmonic yaw motion of the engine.

The reaction and
rebound of a individual cylinder firing moving the crankcase.
I'm not sure but some of it maybe also be attributed to camshaft and
valve spring pressure. Valve train forces are much higher than you
might
first suspect.

The forces generated and reacted within the engine, stay within the
engine. They don't reflect outside. The whole problem of roughness
however, has to be considered with the possibility that the prop or
engine has a first order out-of-balance that is dominating any
observer's attention.

Whit all this fooferaw proves to me is that direct drive may be simple
mechanically, but in terms of vibration it's very complex. Dave
Blanton's belt drive with a general purpose flywheel-on-one-end,
damper-on-the-other crank makes more sense every day.

Blanton's belt drive with a general purpose flywheel-on-one-end,
damper-on-the-other crank makes more sense every day.

If there were say 100,000+ Blanton drives out there like there are 4
cylinder direct-drive A/C engines I'm sure there would be many other
issues for our coffee shop discussions. They are the sort of system
that is probably suitable for a fiddling type experimenter, but would
have their own set of problems when put in the hands of the general
pilot expecting 2000 hr TBOs.

There's a lot to be said for simplicity, although I'll admit the
almost-opposed 4 cyl direct drive scheme does have some very subtle
driveline considerations. The second harmonic yaw motion isn't
recognized by virtually any mechanics, even though it probably has a
lot to do with engine accessory and baffling problems.

At that time, we consulted the service manual, which
states 8-2.

Maybe that's from the Piper legal dept to discourage hand
propping.........!

In rec.aviation.owning nrp wrote:
: At that time, we consulted the service manual, which
: states 8-2.

: Maybe that's from the Piper legal dept to discourage hand
: propping.........!

Possibly, but that part of the book has been there forever. The trouble with
the 8-2 position (from the front) was that is was unobtainable IIRC. It was a nice,
clean 30 degrees off in either direction. Doesn't exactly give one a warm fuzzy when
the official service instructions are impossible to follow.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

wrote:
In rec.aviation.owning nrp wrote:
: At that time, we consulted the service manual, which
: states 8-2.

: Maybe that's from the Piper legal dept to discourage hand
: propping.........!

Possibly, but that part of the book has been there forever. The trouble with
the 8-2 position (from the front) was that is was unobtainable IIRC. It was a nice,
clean 30 degrees off in either direction. Doesn't exactly give one a warm fuzzy when
the official service instructions are impossible to follow.

-Cory


The 8-2 (or more closely 7:30--1:30) IS appropriate for hand
propping. The position given by Service Manuals is at TDC so that the
prop will be at about 10:00 when ready to hand prop with the piston
partway up thye compression stroke.

The three-blade props used on some larger engines can cause
mysterious vibrations unique to them. Some Cessna 185s, for example,
had a habit of throwing off the alternator belt. The problem was
normally cured by removing the prop and rotating it 180 degrees. Don't
ask me why.

Some engines have problems with the pulsating airflow off the prop
blades striking the air intake at just the wrong time and upsetting air
and fuel flows in the carb. (Remember that the airflow through the carb
isn't particularly constant, as it accellerates and decellerates with
intake valves opening and closing.) This is readily apparent on some
older 182s, on which the air cleaner comes off with the cowling, and
running the engine with the cowl off will make for a rough-running
engine. The air cleaner damps some of the pulsation. Re-indexing the
prop might improve smoothness and maybe performance.

Dan

In rec.aviation.owning wrote:
: The 8-2 (or more closely 7:30--1:30) IS appropriate for hand
: propping. The position given by Service Manuals is at TDC so that the
: prop will be at about 10:00 when ready to hand prop with the piston
: partway up thye compression stroke.

The problem is that it's not clear to me whether or not saying "8-2" is with
respect to looking at the prop from the front (as one would be when servicing it), or
with respect to sitting in the pilot's seat (as everything else is always referenced).

: engine. The air cleaner damps some of the pulsation. Re-indexing the
: prop might improve smoothness and maybe performance.

Re-indexing our O-360 prop reduced the perceived vibration... not sure if it
was real or "we did all that work so it *must* be better." Unfortunately, we
reindexed it to make it less conducive to hand-propping.

-Cory

--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************