Prop Indexing

nrp wrote:
I was in a 172M club that had a bad first order shake appear after an
engine overhaul. It turned out the prop had to be indexed to 45
degrees @ TDC to get rid of it. The orientation requirements are in
the Cessna manual, but there is no explanation.

As an ME with some background in dynamics, this is the only reason I
can see for this:

All 4 cylinder horizontally opposed engines with offset (in contrast to
directly opposite) cylinders will have a second harmonic yaw motion
about their CG due to the short connecting rods. Without a double
crank speed counterbalance shaft system, there is no way to eliminate
this. This causes a yaw vibration or swinging of the engine at 80 Hz
at 2400 rpm for example. I think it is also the reason the vibration
environment on the front of a Lycoming engine is so severe.

I think this is called a "rocking couple".

It's an issue with aftermarket Harley engines designed to use offset
cylinders so as not to need female (fork) rods. When Colombo laid out
the "short block" Ferrari V12 he wanted to have the bores on centers
and fork/blade rods but Enzo said since Packard did not to so with the
Twin Six it was unnecessary.

A little bit off topic, as always. Certified a/c are stuck with LyCon
but homebuilders who use 'em are idiots. They belong with the OXX-6,
Pobjoy, Menasco, Hirth, Siemens-Halske, Guiberson and Caminez on a
museum wall.

I have checked a number of engine/prop combinations with prop balancing
equipment and found the overall rock/yaw movement of any Continental or
Lycoming 4-cylinder sensitive to prop index.

There are 6 bolt holes in the crank flange and the prop will cover two. The
most perpendicular to TDC is the worst position. The best is the position
of a blade following TDC, the classic 10:00/4:00 suited for handpropping.
Even the intermediate position with a blade leading TDC will cause more yaw
on the aft of the engine.

My layman's mind attributes this to the firing order. 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.

Most fixed pitch crankshafts do not have a particular index bushing that
constant-speed models have.
Yes, Lycomings have a index bushing for the ring gear position but its not
for a prop necessarily. One can install a fixed pitch prop in any of three
axis.

6 cyl Cont or Lyc cranks have a prop index bushing. Two choices, zero or
180 different. I know there has to be one but I have never seen a fixed
pitch prop on a 6-cylinder myself. I bet if the prop was indexed to the
middle two throws the engine would suffer from more yaw.

Offhand, I know of several instances when manufacturers recommend a change
in this reindexing of constant speed prop index. The first 300 serial
numbers of the Mooney M20J can be converted from the IO-360 A1B6 to A3B6 by
moving a prop bushing one hole counterclockwise which aligns it more closely
to TDC.
There is a Lycoming service bulletin advising about the same for some Aviat
Huskeys.
ANd there is a change thru the evolution of Grumman American service
manuals.

Oddest instance to me was hearing that some Europeans conducting tests on
aircraft with split exhaust systems have found the sound signature is less
when the prop is indexed a particular way, to be out-of-sync of the exhaust
note coming out the pipe.

Kent Felkins
Tulsa

I have checked a number of engine/prop combinations
with prop balancing equipment and found the overall
rock/yaw movement of any Continental or Lycoming
4-cylinder sensitive to prop index.

But is there any evidence that performance can be impacted by prop
indexing?

In rec.aviation.owning Fly kentf AT entergate.com wrote:
: 6 cyl Cont or Lyc cranks have a prop index bushing. Two choices, zero or
: 180 different. I know there has to be one but I have never seen a fixed
: pitch prop on a 6-cylinder myself. I bet if the prop was indexed to the
: middle two throws the engine would suffer from more yaw.

Some Cherokee 235's have a fixed pitch prop. All Pawnee 235's have a fixed
pitch prop (PA-25-235), at least all that I have ever seen. All O-300
Skyhawks have a fixed pitch prop, except for the very last geared ones,
called "Powermatic".

Every O-300 Skyhawk I've seen has the 10:00/4:00 prop positioning. Is
it correct? Is it optimal? Don't know...
--
Aaron C.

Thanks for the reminder Aaron. Your're right.
Boy, that was a dumb statement I made. Heck I was working on a Skyhawk
O-300 today!
Kent Felkins



" In rec.aviation.owning Fly kentf AT entergate.com wrote:
I know there has to be one but I have never seen a fixed
: pitch prop on a 6-cylinder myself.

Some Cherokee 235's have a fixed pitch prop. All Pawnee 235's have a
fixed
pitch prop (PA-25-235), at least all that I have ever seen. All O-300
Skyhawks have a fixed pitch prop, except for the very last geared ones,
called "Powermatic".

Every O-300 Skyhawk I've seen has the 10:00/4:00 prop positioning. Is
it correct? Is it optimal? Don't know...
--
Aaron C.

and someone worked out that the stresses in a crank due to
straight torque are

I had reached that conclusion in an earlier posting - and maybe others.
These things are buried so deep in the technical organizational
underbrush that they never seem to reach the light of day. I sure wish
someone in the real know would post to this group.

Your two blade/three blade thing may be correct. Obviously your
observations are. I have been looking at things from only a
theoretical basis.