Prop Indexing

[Mike Granby]

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.

[Jim Macklin]

Airplanes without starters have the prop indexed for hand
propping, engines with starters have the blades on a two
bladed prop aligned with the crank throw.


--
James H. Macklin
ATP,CFI,A&P

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


"Mike Granby" > wrote in message
oups.com...
|
| 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.
|

[Roy Page]

Jim, we learn something every day !
Is there a reason that the 2 blade prop is aligned with the crank throws ?

--
Roy
N5804F Piper Archer
"I have had some bad landings but I have never missed the runway"


"Jim Macklin" > wrote in message
news:y9nuf.38976$QW2.20013@dukeread08...
> Airplanes without starters have the prop indexed for hand
> propping, engines with starters have the blades on a two
> bladed prop aligned with the crank throw.
>
>
> --
> James H. Macklin
> ATP,CFI,A&P
>
> --
> The people think the Constitution protects their rights;
> But government sees it as an obstacle to be overcome.
> some support
> http://www.usdoj.gov/olc/secondamendment2.htm
> See http://www.fija.org/ more about your rights and duties.
>
>
> "Mike Granby" > wrote in message
> oups.com...
> |
> | 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.
> |
>
>

[Jim Macklin]

I recall something about harmonics and power pulses. For
hand propping, the blade angle should be such that you bring
the piston to TDC with the blade above the shoulder so you
can pull it through as you step back, when hand propping a
plane with a starter, the piston is at TDC with the blade
angle lower and the person is more likely to be in a
position to fall into the prop rotational plane.


Of course, I could have made it all up or simply remembered
some details, IF I cared I'd look up the answer in one or
more texts/web pages from a reliable source.



--
James H. Macklin
ATP,CFI,A&P

--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


"Roy Page" >
wrote in message
ink.net...
| Jim, we learn something every day !
| Is there a reason that the 2 blade prop is aligned with
the crank throws ?
|
| --
| Roy
| N5804F Piper Archer
| "I have had some bad landings but I have never missed the
runway"
|
|
| "Jim Macklin" > wrote
in message
| news:y9nuf.38976$QW2.20013@dukeread08...
| > Airplanes without starters have the prop indexed for
hand
| > propping, engines with starters have the blades on a two
| > bladed prop aligned with the crank throw.
| >
| >
| > --
| > James H. Macklin
| > ATP,CFI,A&P
| >
| > --
| > The people think the Constitution protects their rights;
| > But government sees it as an obstacle to be overcome.
| > some support
| > http://www.usdoj.gov/olc/secondamendment2.htm
| > See http://www.fija.org/ more about your rights and
duties.
| >
| >
| > "Mike Granby" > wrote in message
| >
oups.com...
| > |
| > | 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.
| > |
| >
| >
|
|

[nrp]

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.

A two blade propeller has to present the same yawing moment of inertia
in response to the 80 Hz engine swing. It can do so only if it is
phased such that it is oriented 45 degrees to the plane of the crank
throws.

If it is phased in the plane of the crank throws, the engine swinging
will present different yaw inertias to alternating second harmonic
inputs ( i. e. every other one), causing a first order response of the
system. This first order response will appear as a shake at crankshaft
frequency.

This would not be a problem we had three blade propellers, or 6
cylinder engines.

Anybody want to challenge this? I'd be interested in further
discussions on this topic

[George Patterson]

Jim Macklin wrote:
> Airplanes without starters have the prop indexed for hand
> propping, engines with starters have the blades on a two
> bladed prop aligned with the crank throw.

Some do, some don't. A four-cylinder engine will stop with all of the pistons
very close to half-way down the bores. If the prop is aligned with the crank
throw, it will stop in a vertical position. My Maule did this, my C150 did not.

It is not possible to align a two-bladed prop with all the crank throws of a six
cylinder engine.

George Patterson
Coffee is only a way of stealing time that should by rights belong to
your slightly older self.

[Roy Page]

This is a very interesting discussion.
I have never had my prop off the Archer.
Is it possible to fit a prop on a PA28 indexed incorrectly or is there a
definite radial locator ?
--
Roy
N5804F Piper Archer
"I have had some bad landings but I have never missed the runway .... yet"

"George Patterson" > wrote in message
news:37xuf.19929$z45.8314@trnddc02...
> Jim Macklin wrote:
>> Airplanes without starters have the prop indexed for hand propping,
>> engines with starters have the blades on a two bladed prop aligned with
>> the crank throw.
>
> Some do, some don't. A four-cylinder engine will stop with all of the
> pistons very close to half-way down the bores. If the prop is aligned with
> the crank throw, it will stop in a vertical position. My Maule did this,
> my C150 did not.
>
> It is not possible to align a two-bladed prop with all the crank throws of
> a six cylinder engine.
>
> George Patterson
> Coffee is only a way of stealing time that should by rights belong to
> your slightly older self.

[nrp]

>it will stop in a vertical position. My Maule did this, my C150 did
not.

It is not possible to align a two-bladed prop with all the crank throws
of a six
cylinder engine.<

My 172M (a different one from my earlier posting) stops with the prop
@ 45 degrees, and is at TDC @ 135 degrees.

I don't think a 6 cyl engine has any reason to be sensitive to prop
orientation when it comes to smooth operation, although I can
appreciate that they might for some other unknown reason.

[Mike Granby]

> I'd be interested in further
> discussions on this topic

Me, too! My original post came as a result of a discussion I'm having
elsewhere. My assertion was that since most service manuals specify
10-4 as the right position, they are simply repeating "old lore" with
respect to hand propping. My interlocutor feels that whatever the
manual specifies is as a result of wanting to get the best performance
from the airplane. My response to this is that performance won't be
impacted, and that if anything, vibration would be the other issue, but
that this would be better addressed with a 12-6 positioning.

[nrp]

Sorry guys -

In reflecting my earlier post, I now think my earlier reasoning is
invalid. Yes there is a second harmonic (80 Hz) yaw moment generated
within the engine, but the rotating propeller will also change the
reflected yaw inertia at 80 Hz.

Therefore, now I don't see how this coupling could make a 40 Hz shake.

Can somebody shed some more light on this? I know the orientation is
critical on a 172M, and I've seen a couple of homebuilts that had
similar prop orientation problems too.

[Bret Ludwig]

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.

[Fly]

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

[Mike Granby]

> 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?

[Aaron Coolidge]

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.

[Fly]

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.

[nrp]

>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.