Proping Question

On 2007-05-15 16:28:54 -0700, EridanMan said:

Interesting topic, interesting discussion... but if I may expand on
the original question a bit?

The Original piper POH that came with my bird ('67 PA-28-140) actually
recommends that that prop be pulled through two complete rotations
backwards any time the engine is started after sitting for a long time
(it actually says its a good idea for every flight, but should be
mandatory any time the engine's been sitting for a while).

I've NEVER heard or seen this advice anywhere else... I've never
practiced it... It seems almost insane to me... but the book says
what the book says.

Anyone have any clue why?

I can get the exact wording tpmogjt ... its in the 'preflight and
takeoff' section of the book.

This is also recommended in at least Cessna 172 manuals for cold
weather starts. The idea is that oil that is cold or has been sitting
awhile is thicker and more sluggish. Pulling the prop through limbers
up the oil some, so the engine does not have to work as hard while
starting. It can make all the difference in whether you are able to
start the engine or not.
--
Waddling Eagle
World Famous Flight Instructor

On Tue, 15 May 2007 12:59:05 GMT, "Doug Palmer"
wrote:

The reasonings ranged from "you should not turn an engine backwards" to
"turning the engine backwards disarms the impulse coupling", to several
issues in between.

The local airport owns two Cubs that are of course always propped. The
one I fly has impulse coupling; I think the other does as well. At
various times I've been propped by flight instructors, the airport
manager, and the airport owner. I've never moved the prop backwards
myself, but I've often seen them do it repeatedly -- I think to unload
the carb because they've flooded it with repeated pulls. I've never
heard anyone remark that it shouldn't go backwards.

However! I've never moved a prop unless someone was standing on the
brakes, and since if I'm propping the plane it's usually because the
guy at the controls is a stranger who probably knows little about
Cubs, I always prop from behind.

Blue skies! -- Dan Ford

On May 16, 9:41 pm, Cubdriver usenet AT danford DOT net wrote:
On Tue, 15 May 2007 12:59:05 GMT, "Doug Palmer"

wrote:
The reasonings ranged from "you should not turn an engine backwards" to
"turning the engine backwards disarms the impulse coupling", to several
issues in between.

The local airport owns two Cubs that are of course always propped. The
one I fly has impulse coupling; I think the other does as well. At
various times I've been propped by flight instructors, the airport
manager, and the airport owner. I've never moved the prop backwards
myself, but I've often seen them do it repeatedly -- I think to unload
the carb because they've flooded it with repeated pulls. I've never
heard anyone remark that it shouldn't go backwards.

However! I've never moved a prop unless someone was standing on the
brakes, and since if I'm propping the plane it's usually because the
guy at the controls is a stranger who probably knows little about
Cubs, I always prop from behind.


I always 'walk through'.
photo is of H M Jenkins prop starting a Rallye. As he and I were the
only ones there that day who were able to take the place of starter
motors :-)
http://imagebank.org.nz/is.php?i=264..._prop_star.jpg

However! I've never moved a prop unless someone was standing on the
brakes, and since if I'm propping the plane it's usually because the
guy at the controls is a stranger who probably knows little about
Cubs, I always prop from behind.

Blue skies! -- Dan Ford


I flew cubs for awhile 30+ years ago. If there was no one around to
prop yours, the drill was to stand behind the prop on the right side
and flip it with your right hand, The left was poised to adjust the
throttle when it caught. I recall that they were amazingly easy to
start (usually on the first or second try).

David Johnson

On 16 May 2007 17:22:39 -0700, Dave wrote:

I flew cubs for awhile 30+ years ago. If there was no one around to
prop yours, the drill was to stand behind the prop on the right side
and flip it with your right hand, The left was poised to adjust the
throttle when it caught. I recall that they were amazingly easy to
start (usually on the first or second try).

They still are, if the engine is warm, and that's the only occasion
when I have to prop the plane myself.

I hold onto the window frame with my left hand and, like you, pull
down with the right hand. I bounce it lightly a couple times in
advance, and one knows exactly when to pull down hard after it springs
back up.

At a minimum, I have a pair of chocks on a length of parachute cord. I
can climb into the back seat, give the cord a tug, then reel in the
chocks.

Blue skies! -- Dan Ford

On 2007-05-15 05:59:05 -0700, "Doug Palmer" said:

Several pilots found themselves in a debate at our field yesterday. The
issue is weather it is safer to move the propeller on a (parked) aircraft in
the direction of usual engine rotation, or opposite usual rotation. This is
assuming that the propeller needs to move for some reason.

The reasonings ranged from "you should not turn an engine backwards" to
"turning the engine backwards disarms the impulse coupling", to several
issues in between.

Any thoughts from the groups collective wisdom?

An engine can be made to run backwards. Model airplane engines do it
all the time, usually as a result of mixture that is too rich. Granted,
real airplane engines are different and have more safety systems, but I
could not say that it is impossible, especially given the enormous
variety in types of engines, magnetos, starters, and fuel systems you
see on airplanes.

I have never heard of an accident involving an engine running backward.
I haven't found one, either. You can be the first! :-)
--
Waddling Eagle
World Famous Flight Instructor

"C J Campbell" wrote in message
news:2007051622171050878-christophercampbell@hotmailcom...

An engine can be made to run backwards. Model airplane engines do it all
the time, usually as a result of mixture that is too rich. Granted, real
airplane engines are different and have more safety systems, but I could
not say that it is impossible, especially given the enormous variety in
types of engines, magnetos, starters, and fuel systems you see on
airplanes.

I don't see how a four cycle engine can run backwards. If the crank is
turning backwards, the intake valve would be open while the piston is going
up, pushing the contents of the cylinder into the intake manifold. While the
exhaust valve is open, the piston would be going down, sucking whatever is
in the exhaust manifold into the cylinder. there would have to be a source
of fuel in the exhaust manifold for the engine to run.

Model engines are two stroke, so they can run backwards.

A long time ago, I bought a moped (also a two cycle). The timing was so far
retarded that it ran better backwards than forwards until I had it timed. It
couldn't pull me up the hill because it ran so badly. Once, it fired up
backwards and ran great. I rode it up the hill backwards.

"C J Campbell" wrote in message
news:2007051622171050878-christophercampbell@hotmailcom...
On 2007-05-15 05:59:05 -0700, "Doug Palmer"
said:

...
An engine can be made to run backwards. Model airplane engines do it all
the time, usually as a result of mixture that is too rich. Granted,

Those are two strokes and don't rely on valve timing to run. A buddy had a
Bultaco motorcycle that liked to do that - it was pretty funny when he
dumped the clutch not realizing that it was running backwards.

real airplane engines are different and have more safety systems, but I
could not say that it is impossible, especially given the enormous variety
in types of engines, magnetos, starters, and fuel systems you see on
airplanes.

If you turn a conventional four stroke engine backwards, what would have
been the exhaust stroke is now an intake stroke, and what was the intake
stroke is now an exhaust stroke - the air will flow backwards through the
engine from the exhaust to the intake so fuel will not find it's way in to
sustatin combustion. On the other hand, an engine can "kick back" for a
revolution or so - and that's enough to do the damage...

To make a conventional four stroke run backwards, you have to re-arrange the
location of the lobes on the cam.

Trivia: Kettering developed his electric starter after a friend was killed
when an automobile engine kicked back while he was starting it with a hand
crank...

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

On 2007-05-17 14:26:01 -0700, "Capt. Geoffrey Thorpe" The Sea Hawk at
wow way d0t com said:

"C J Campbell" wrote in message
news:2007051622171050878-christophercampbell@hotmailcom...
On 2007-05-15 05:59:05 -0700, "Doug Palmer"
said:

...
An engine can be made to run backwards. Model airplane engines do it all
the time, usually as a result of mixture that is too rich. Granted,

Those are two strokes and don't rely on valve timing to run. A buddy had a
Bultaco motorcycle that liked to do that - it was pretty funny when he
dumped the clutch not realizing that it was running backwards.

real airplane engines are different and have more safety systems, but I
could not say that it is impossible, especially given the enormous variety
in types of engines, magnetos, starters, and fuel systems you see on
airplanes.

If you turn a conventional four stroke engine backwards, what would have
been the exhaust stroke is now an intake stroke, and what was the intake
stroke is now an exhaust stroke - the air will flow backwards through the
engine from the exhaust to the intake so fuel will not find it's way in to
sustatin combustion. On the other hand, an engine can "kick back" for a
revolution or so - and that's enough to do the damage...

To make a conventional four stroke run backwards, you have to re-arrange the
location of the lobes on the cam.

Trivia: Kettering developed his electric starter after a friend was killed
when an automobile engine kicked back while he was starting it with a hand
crank...

Sounds like it was running backwards to me. Maybe it would not keep
running, but the prop only has to hit you once.
--
Waddling Eagle
World Famous Flight Instructor

On May 18, 7:38 am, C J Campbell
wrote:
On 2007-05-17 14:26:01 -0700, "Capt. Geoffrey Thorpe" The Sea Hawk at
wow way d0t com said:





"C J Campbell" wrote in message
news:2007051622171050878-christophercampbell@hotmailcom...
On 2007-05-15 05:59:05 -0700, "Doug Palmer"
said:

...
An engine can be made to run backwards. Model airplane engines do it all
the time, usually as a result of mixture that is too rich. Granted,

Those are two strokes and don't rely on valve timing to run. A buddy had a
Bultaco motorcycle that liked to do that - it was pretty funny when he
dumped the clutch not realizing that it was running backwards.

real airplane engines are different and have more safety systems, but I
could not say that it is impossible, especially given the enormous variety
in types of engines, magnetos, starters, and fuel systems you see on
airplanes.

If you turn a conventional four stroke engine backwards, what would have
been the exhaust stroke is now an intake stroke, and what was the intake
stroke is now an exhaust stroke - the air will flow backwards through the
engine from the exhaust to the intake so fuel will not find it's way in to
sustatin combustion. On the other hand, an engine can "kick back" for a
revolution or so - and that's enough to do the damage...

To make a conventional four stroke run backwards, you have to re-arrange the
location of the lobes on the cam.

Trivia: Kettering developed his electric starter after a friend was killed
when an automobile engine kicked back while he was starting it with a hand
crank...

Sounds like it was running backwards to me. Maybe it would not keep
running, but the prop only has to hit you once.
--
Waddling Eagle
World Famous Flight Instructor- Hide quoted text -

- Show quoted text -

The engine can fire when pulled through forward, but if the
prop has too little momentum to carry the piston past TDC or the
firing is advanced enough, it will kick backward. One blade,
travelling at the speed it does, will split your skull. I work on
these things all day, and when I forget that the prop is there and I
stand up after doing something on the engine and whack my head on a
stationary prop blade, it hurts big time. When it's swinging after a
kickback it's moving faster than an axe and weighs a lot more. Whether
dull or not, that trailing edge will do plenty of damage. You learn
respect for these things when you bash your head occasionally.
I used to own a '78 Dodge pickup truck, the worst vehicle I
ever encountered. It would "diesel' on after I turned the ignition
off, ignition being caused by hot carbon points in the cylinder head.
Some cylinders would fire, some wouldn't, and the result was an
exhaust system full of fuel vapours. An auto's exhaust system is a lot
longer than a lightplane's, and it can store plenty of vapour. Sooner
or later the engine would kick over backward, the exhaust would get
sucked into a cylinder, and it would run backward for a half-second or
so and finally die when the fuel vapours ran out. The hot carbon was
still doing the igniting, like a glow plug.
If an airplane does that it won't run so long, and it only
does it if there's hot carbon in the head which means that it's been
running. And that's why I consider a prop most dangerous immediately
after shutdown.

Dan