Proping Question

"Andrew Sarangan" wrote in message
oups.com...
snip

Although I agree with the dangers of accidental engine starts, what I
would like to know if there really have been any cases of inadvertant
engines starts when the prop is turned half a rotation with the
mixture in cut-off even if the magnetos were on. All the cases I am
aware of are related to hand-propping, which is not the same as
turning the prop to reposition the blades.

I had a friend who claimed that he had a warm engine start in his hangar
when he repositioned the prop. The hanger door was closed and he had
nowhere to go. Claimed his back was against the door and he held his hand
on the center of the spinner to stop the aircraft from moving forward.
Luckily the fuel was shutoff and it only ran for a very short time. I
forget where the gas came from. He may have been spinning a tale but he
really wasn't that kind of guy.

Happy landings.

In rec.aviation.student EridanMan wrote:
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.

My 63 Musketeer (BE-23) POH says the same thing:

'Always pull the propeller through by hand, opposite the direction of
rotation, several times to clear the engine and "limber up" the cold,
heavy oil before using the starter. This will also lessen the load on
the battery if external power is not used'

An old timer saw me doing that and nearly bit my head off saying that
I was going to break the vacuum pump. I felt I needed to do that in
cold weather because I was having a lot of trouble starting the
engine. I'd get maybe two revolutions before the battery died. Turned
out I needed a new starter and I had some electrical shorts too.

The vacuum pump did give up the ghost last year, but I don't think
it was related as I meekly switched to "limbering up" in the suggested
direction.

With the new starter, I don't really have any trouble starting anymore,
but I will pull it through a few times on the coldest days.

Followup set to r.a.p

--
Don Poitras

"Private" wrote in message
news:0Ft2i.188319$aG1.182030@pd7urf3no...

I had a friend who claimed that he had a warm engine start in his hangar
when he repositioned the prop. The hanger door was closed and he had
nowhere to go. Claimed his back was against the door and he held his hand
on the center of the spinner to stop the aircraft from moving forward.
Luckily the fuel was shutoff and it only ran for a very short time. I
forget where the gas came from. He may have been spinning a tale but he
really wasn't that kind of guy.


Interesting. I used to know an old timer that told the same tale.

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 15 May 2007 14:35:47 -0700, "Robert M. Gary"
wrote:

In the C140, the Aeronca, and the J-3 the carbs (probably all
Stromburg), pulling the mixture all the way out at idle had no effect
at all on the engine.

I fly a J-3, and I've never seen one with a mixture control.

Blue skies! -- Dan Ford

On 15 May 2007 09:19:59 -0700, "Robert M. Gary"
wrote:

So I would pull the prop through with an
open palm hand such that if the prop did start I would be ok (some of
the old guys actually start the plane this way)

That's what I was taught to do, and what I do if I'm propping from the
front (rarely). As it happens, I was 68 at the time, but my instructor
was 21. Blue skies! -- Dan Ford

"Private" wrote in message
news:0Ft2i.188319$aG1.182030@pd7urf3no...

"Andrew Sarangan" wrote in message
oups.com...
snip

Although I agree with the dangers of accidental engine starts, what I
would like to know if there really have been any cases of inadvertant
engines starts when the prop is turned half a rotation with the
mixture in cut-off even if the magnetos were on. All the cases I am
aware of are related to hand-propping, which is not the same as
turning the prop to reposition the blades.

I had a friend who claimed that he had a warm engine start in his hangar
when he repositioned the prop. The hanger door was closed and he had
nowhere to go. Claimed his back was against the door and he held his hand
on the center of the spinner to stop the aircraft from moving forward.
Luckily the fuel was shutoff and it only ran for a very short time. I
forget where the gas came from. He may have been spinning a tale but he
really wasn't that kind of guy.

Happy landings.


I hope he was wearing gloves!

Actually I have either heard or read the same story a couple of times over
the years, and I really do have my doubts.

In article ,
Cubdriver usenet AT danford DOT net wrote:

On 15 May 2007 14:35:47 -0700, "Robert M. Gary"
wrote:

In the C140, the Aeronca, and the J-3 the carbs (probably all
Stromburg), pulling the mixture all the way out at idle had no effect
at all on the engine.

I fly a J-3, and I've never seen one with a mixture control.

That's because the carburetor is hard to find now a days.
The ones that out there in the fly markets are high priced and are
pretty much junk. The people selling them are hoping to find a sucker
that only has the model number and doesn't know what to look for.
I am still looking for one to put on a 7AC Champ.

On May 16, 6:40 am, john smith wrote:
In article ,
Cubdriver usenet AT danford DOT net wrote:

On 15 May 2007 14:35:47 -0700, "Robert M. Gary"
wrote:

In the C140, the Aeronca, and the J-3 the carbs (probably all
Stromburg), pulling the mixture all the way out at idle had no effect
at all on the engine.

I fly a J-3, and I've never seen one with a mixture control.

That's because the carburetor is hard to find now a days.
The ones that out there in the fly markets are high priced and are
pretty much junk. The people selling them are hoping to find a sucker
that only has the model number and doesn't know what to look for.
I am still looking for one to put on a 7AC Champ.

The Stromberg carb on that Cub will either have a mixture
control that's lockwired in the full rich position, or will be missing
the mixture mechanism and have a cover plate over it. My A-65 had the
cover plate, and since I was a machinist in a former life, I machined
the mixture control parts and put them in. It works fine, but I seldom
use it. The CHTs go up too much if I lean it.
The mixture control in that engine doesn't directy control
the fuel flow like the Marvel Schebler/Precision Aeromotive carb does,
and so it can't shut the fuel right off. The float bowl is vented
through a cavity in the top cover that has two other passages, one
leading to the dead airspace behind the venturi where the air pressure
is more or less ambient, and the other into the venturi itself.
Without the mixture parts installed, the venturi will draw a tiny bit
of air from the dead airspace, but not enough to drop the bowl
pressure. With a mixture control, leaning the engine reduces the
airflow from the dead airspace by gradually shutting it off, and the
venturi's much lower pressure begins to drop the atmospheric pressure
in the float bowl. Since fuel flow is dependent on the difference
between venturi pressure at the fuel nozzle and the bowl's vented
pressure, the flow decreases as the bowl's pressure comes closer to
the venturi's pressure. In other words, the venturi suction holds the
fuel back. At idle, there's not much airflow through the venturi and
it doesn't generate any suction, so the mixture has no effect on fuel
flow.
Turning a prop backwards during hand-propping reduces the
mixture ratio in the cylinders if the engine has been overprimed. Air
is drawn through the exhaust and will absorb the fuel, carrying it
back through the carb and out. The carb will often drip fuel during
such an event. Combustible mixtures range from 8:1 (rich) to 18:1
(lean), and overpriming will make the air/fuel mixture much richer
than 8:1 and the engine won't fire. When an electric starter is
available, we can pull the mixture, open the throttle and crank until
it leans out enough to catch, but when handpropping this could take
hours and cause a heart attack or something.

Dan

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