Proping Question

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?

"Doug Palmer" wrote in message
ink.net...

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?

There is potential for damage to a vacuum pump if turned backward.

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.


It doesn't "disarm" it, it just won't fire. This means than the engine
is less likely to start if the mags are hot (since the impulse coupling
aids in the starting). However, NEVER trust a propeller. Even turning
them backwards can generate a spark and even if the engine doesn't start
it can kick the prop over enough to do damage.

The argument against it is that people claim it hurts the vacuum pump.
While some dry pumps are designed to only turn one way, that's at
operating speed. The vanes are in there loose enough when it's
not spinning to not be a problem.

Except when absolutely necessary (like to get the tow bar connected)
you shouldn't be turning the prop at all. There's no good reason to
justify the dangers.

Except when absolutely necessary (like to get the tow bar connected)
you shouldn't be turning the prop at all. There's no good reason to
justify the dangers.

Partly, just to be a smart-ass, and also looking at the geometry, that could
be one of the worst times to turn the prop...


In reality, as already pointed out, that is one of the times when it could
be necessary. At the very least, I would verify that all controls
(especially throttle and mixture) are all the way back and that all
appropriate switches are off--and then try to stay out of the prop arc while
working. That should minimize the personal danger and (hopefully) eliminate
the possibility of one of those "Voracious Airplane Eats Tractor" type
accidents that circulate forever on the internet.

Peter

Ron Natalie wrote:

The argument against it is that people claim it hurts the vacuum pump.

That was what I was taught many moons ago. I have never confirmed it,
but my brain tells me that the engine is designed to turn in one
direction, so don't turn it in the other. No real proof (I'm not an A&P
and have never played one on TV), but just my own logic.


Except when absolutely necessary (like to get the tow bar connected)
you shouldn't be turning the prop at all. There's no good reason to
justify the dangers.

If the engine is properly shutdown (boost pump off, throttle slightly
above idle, and mixture to cut off) the chances are greatly reduced than
if you just kill the engine by turn off the mags. Additionally, some
people recommend checking your mags before shutdown to make sure you
don't have a broken P lead which would also help. A flying club I used
to belong to had a "policy" (not a rule, just a suggestion that it would
be nice if you did it) of turning the prop vertical after parking it to
keep the birds from sitting on it and pooing on the plane.

Obviously the safest way to prevent an accidental fire while hand
turning the prop is to not do it. When I do turn a prob by hand, I try
to do it in the direction it turns while running, turn slowly to
minimize any compression (don't know if that is true either, just what I
was taught), and only use the palms of my hand on the face of the prop
to minimize the possibility of the prop smacking the back of my hand
should it fire. YMMV.


-m
--
## Mark T. Dame
## CP-ASEL, AGI
## insert tail number here
## KHAO, KISZ
"A programmer and his mind are soon parted."

The magneto cannot generate a spark when turned slowly
backward, and it's not just because the impulse coupling doesn't work
backwards. The magnet could generate a flow in the primary winding,
but the points are closing rather than opening at the right time and
the arresting of the flow, a requirement for spark, isn't there.
Further, the distributor is geared to send a spark to a cylinder based
on a particular direction, and turning some reversible mags backward
can generate a spark but it happens when the distributor finger is in
the wrong place. Besides, the mag has to be turning at a good clip to
make any spark without the impulse coupling.
Worn-out vacuum pumps could indeed fail if turned
backward, but they don't belong on the airplane anyway. We use the
pumps with the wear inspection ports so that they never get to that
vulnerable stage. A good pump can be turned backward without fear of
breaking anything.
So we teach our students to turn the prop backward, but
that it must be done with great caution as a matter of course. Most of
them don't understand magnetos or engines or anything else and might
someday turn the thing in the wrong direction, and you have to make
such rules so that they learn that props can kill and so they will
keep their friends from fooling with it. We never lose a vacuum pump,
have never had an inadvertent firing. The most dangerous time is right
after the engine is shut down, when hot carbon in the cylinder head
could fire any vapors still present. We also check the mag grounding
at idle just before mixture cutoff.

Dan

On May 15, 8:20 am, wrote:
We also check the mag grounding
at idle just before mixture cutoff.

Dan

What type of plane are you handpropping that has mixture cutoff? Must
not be the traditional Stromburg carb.

-Robert

"Robert M. Gary" wrote in message
ps.com...
On May 15, 8:20 am, wrote:
We also check the mag grounding
at idle just before mixture cutoff.

Dan

What type of plane are you handpropping that has mixture cutoff? Must
not be the traditional Stromburg carb.

-Robert

This could be just a nomenclature issue. I was taught to call the lean
position of the mixture control "idle cut off" even though it really doesn't
cut anything off. However, it is too lean to keep the engine running at
1000 rpm.

I'm curious about what others think

Peter

On May 15, 10:21 am, "Robert M. Gary" wrote:
On May 15, 8:20 am, wrote:
We also check the mag grounding

at idle just before mixture cutoff.

Dan

What type of plane are you handpropping that has mixture cutoff? Must
not be the traditional Stromburg carb.

-Robert

We were talking about turning the prop backward to reposition
it, not handpropping to start it. All our training airplanes have
starters and idle cutoff mixture controls. My own old Jodel with its
A-65 has to be handpropped, and has the old Stromberg with cruise
mixture control that has no effect on idle mixture. I shut the fuel
off and wait a bit for the bowl to run dry when I shut it down.

Dan

A flying club I used to belong to had a "policy" (not a rule, just a
suggestion that it would be nice if you did it) of turning the prop vertical
after parking it to keep the birds from sitting on it and pooing on the
plane.

We turn the prop vertical in the winter to prevent water from pooling inside
the spinner and then freezing into a block of ice. This happened to me once,
and the vibration it caused was impressive. We shut down, pulled the plane
into a heated hangar, turned the prop vertical, and waited for the ice to melt
and drain out.