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Pulling through 7, 8, 9 or 10 blades, or 20, 21?

 
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psb777



Joined: 12 Aug 2009
Posts: 34
Location: UK

PostPosted: Wed Sep 30, 2009 5:58 am    Post subject: Pulling through 7, 8, 9 or 10 blades, or 20, 21? Reply with quote

Pulling through 7, 8, 9 or 10 blades, or 20, 21?

[Assuming two blade prop!)

Recently on the Yak list there has been discussion
on the best way to prime/start a M14P.

Before buying my aircraft I read some detailed and
frightening explanations of the cause and consequences
of hydraulic lock. The clearest explanations were
written, I think, by Richard Goode.

We're all justifiably scared of the cost and
other consequences of the engine failure which is
ultimately if not immediately caused by hydraulic
lock. We're also lazy so few if any of us follow the
engine manual's advice - we do not routinely (if ever)
remove the bottom spark plugs. (Although many of us
have installed the inlet drains to almost eliminate
the need to do so.)

One 18T check list I saw said to pull the engine
through 7 blades.

I re-wrote that in my head to read "at least 7
blades". Soon I was pulling 8 blades. It was 7 plus
1 for luck and also because 8 is more than 7 and the
script in my head was "at least 7". If 7 is enough,
then 7 should be enough. I felt silly pulling 8
blades just to satisfy some netherworld spirit.

Later I read "7 or 8 blades" in another M14P
checklist. I don't know how to distinguish between
the necessity to pull 7 blades sometimes and 8 at
other times. If 7 is enough, then 8 is unnecessarily
one too many. If 8 really is needed, sometimes. then
why say "7 or 8"? Say "8". Nevertheless, I decided
to follow the advice which contained the higher
number: I continued to pull 8 blades but no longer
did I do so out of superstition: 8 is indeed "7 or
8". and, to my considerable relief, 8 is no longer
unnecessarily one more than "at least 7".

But the upward pressure on the number of blade
pulls continued. Yet another excellent and usable
18T checklist advised 8 or 9 blades. A pilot I
respect advised similarly. So whereas my behaviour
was compliant it was obvously not good enough: I
was pulling 8 blades and 8 is "8 or 9" but that felt
inadequate - soon I was pulling 9 blades. I'm scared
of hydraulic lock.

For a while the number of blades I pulled was steady
at 9. But to make a lie of my assertion that I
am a rational man, recently I found I was pulling
10 blades. This feels stupidly wasteful of effort,
at best; or superstitious, at worst. It was with
some initial relief, therefore, to read here recently
Didier's advice at the Yak forum to pull through 10
blades. I was not alone! But my heart sank when I
read the supporting reasoning: one for each cylinder,
and another for luck. But the advice is flawed for
two reasons: superstition, and the engine is geared.
Sorry, Didier, one blade is not one engine stroke.

I am relieved that 10 blades will not be established
as the norm because I know, were that to happen,
I would soon be pulling 11 blades to be on the safe
side; or because 11 is without argument "at least 10";
or because the number of blades I pull has become so
large I may have miscounted, so I'll add one for luck.

The number keeps on going up because we are
justifiably scared of hydraulic lock. Bakers would
once be severely fined for being underweight on a
dozen bread rolls and hence the Baker's Dozen is 13.
That's what's happening here. And now you have just
read "13" in an article to do with the correct number
of pre-start blade pulls on an M14P, doubtless some
nagging of your subconscious will soon have you pull
13 blades on your beloved M14P. It will soon be
standard practise as a Google search will highlight
part of the previous sentence "pull 13 blades on
your beloved M14P". But only for a while, because
inevitably this will become 14 blades, as this is
"at least 13".

I had resolved therefore to stop pulling 10 blades and
to go back to pulling 9 blades but then a follow up
posting from Richard Goode made me realise his advise
to pull the prop through 10 "revolutions" had not been
a misprint. As he clarifies later with "20 blades".
No, I misquote, it's "at least 20 blades". 21, then?

By far the least ambiguous, best reasoned, and most
usable Yak and M14P advice I have read is from
Richard. And my understanding of hydraulic lock
derives from his writings on the subject and you
are likely better off reading that. Nevertheless,
I gave it a think. And this is what I thunk:

EXAMPLE 1 - the engine is on a tailwheel aircraft and
the engine is NOT fitted with inlet manifold drains.

There is oil which has leaked past the rings into
the cylinder and past a slightly open inlet valve
into the inlet manifold. Upon starting to pull the
engine through, the cylinder may be at the start of
its inlet stroke. The inlet stroke occurs and air
bubbles past the oil in the inlet manifold, leaving
the oil in the inlet manifold when the inlet valve
closes tight, after 180 degs. The engine is pulled
through a compression stroke, 360 degs, and there
is perhaps not quite enough oil in the cylinder
to prevent this. And the engine is pulled through
a power stroke, 480 degs. Then an exhaust during
which the cylinder head oil is pushed out, 720 degs.
This is seen by the operator. On the following
inlet stroke air once again bubbles past the inlet
manifold oil which still does not enter the cylinder,
900 degs. But enough! You can do this all day and
although no more oil emerges from the exhausts there
will still be oil in the inlet manifold to be sucked
in to cause hydraulic lock when the engine is started.
20 BLADES IS NOT ENOUGH.

If you don't have inlet manifold drains 20 BLADES IS
NOT ENOUGH.

EXAMPLE 2 - the engine is on a tailwheed aircraft and
the engine IS fitted with inlet manifold drains but
possibly they were [mistakenly] left closed overnight

There is oil which has leaked past the rings into
the cylinder and past a slightly open inlet valve
into the inlet manifold. The inlet manifold drains
are now opened. The oil in the inlet manifold flows
out and this is obvious to the operator, who waits
for the oil to stop dribbling. Upon starting to pull
the engine through, the cylinder may be at the start
of its inlet stroke. After the inlet stroke there
remains oil in the cylinder head and the engine has
turned 180 degs. The engine is pulled through a
compression stroke, 360 degs, and there is perhaps
not quite enough oil in the cylinder to prevent this.
And the engine is pulled through a power stroke,
480 degs. Then an exhaust during which the cylinder
head oil flows out, 720 degs. All the oil should now
have gone. 720 engine degrees is 1200 prop degrees
which is 6.6 prop blades.

Haha! The advice should be to pull "at least 6.6
prop blades". That is why the magic number is 7.
7 blades is enough.

BUT BECAUSE I AM SUPERSTITIOUS/PARANOID I WILL
PULL 8 BLADES AS THIS IS WITHOUT ARGUMENT "AT LEAST
7" BLADES. AND IF ANY OIL FLOWS FROM THE MANIFOLD
DRAINS OR THE EXHAUST I WILL WAIT AND DO IT ALL AGAIN.

Conclusion: If 20 blades is necessary then it
isn't enough. Otherwise 7 should be fine.

After that, of course, you still need to prime. But,
as demonstrated, more than 6 propeller pulls for the
prime will just blow fuel out the exhaust.

My starting procedure (summarised to leave out
the always done procedural and safety checks such
as mags off, masters off, pen in dashboard clip)
is the Richard Goode procedure with the amendments
listed here.

* Pull through EXACTLY 7 (or, if you lose count, 8) blades. If oil emerges, wait and repeat.
* Close manifold drains (they were open, weren't they?)
* Get in the a/c
* Eliminate any vapour by priming first to the system side. (thanks Richard, great tip!)
* Prime cylinder side 3 strokes (or 6 if cold).
* Get out and turn prop EXACTLY 6 (or, if you lose count, 5) blades.
* Momentarily depress manifold drains (to release any liquid fuel to avoid another cause of HL).
* Get in and prime cylinder side 1 stroke (or 2 if cold).
* Immediately start, do not releasing start button too soon (my most common mistake despite Richard's advice not to use the air system as if it were a starter motor).

Paul Beardsell
HA-YAB http://beardsell.com/flying/ha-yab/
Yak-18T http://beardsell.com/flying/yak18t/


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psb777



Joined: 12 Aug 2009
Posts: 34
Location: UK

PostPosted: Wed Sep 30, 2009 6:18 am    Post subject: Re: Pulling through 7, 8, 9 or 10 blades, or 20, 21? Reply with quote

Aaargh! 720 engine degrees is NOT 1200 prop degrees. It's 430 prop degrees. Is that right? Which is less than three (2-bladed) propeller blades pulled through.

Thus pop goes my arithmetic but what about the reasoning? If I turn the engine thru 720 degs I have pushed out all the oil in the cylinder head. If I pull the engine thru 7 blades I have done that more than twice. 7 has to be enough. 3 might be enough.


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MONTY(at)bpaengines.com
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PostPosted: Wed Sep 30, 2009 10:55 am    Post subject: Pulling through 7, 8, 9 or 10 blades, or 20, 21? Reply with quote

All theories of how much to pull the engine thru go in the toilet if
there is a slug of cold oil in one of the induction pipes. The engine
will start, the oil gets warm in the inlet pipe, then a big slug of oil
enters the cylinder.

Monty Barrett

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