jill(at)m-14p.com
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 Posted: Sat Jun 03, 2006 8:02 am Post subject: Sudden Stoppage Reply |
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Group,
There was a thread running a week or so ago about sudden stoppage,
"shock loads" and engine teardowns. Our opinion differs from a
previous posting.
Carl submits the following:
To add our experience on the “sudden stoppage” question, I offer the
following with respect to M-14P engines and V-530 props:
While we have not sold hundreds of aircraft, since 1993 we have
repaired more engines than anyone in the West, including complete
teardowns. We are also certified by OKBM for repair and overhaul of
the M-14P.
Let’s start by defining terms. A “prop strike” alone says nothing
about forces experienced by the engine. A “sudden stoppage” defines a
reduction in engine RPM from some external or internal cause. The rate
of deceleration is the key to how much damage may have occurred. We
have seen only four sources of stoppage:
1. The engine had oil in a cylinder that did not compress with the air
start turning the engine.
This will usually result in a bent link rod. A teardown of the power
section is necessary to check crank
runout (necessary) and is perhaps the easiest way to change the link
rod as any other way.
2. The engine had oil in a cylinder or sucked it in from the intake
tube and did not compress after
the engine fired. The first thing to check, if the engine will turn,
is the magneto timing. This tells if the
accessory shaft is twisted. Next, a teardown of the power section. We
usually find the crank rear
counterweight has rotated on the bearing shaft. This is a quick, cheap
fix. Continued rotation, however,
results in broken teeth on the cam drive gear and cam idler gear.
Broken teeth on the generator drive gear
and damaged accessory shaft are very possible. A new link rod and
probably a new cylinder will also be needed.
3. A previously bent link rod fails in flight. The prop shaft will
shear. In the worst case we have seen,
there were two pistons in one cylinder. The entire engine will be
trash.
4. The rotating prop hits something. If you taxi into something solid
like a fire hydrant, the resultant
damage will be the same as in #2 above, except for no damage to a link
rod or cylinder. If you experience
a gear up landing you have to make a decision. At landing speed, the
aircraft travels 3.5 feet for each blade
in contact with the ground at initial point of contact. As the engine
slows, this increases. Consequently,
the rearward deflection of a very thin blade exceeds the rotational
shearing. Additionally, the reduction
of blade length per rotation with a normal flare landing is less than a
tenth of an inch. In other words, the
wood fails well before any engine parts are stressed to the +400
foot/lbs of torque design load.
We had many conversations with the Russian overhaul factory during the
90’s and with OKBM in 2001. They require a teardown under their system
(labor is cheap) but no one could recall damage from a gear up landing.
We have sold 20 – 30 sets of blades for replacements on gear up
landings with no known subsequent problems with the engines. Having
overhauled many V530 prop hubs, there is no doubt that it will fail
last.
If you land gear up, you must weigh the costs, down time, damage
probabilities and future consequences. If I landed my Yak 52 gear up
in a n normal landing configuration, I will install new blades and keep
flying.
Carl Hays
Continuing in Doug Sapp's vein, we are also committed to the long haul
of keeping your aircraft flying. Every day we address new R&D
challenges, shortages, bureaucratic entanglements, etc. This business
is not for the faint of heart. I think as Yak owners, we have enjoyed
the relatively cheap joys of owning and operating these aircraft. The
pendulum is swinging the other way. I don't begrudge the Russians
asking for high prices, right now. Eventually this market will have to
normalize itself. That means some factories in Russia will flourish,
some have folded or will fold. I have a choice to buy their parts or
find my own solutions. I have spent several thousand dollars to
manufacture dies to stamp out gaskets and more money stuck into the
inventory of those gaskets. I appreciate every single sale I make.
There are easier ways to make more money and at far less risk, but I
personally love what I do and enjoy (most) people in this business.
Years ago, we heard of a small group of entrepreneurs in Russia that
had purchased a factory that had closed down. They began a rabbit meat
processing plant and used the fur byproduct to make ushankas - the
classic Russian fur hat. Our partners in Moscow asked if we would be
interested in trying to establish a market over here. We
wholeheartedly agreed to give it a whirl and ordered 100 ushankas. I
sold some to customers, outfitted family members in Wisconsin and New
York but I still have a bunch left here in Arizona (big market). We
never expected to make a fortune on this venture. In fact, if we broke
even we'd be happy. We did have an interest in helping a company that
was trying in an business climate where that is still difficult.
The bottom line is that M-14P Inc. is in business to make sure your
airplane is flying, make a little money to keep the lights on and have
a little bit of fun along the way.
Jill Gernetzke
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