Message-ID: <20000205065704.20269.qmail@web125.yahoomail.com>
Date: Fri, 4 Feb 2000 22:57:04 -0800 (PST)
From: BILL HANNAHAN <wfhannahan@yahoo.com>
Subject: RELIABILITY
To: MAIL LANCAIR <lancair.list@olsusa.com>
Mime-Version: 1.0

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Once in a while I get an irresistible urge to
stir up a hornets nest by challenging a
longstanding and widely cherished belief.  I
should know better, having read what happened to
Galileo, but here goes.

Consider the following statement. Most engine
failures occur at the first power reduction after
takeoff, therefore it should be delayed till a
safe(?) Altitude of 1000’ feet is reached. True
or false? 

 Now consider this statement. While driving about
the country you will occasionally come across a
serious traffic accident. In most cases there
will be police cars and ambulances at the scene,
therefore police cars and ambulances cause most
serious traffic accidents.  True or false? The
logic is the same in both cases so your answers
are the same, right?

 Uhaa, what’s that buzzing noise?

Now consider this example. An underground
coalmine has nine levels. A coal bucket weighing
1000lb empty is lowered to the bottom, level 9,
where 1000 lb. of coal is dumped in. The bucket
is raised to level 8 where another 1000 lb. of
coal is added. The process continues at each
level so that when the bucket reaches the surface
it contains 9000 lb. of coal for a total weight
of 10,000 lb. The bucket is suspended on a cable
with a breaking strength of 20,000 lb. when new,
however each complete cycle reduces the strength
of the cable by 1 lbf.  When and where will the
cable break? Neglecting a few details like the
weight of the cable, it will fail at level 1 when
the last pound of coal is added on cycle 10,001 .

That new engine you mounted on your firewall,
like the new cable, is brutally strong, capable
of absorbing almost any punishment you deal out.
But with each cycle, parts of your engine become
less strong and more brittle than they were. At
some point the margin between the strength of a
part and the load it is subjected to may shrink
to zero and the reassuring sound of raw power
changes to a brief rendition from the hammers of
hell followed by the thought provoking serenity
of glider flight.  This transition is most likely
to occur when the engine is being subjected to
maximum stress.

Engine stress can be divided into two categories,
mechanical and thermal.

One source of mechanical stress is the
acceleration loads on rotating and reciprocating
parts. It is proportional to the square of RPM.
At 2700 rpm the loads are 26% higher than at
2400. Another source of stress is pressure in the
cylinder, that generates thrust load on the
piston, connecting rod, crankshaft and the
cylinder itself.

Thermal stress is partly mechanical stress
induced by temperature changes in the engines
parts combined with the changes in material
properties at high temperatures.

Preignition and detonation are phenomenon that
can age an engine rapidly as described in an
article on the Lycoming web site.


Our airplanes perform better at 80% power then
most certified piston poppers running wide open. 
We have the option to trade a bit of performance
for an improvement in reliability. To slow the
aging process and enhance reliability I normally
take off at 26 square.  When the plane is cleaned
up and accelerated to cruise climb speed I back
off to 23.5 squared.  When the CHT stops
climbing, mixture is pulled from extremely rich
to very rich, than leaned in small increments at
one to two minute intervals depending on the
actual temperature.

Some folks say injected engines have an extra
rich setting for takeoff power.  I have no
knowledge or advice on this. It would be
interesting to hear from an expert, a) how it
works, b) how it differentiates takeoff from full
throttle cruise, especially at high altitude
airports, and c) which injected engines have this
characteristic.

I make full power takeoffs at airports with
runways that are very long and numerous. The idea
is to stress the engine as hard as possible to
detect any developing weakness under circumstance
where a landing straight ahead or a circle to
land on a crossing runway is possible. Throttle
back before exceeding gliding range to the
airport.

For a second opinion read Old Wives Tales from
the Lycoming web sight, copied below for
convenience.

OK hornets, sting me if you can!

Bill Hannahan


>From the Lycoming web page which has many
interesting articles on the maintenance and
operation of engines.


Http://www.lycoming.textron.com/support/publications/old_wives_tales.html

 	  
A Review of Old Wives TalesTale Number One—"The
most likely time for an engine failure to occur
is at the first power reduction after takeoff."
Every individual who pilots an aircraft has
probably heard this statement at some time. Is it
a true statement? We will venture a guess and say
that perhaps it may have been at some time in the
distant past.Several years ago this question was
asked of me and it led to questioning some FAA
employees and a number of other pilots about
where the justification for this statement might
be found. After several weeks of poking into this
subject, it was finally necessary to conclude
that we could find no justification - that it was
simply an "Old Wives Tale."A letter which
recently came from a Flyer reader takes this one
step further. First it appears that there are
many who continue to repeat this tale. This
caused our reader to delve into the subject a
little deeper - perhaps a little more
scientifically than I did. Our reader studied a
computer readout which had data on incidents of
engine failure over a recent three year period.
Based on the material in that report, this reader
concluded that engine failures during takeoff are
quite rare, and that failures during cruise are
far more common. This does seem logical since the
engines of fixed wing aircraft run a majority of
their operating life in the cruise power
range.Our reader also had a very believable
theory about how this tale may have gotten
started. He wrote, "It seems likely to me that
this idea got started when twin engine flight
instructors would simulate an engine out during
takeoff - right about the time that the student
put his hand on the prop control to reduce
power.... Gradually the idea was propagated that
this was the most likely time for an engine
failure, when in reality it was a likely time for
an instructor to simulate a failure."From these
two searches for justification - with none being
found in either case, I believe it is fair to
conclude that "the idea of an engine failure
being most likely to occur at the first power
reduction after takeoff" is in fact an old wives
tale. For the sake of safety, lets stop repeating
this false tale and start promoting the idea that
we should be ready to deal with power failure at
any time.A second old wives tale is still being
promoted by some individuals. This tale involves
the constant speed propeller and goes like this:
"The RPM in hundreds should not be exceeded by
the manifold pressure in inches of mercury."
Referred to as a "squared power setting" (i.e.
2400 RPM x 24 inches of MP), it appears that this
tale may be the result of a carry-over from some
models of the old radial engines which were
vulnerable to bearing wear at high power
settings. Changes in engine design along with
improved metals and lubricants permit changes in
the operation of modern flat, opposed cylinder
power plants.Any pilot who believes that squared
power settings continue to be necessary should be
urged to read and understand the information in
the Pilot’s Operating Handbook (POH). While there
are limits to the power which should be taken
from most engines, particularly those which are
turbocharged, the combinations of RPM and MP
listed in the power charts of the POH have been
flight tested and approved by the airframe and
powerplant engineers. For example, if the POH
chart lists 2200 RPM and 26 inches of MP as an
approved power setting, pilots should not be
apprehensive about using that setting if it meets
their needs.Isn’t it strange that some bits of
information come to be believed by large segments
of a population even when they are untrue? The
two issues discussed above are good examples.
Will it ever be possible to get all of our fellow
pilots to reject the two false ideas outlined
here? Let’s keep trying.

=====
BILL HANNAHAN
WFHANNAHAN@YAHOO.COM

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