One reason for not running a car or airplane for short periods is that you
have initial startup wear. Most of the wear in cars, and perhaps airplanes
too, happens at startup, when there is zero oil pressure, minimal oil on the
surfaces, and the presence off some condensed moisture. If you could only
run an engine on the ground for 5 minutes at a time, you would definitely
not want to run it once a day. On the other hand running it like this 4
times a year would probably increase the life by making sure there is some
oil on the interior surfaces. I don't know of any data that will tell us
where the sweat spot is that causes the least wear. One point of evidence we
do have is that engines that are not run for extended times do corrode in
tell tale ways. You never want to buy a plane that has sat for extended
periods.

wrote in message
oups.com...
I just watched a program about the huge Oresund bridge between Denmark
and Sweden, and there was a segment about how the corrosion problem for
the bridge was dealt with. Instead of painting, they use sealed
compartments that have the air humidity kept below 60%. This they said
eliminates corrosion. This started me thinking about the corrosion we
aircraft owners are told to guard against, since I have personally been
told by an overhaul shop, that half of all the engines he sees that
need work, need it because of corrosion. There are several "facts" that
I am beginning to question as to their validity. The ones that come to
mind a

1) You have to get the oil up to 180 deg F or the water in the oil
won't evaporate.

2) Starting and ground running the engine for a minute or so is the
"worst" thing you can possibly do.

3) Flying for an hour will "clean" the oil (or at least evaporate the
water, preventing acid formation) so that it doesn't turn to acid and
dissolve the engine while sitting idle.

There are probably some others, but these three stand out the most
to me. Now I am sure that what I am about to say will not go over well
with some people, but I have the asbestos suit ready and waiting.
My take is that these three "facts" are a bunch of poppycock. Why
or how they got started is anyone's guess, but the reasoning behind
some of them is understandable, for others I wonder what they were
smoking at the time. My thoughts are along these lines, and I admit I
could be wrong, but I don't think so.

Concerning fact #1...Why does someone think that the water has to be
brought to a boil before it will evaporate. Water evaporates very well
even at sub-freezing temperatures,much less at the warm to hot temps
created in a running engine. And at say 140 F, I can't help but believe
that any water or moisture in the engine will be purged quickly. With
water at that temp you can literally see clouds of vapor escaping, and
this is well below boiling. Not that the hotter the engine gets the
water doesn't evaporate more quickly, it does I'm sure. It's just that
in the engine cases which are open to the air at the breather tube and
elsewhere, any heat above say 85 F or so will be more that enough to
dry out the oil in the crankcase. As evidence of this, I ground run my
engine all the time and have for many years. I live in a VERY humid
climate. The oil analysis reports I have done on a regular basis by
Blackstone have never shown any trace of water or moisture. I recently
tore done the engine for rebuild after more then 15 years of perfect
service, and the cam lobes, lifter faces, and every part in the engine
was shiny and totally free from rust or any other corrosion.

Concerning fact #2.... We all constantly clean and oil many of the
things we own such as tools, etc. It is the layer of oil that prevents
the rust. I accept that if the oil is too acidic it could corrode or
"chem mill" the metal, but it takes highly acidic liquid to do that,
and the oils I use have acid buffers in them to deal with mild acidity.
Running is the only way to re-oil all the parts in the engine,
especially the cam and lifters. Just like oiling the machinists tools
that I own keeps them rust free, oiling the engine keeps it rust free I
would think.

Concerning fact #3.... This is the one that really makes me wonder. If
oil needs to be changed every 25-50 hours, how does flying for an hour
clean it? I can't help but think that the longer the oil is used the
dirtier it gets. I guess they think that it's "really" dirty just after
starting, and you "clean" it as you fly.

In closing, it seems to me that many of the things we are told are
contradictory on this subject. I have witnessed many OWT come and go in
my time, so common knowledge isn't always correct, lean of peak
operation comes to mind. Some blockheads still don't think George Braly
has it right.


Blue Skies
Rusty