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MISCONCEPTIONS
By Peter G. Tanis
In the book Stick and
Rudder. Wolfgang Langweische pointedly
differentiated airplanes from cars: "It may sound
like one and smell like one, and it may have been
interior decorated to look like one; the difference
is - it goes on wings. Langweische's point also
applies to an airplanes engine - it not like a
car's either.
The reason an aircraft's engine
needs different treatment from a car lies in the
basic construction of the engine. In an auto, the
engine block is usually cast iron and the
crankshaft is steel; these metals expand at
approximately the same rate when heated. In
contrast, the horizontally opposed aircraft engine
has an aluminum crankcase supporting steel
components such as the crankshaft and the camshaft.
The cylinder barrels are steel with aluminum
cylinder heads tightly attached.
The rate of expansion for
aluminum, as it is heated, is twice that of steel.
This also applies as it cools - aluminum shrinks in
size twice as much as steel.
Because the engine was designed and assembled at
room temperature, its clearances between parts can
shrink dramatically when the severe cold of winter
jets in. At temperatures as low as -11 degrees
Fahrenheit, one popular engine can completely lose
crankshaft bearing clearance. No wonder they turn
over hard when they're cold! Even warm oil can't
help when there isn't any bearing clearance. Many
pilots have ideas on how to operate their engines
in cold weather that come from their experience
with their automobiles. As a results there are some
popular misconceptions. Misconception: If You Can
Start It
...
Myth: ''The main purpose of preheating is to
start the engine: therefore, if you can start it,
you don't need preheating.''
Fact: An automobile engine survives quite well when
cold-started, but an airplane engine can be
severely damaged.
Because of poor fuel vaporization, an engine with
cold cylinders is hard to start. But if it is
started while they are cold, the cylinders are
easily damaged. The top end of the cylinder bore is
smaller than the base end-this is called ''choke".
It's designed to allow a nearly straight cylinder
wall once the engine is at operating temperature.
The choke has little effect at start-up in moderate
ambient temperatures. But the colder the
temperature, the more the cylinder is choked. When
the cold engine is turned over, the piston is
forced into the smaller-than-normal top end of the
cylinder.
Another thing happens when the cylinder is cold. It
concerns the wristpin, which in normal operation
floats freely, axially within its bore in the
piston. But the differences between metals in cold
weather may change that. The piston, being
aluminum, grips the steel wristpin. When a piston
which last stopped at the bottom of its travel
cools down, the wristpin end may be locked against
the cylinder wall. When this engine is started, the
wristpin end may wear against the cylinder wall.
The piston-to-connecting rod juncture also becomes
stiff in a cold engine, causing the piston to tip
at an angle as the engine is started. The first few
times the piston travels in the cylinder, it may do
so with its piston rings cocked at an angle and the
piston skirt contacting the cylinder wall.
As if this weren't enough damage to the cold
cylinder, one more thing occurs once the engine
starts; the aluminum piston grows at a faster rate
than the cylinder diameter. The result is scuffing
of the cylinder wall by the piston until the
temperatures equalize. This all might be enough to
make the owner of $12,000 aircraft engine grimace
to think of the pain his engine already may have
undergone. But there are worse effects from
cold-starting. Starting the engine with cold
cylinders may result in excessive wear-starting
with a cold crankcase could cause main bearing
failure.
Misconception: Warm Oil
Equals Warm Engine
Myth: "When preheating. the
most important thing is warm oil.'' This idea is
similar to some early preheating methods in
automobiles, which even in autos were not too
successful. The "dipstick heater'' heated
automobile oil but didn't help greatly in producing
the start.
Fact: Warm oil may not even
help if the rest of the engine is cold. Looking
again at the automobile engine, the most successful
means of preheating is the "inblock" type of
heater, which heats the coolant. This automobile
preheater heats only the cylinders and the block
areas--the oil isn't heated at all. It relies on
multi-viscosity oil to flow once the engine start
occurs. If oil heat were the only significant thing
in an aircraft engine, then the new multi-viscosity
and synthetic oils would be the only precaution
need. But actually, the aircraft engine with a cold
crank case, may have reduced bearing clearance
which won't accept any oil at all--hot, cold, or
synthetic. The bearings and journals may be in
metal-to-metal contact at the first instant of
motion.
Consider the damage that might be done by someone
who pulls the prop through a few times on a cold
engine to "free it up."
Complete
Tanis on-aircraft system includes heater
element for oil sump, plus elements for each
cylinder head, external power plug, pilot light.
It can be left plugged in continuously, keeping
engine safely warm for starting even in sub-zero
weather.
Cozy Cylinders
Myth: "If the cylinders are warm, you're
preheated." Some methods of preheating heat
only the cylinders. These engines start easily and
it appears that the plane is "home free."
Fact: While a car lives quite well this way, an
airplane may be in trouble because it has reduced
bearing clearances due to that cold crankcase.
Many failures have occurred in aircraft engines
over the years that have been the result of
improper preheating. It's my belief that most of
these may have been blamed on other things because
the nature of the problem was not understood. Some
types of failures that are caused or aggravated by
improper preheating are as follows:
Cold crankcases may "burn" or excessively wear main
bearings even though the engine has warm oil or
cylinders. This is usually incorrectly blamed on
"stiff oil" or a congealed oil cooler. In extreme
cases the bearing insert may rotate, blocking oil
flow in the entire crankshaft and thereby causing a
massive failure of the engine. In less severe
cases, the engine may exhibit poor propeller
control due to oil pressure losses in the worn main
bearings. Twins might demonstrate this by
propellers that won't stay "in synch" and won't
respond to cures such as overhauling the props and
governors. Other clearance related problems may
occur within an engine with a cold crankcase, such
as improper fit of the camshaft and the valve
lifter bodies.
If an engine has a warm "top end" and cold oil,
this may create its own problem. When using a
straight grade of heavy "summer" oil, the oil
system may not be able to draw oil to lubricate the
engine once start-up occurs. The newer
multi-viscosity and synthetic oils do a much better
job in this respect.
Drier Consequences
Myth: "since the engine is a closed system,
moisture is not a problem in preheating."
Fact: The engine is not a closed system. Moisture
is produced whenever the engine is run, and any
preheater vaporized this moisture. Regular flying
of the aircraft is necessary to clear out moisture
whether it is summer or winter.
This is an area not commonly understood. If only
the engine's lower end (oil sump) is heated, the
moisture vapor rises and condensed on the cold
parts such as the crankshaft and cam. (One can see
the same thing occur on basement water pipes in the
summer.) This moisture will produce rust and
acids.
But also, under the right conditions, it may freeze
in the oil breather tube, blocking the breather. If
this occurs, the crankshaft nose seal can in many
cases be blown out of the engine, followed by the
entire supply of oil.
The only way to avoid such problems is to assure
that the preheater system preheats the entire
engine and that the pilot has taken the proper
precautions to winterize the engine.
A list of winterizing items should include winter
grade oil, an oil cooler cover, and insulated
breather tube with an alternate hole, and a check
to see that the engine's baffle strips are in
place. A winter front should also be used if
approved for the airplane. (This is also a good
time to check the cabin heater for exhaust
leaks!)
Certification
Myth: "Since the airplane is FAA approved, it
should operate well under any temperature
condition." How could such an expensive device as
an airplane engine exhibit such poor
characteristics? Didn't FAA approval require it to
operate in these conditions?
Fact: The FARs under which the engine was certified
didn't require it to meet standards for
low-temperature operation. This isn't all bad, but
the engine's operator should be aware and take some
precautions.
In below-zero weather, the engine develops more
horsepower than it was certified to
develop--possibly as much as 15 percent more! To
counteract this, it's the practice of many cold
weather pilots to add carb heat once the throttle
is opened full (and to remove it when the reduce
power). Also, they don't sit on the runup pad with
the carb heat on for long periods, since it may
raise the temperature just enough to cause frost in
the induction system. This could cause the engine
to die when the throttle is opened.
"One Heater's Like
Another"
Myth: "Any preheater that is FAA approved or
that has "No Hazard Approval" will preheat my
engine properly." When a preheater is advertised as
FAA approved, doesn't that mean that it will do a
proper job of heating?
Fact: Since the FAA has no regulatory standards for
cold weather operations, to gain approval a
preheater may not have to meet any standards--it
may not even work. Many preheaters don't have any
kind of "approval", nor are they required to
because they are not installed on the aircraft.
The crankcase, the part of the engine most
critically in need of preheating, is the most
difficult part to preheat. This is because of a
kind of "wind chill factor", analogous to what
people experience in cold, windy weather, which is
transmitted through a not-so-obvious mechanism--the
propeller. Typically, the propeller accounts for the largest
heat loss on an engine being preheated. It sits
outside the cowling in the wind, drawing heat from
the crankshaft and case. While this "wind chill" can't cool an engine to lower than the outside air
temperature, it will demand more heat output from
the preheater to warm the engine to a given
temperature. Because of this, an insulated cover
for the cowl and propeller is desirable when trying
to preheat.
Every preheater has limits as to how much wind
chill it can handle. At a given sub-zero
temperature, some preheaters don't have enough
output to heat properly on a calm day--when wind
chill is added, even the best at some point will no
longer do the job. When a aircraft owner is
shopping for a preheater, he should find that the
reputable manufacturer of the unit is able to
discuss what temperatures his unit will produce--as
measured at the crankcase of the particular
engine--and the effect wind chill will have on this
performance.
Manufacturers of preheaters make many claims in
their advertising--some claim BTU's of heat, others
watts of power, and still others that you can "start in only 10 minutes" (or 15, or 20). But
considering that the information really needed is
whether the preheater will produce safe starting
temperatures, it's enlightening to compare
preheaters by the same standard.
One can try converting manufacturers' claims to the
same measurements. Conversion factors are available
in any high school physics book for such things as
watts to BTU's (One BTU equals about 0.293
watt-hours). One thing to remember is that there
are losses every time energy changes form or is
transferred to some other object.
Obviously, the best standard to use would be
temperature within the core of the engine. In the
absence of the ability to measure this precisely,
spending more time preheating, assuming
adequate preheater output, can be like buying extra
engine insurance.
Once the threshold of output has been met, there
are some other differences among types of
preheaters.
Most preheaters sold today are of the "air blower" type. Through one means or another, air is heated
and then blown around the engine compartment. Since
heat rises and it's often hard to position the
blower to be sure that hot air travels by all the
cylinders as well as the crankcase, there are often
parts of an engine that are extremely cold after
what seemed to be a hearty preheating session.
Sometimes, the blown air is simply misapplied, and
never gets to the critical engine parts. If it's
applied without proper engine covers in windy
conditions, the blown hot air just blows uselessly
away. But the most common mistake is to believe
that all that hot air applied for 15 minutes equals
a warm engine, when it could take hours to do the
job, with some heaters.
Provided these pitfalls are avoided, there are some
hot air preheaters which can do a reasonable job,
when properly employed.
Another type of heater being sold today heats the
oil pan electrically. However, it does nothing for
the "upper" engine, particularly the cylinder
heads. There are also dipstick-style heaters, which
we can dismiss for reasons mentioned above.
And there is the Tanis preheater, in which electric
heating elements are installed in strategic places
around the engine--not only an element on the oil
sump, but other elements on each cylinder--and the
airplane is simply "plugged in" to 110-volt outlet
for about 5-6 hours prior to being started. (It can
be left plugged in continuously, keeping the engine
constantly ready for starting).
It has been my experience that any "air blower" with less than 50,000 BTU is just too anemic to
work well. Since the total BTU energy in a typical
16-ounce propane bottle sold with many air blower
pre-heaters is approximately 8.700 BTU, there does
not appear to be enough heat in the bottle to do
the job--even if air transfer of the heat were 100
percent efficient (which is definitely not the
case.)
Yet an installed system such as the Tanis TAS100
will do an acceptable job on as little as 250
watts. Why is this? This particular systems
operates for a longer period of time, makes fewer
exchanges of energy, and transfers heat by
conduction, which is very efficient.
Whatever type of preheater is employed, its
effectiveness can be vastly enhanced by using
thermal blankets around the cowl to keep the heat
from being blown right out of the engine
compartment. Again, there are conditions of wind
and temperature which can make it impossible to
preheat the engine to the extent needed for a safe
start.
Properly
bonneted Warrior gets Tanis system
preheat using a portable generator, on a frozen
lakebed in the northern
states.
Ultimate Question
At this point, one may consider another
question: When the temperature or wind chill drops
into the negative teens, should we really be
flying? If a pilot had a forced landing, could he
survive long enough to reach shelter? When the wind
chill reaches -30 degrees and lower, a pilot who is
normally systematic and safety-conscious can be
turned into a madman whose only concern is to get
the door closed, the engine started, and the cabin
heater turned on. Some cold country pilots don't
fly much below -20 degrees unless it's an
emergency. At these temperatures a pilot may have
trouble keeping cylinder temperatures up, and as a
result produce more cylinder wear.
To summarize, when the temperature is below 20
degrees, be sure to thoroughly preheat the engine.
If you want to determine the "quality" of your
preheat, the cylinders, nose case, and the oil
should all be warm to the touch. If they aren't,
don't start.
Not all cold days are bad. On some cold days we may
climb into warm air and have a beautiful flight.
Cold weather flying can be some of the most
enjoyable of all flying. The air is smooth and the
airplane will perform well. Cold moonlit winter
nights can be great flying.
The beautiful thing about winter flying is that
once we understand it, we can properly prepare for
it. A pilot can dress properly for winter and be
comfortable. Try that for hot
weather--impossible!--I much prefer
winter.
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