thIs mOnth I’ll discuss heat, and how it
affects your engine.
I may not cover each circumstance,
because there are so many different
combinations that can combine to create
problems. Likewise, you can make a series of
errors that are bad enough in the short term to
hurt the engine run.
However, this discussion is extremely
important; if you want your equipment to
perform the way it should, read on.
before I get into some problems and
solutions, general discussion on
engine/cooling configurations/installations is
necessary.
Probably the most important factor in a
Stunt engine that runs well is a stable
mounting system.
The mounting surface should be flat, and
the engine should mate perfectly to the
surface. The surface should be a hard
material, such as aluminum or pheonolic, to
transfer load, heat, and vibration, and should
last indefinitely. There must be sufficient
mass to absorb and transfer vibration to the
main structure.
The design of the internal structure should
be carefully analyzed, to ensure that it has a
direct line to the exterior skin of the airplane.
If you don’t examine this, you can expect a
stress crack where the continuity stops.
Next, look at how you have provided for
airflow around your engine/muffler and pipe.
For Profiles, the engine is generally “hanging
out in the breeze.”
However, some do enclose the crankcase,
fairing it to a spinner. Be careful with this; the
“bottom end” can get hot enough to cause
problems on really hot days. It’s best to
provide some venting to the top of the nose; a
1/4-inch “blow hole” is sufficient, as long as it
allows the hot air to rise and escape.
The same approach holds true for the fully
cowled engines; it’s best to provide a crankcase
vent, although quite a few airplanes don’t have
one. It has a great deal to do with the normal
ambient temperature your model faces and the
repetitive rate at which you practice.
Use the hole, but don’t make it larger than
1/4 inch.
On to fully cowled models. As a general
rule, nothing should touch the airplane except
where you must mount the engine, muffler, or
pipe. This is for vibration transfer as much as
heat.
For heat, a surface in contact will create a
“hot spot,” and will cause a problem with
your model’s run characteristics.
If you think you don’t have a problem, go
back and re-evaluate!
Another point is stagnant air in your
model’s cowling. This is a killer, which can
quickly damage the engine.
Look carefully at the airflow from entry to
probable exit. You can get into trouble by
assuming that many cooling holes will get the
job done. That isn’t necessarily so; it depends
on what you want to cool. What I’m referring
to is a tuned-pipe installation.
I’ve seen and done installations where I
wanted to make sure I had the coupler and the
front of the pipe addressed. To make sure, I
had those areas cooled with outlets in close
proximity to those points.
I did get reasonable engine runs, but I
quickly learned that the pipes were producing
leaks. I did some airflow testing at that point,
spurred on by a close look at Bill Werwage’s
installation.
By blowing air with a hair-dryer and
checking with a shredded pipe cleaner, I
learned that most of the air went out the
CONTROL LINE AEROBATICS
Frank McMillan, 12106 Gunter Grv., San Antonio TX 78231
Another Al Rabe Mustang—this one by Nationals competitor Dave Hemstrought. Has
molded shells from the original molds. Many hours were invested in this model.
Tom Morris presents the component kit of the classic Cavalier design. Comes complete
with hardware package in a hardwood shipping case. An excellent value.
January 2001 151
Bare bones of Gerald Clark’s Wind Plane. Profile fuselage is not covered; remains open
in center. Flies well in wind! Rudder varies offset with line tension.
nearer exit. That meant everything aft
received virtually no airflow. (Remember
stagnant air?)
When I learned this was happening, I
closed off each hole. To my surprise, more
air came out the next hole, and so on, until
all but the last major opening over the barrel
of the pipe was closed. This is exactly the
configuration that Bill uses!
The basic layout is the approximately 3/4
x 11/2-inch inlet, with an outlet opening that
is the length and width of the main pipe
barrel, and at least an inch overlapping each
end for adjustment.
That isn’t the whole story! I was
discussing airflow, and now you have a
tunnel with an entry and an exit, which
contains a most important and expensive
package. You need to protect it with
smooth airflow in the tunnel.
Give the engine/pipe combination the
most physical clearance you can, then very
carefully add soft balsa filler blocks, so you
can control to smooth out the flow. Avoid
any sharp edges that can create turbulence.
Spend some time here; it will pay off in
longevity and durability for your equipment.
Also consider how you are going to finish
this area. It must be fuelproof, because
accidents do happen and raw fuel has a way
of finding cracks and pinholes. Sometimes
you will have to finish a part off the airplane,
because you can’t conveniently reach a
certain area after final assembly. Think ahead.
A couple thin coats of Smooth and Easy
epoxy heated and wiped off will be light and
effective. Butyrate dope requires quite a few
coats, to ensure that all pinholes are covered.
You must fuelproof the internal access areas.
The same basics apply for conventional
setups: clean interior cowling shapes, at
least three times the outlet areas to intake,
and nothing touching.
the basic point of this column is heat, but
can you have too much cooling?
You bet you can—big time!
It often manifests as a rough run that
starts as the airplane breaks ground and
settles into a four-cycle. Other times it
shows up after the engine has come to a
point in the pattern where it isn’t working
hard, such as outside rounds. Sometimes
it shows as the engine getting off its
“curve,” or slowing down.
Fixing the problem is a matter of trying
solutions until you eliminate the unwanted
characteristics.
Try a hotter plug. Then try a heat shroud
on the engine with two stages—one to direct
the airflow. If that doesn’t work, fabricate a
shroud that blocks off airflow to the cooling
fins. This will solve most problems; if not,
block the intake air incrementally.
As a last resort, try propylene oxide—an
ignitor—to raise the ignition temperature. One
ounce at a time per gallon is a good approach. I
mix my own fuel, and I typically run two to
three ounces per gallon. This mix offers smooth
running in a broad range of conditions.
Back to the heat.
Several symptoms will indicate that you
need to look at cooling the package. Some
“signs” may be subtle.
Be aware of how your engine reacts, or
“cycles” in the maneuvers. If you have
problems controlling the preciseness of your
shapes, or the airplane is getting ahead of you,
one cause might be heat.
Each engine setup has a “zone” it wants to
run in. Many factors contribute—propeller,
fuel, drag lines, etc. Your task is to find that
zone and keep your engine in it.
The good thing about cooling/heat is that
once you find the installation configuration, it
tends to hold well.
I received a letter from the Hillsdale Flyers
executive officer George Yatsko, concerning
training beginners in Control Line, and
eventually moving up to Stunt. George’s thrust
was to use electric power, because of the
advantages of noise and flying-site availability.
There are development issues of course,
but great strides are being made with electric
power in other modeling disciplines.
There is word that John Brodak has a
development effort under way. If any readers
are interested in working with the Hillsdale
Flyers, contact George Yatsko at Box 32,
Hillsdale NJ 07642. MA
